Effect of Land Use, Climate and Soil Structure on Soil Organic Carbon in Costa Rican

Ecoregions

Thesis

Presented in Partial Fulfillment of the Requirements for the Degree Master of Science in

the Graduate School of the Ohio State University

By

Paula Chacón Montes de Oca

Graduate Program in Natural Resources

The Ohio State University

2009

Thesis Committee:

Rattan Lal, Advisor

Frank Gilbert Calhoun Jr

Norman R. Fausey

Copyright by

Paula Chacón Montes de Oca

2009 ABSTRACT

Abrupt climate change (ACC) is an unprecedented global threat. Cost effective strategies to mitigate ACC include offsetting anthropogenic emissions through terrestrial carbon

(C) sequestration. Cultivated tropical soils may contain 40 % less soil organic carbon

(SOC) than their potential capacity. Consequently, the conversion into appropriate land use and management practices could increase the SOC pool and mitigate ACC. The identification of appropriate soils and land use options for establishment of terrestrial C offsets necessitate establishing what proportion of the total C pool is securely stable within the soil aggregates and adsorbed to the silt and clay particles of the soil.

A field study was designed to understand mechanisms of C sequestration in soils of the

Neotropics and their capacity to mitigate ACC under different management schemes.

This study focuses on characterizing the SOC pool up to 1-m depth in 12 land uses, distributed in 3 contrasting ecoregions of Costa Rica. Specific objectives were to: (i) to determine the effect of land use on SOC pool, (ii) to establish the role of climatic parameters on SOC pool and, (iii) to establish the role of primary and secondary soil particles on the physical protection of SOC. The hypothesis tested was that SOC pool is significantly influenced by climate and land use, and its stability is strongly dependent on the physical protection provided by soil aggregates and primary particles.

Results show that the mean SOC concentration was higher in the order Montane > Pacific

Dry > Atlantic Moist ecoregion. The total SOC pool ranged from 114 – 150 Mg C ha-1 in

ii the Atlantic Moist ecoregion, 76 – 165 Mg C ha-1 in the Pacific Dry ecoregion and 166 –

246 Mg C ha-1 in the Montane ecoregion. The estimated C sink capacity was of 18.1 -

36.7 Mg C ha-1, 14.1 – 88.6 Mg C ha-1 and 9.4 – 80.7 Mg C ha-1 in the Atlantic, Pacific and Montane ecoregions, respectively. The SOC was significantly correlated to bulk density, field moisture content, mean annual temperature, mean annual precipitation and altitude. There was a physical protection capacity of 6 to 60% in the silt plus clay fraction of some of the land uses studied. The land uses with heavier texture had a larger SOC storage capacity than those with lighter texture, which had already attained their estimated capacity. However, some soils were significantly deviated from their capacity level, which indicated that given the high SOC contents and greater proportion of silt plus clay, the models suggested by the literature are not reflective of the real attainable SOC storage capacity. Finally, the aggregate properties were significantly related to SOC and differed significantly among land uses and ecoregions. It was concluded that the ultimate

SOC sink capacity is determined by the protective capacity in the silt and clay fraction, rather than the SOC sink capacity on total soil basis.

iii DEDICATION

Dedicada al campesino costarricense, en especial a aquellos campesinos que han sido parte de mi formación personal y profesional.

A mis abuelitos y mis papás, quienes con su ejemplo me enseñaron la importancia de la responsabilidad, la honestidad y la lealtad, a ellos dedico este título y todos mis logros personales y profesionales.

iv ACKNOWLEDGEMENTS

I wish to thank my advisors, Dr. Rattan Lal, Dr. Norman Fausey and Dr. Frank Calhoun for their advice and guidance throughout this process. I have special gratitude to Dr.

David Hansen, who was the first contact from OSU I met in Costa Rica and has been very supportive ever since.

Special thanks go to my colleagues at C-MASC, especially Ji Young, Josh Beniston and

Klaus Lorenz for their support and opportune advice. Also, I am very grateful to Mr.

Basant Rimal and Mr. Sandy Jones for their support at the lab facilities.

I want to thank the administrative staff at SENR: Mrs. Pat Polczynski, Mrs. Amy

Schmidt, Mrs. Pat Patterson, Mrs. India Fuller, Mrs. Betsy Poeppelman, Mrs. Theresa

Colson and Mrs. Mary Capoccia. Also, I am very grateful to Dr. Don Eckert and Dr.

Jerry Bigham, they have been very supportive and helped me in many occasions.

At EARTH University, I want to thank everyone who helped me conduct my research:

Dr. Humberto Leblanc and Dr. Ricardo Russo, Mr. Ricardo Palacios, and Mr. Carlos

Sandi, Mr. Herberth Arrieta and all the staff at the soil science laboratory, and specially, my field worker Don Joaquin. Special thanks go to the families and administrators at the farms I visited: the González-Sotela family, Mr. Alexander Bolaños, Mr. Róger Blanco,

Mr. Henry Guerrero & Family, Mr. Ramón Quesada and his son, the Araya-Murillo family and Mr. Diddier Campos.

v Finally, I want to thank my fiancé Daniel, for his daily support and encouragement. His company has been a great source of comfort and strength during these years.

vi VITA

B.Sc. in Agricultural Sciences. EARTH University, Costa Rica (01/03 – 12/06).

Graduate Research Assistant. Carbon Management & Sequestration Center, The Ohio

State University (01/08 – present).

Teaching Assistant. School of Environment and Natural Resources, The Ohio State

University (03/09 – 06/09).

Research Associate. Carbon Management and Sequestration Center, Ohio State

University (05/07 – 12/07).

Research Associate. EARTH University, Costa Rica (01/06 – 12/06).

Research Associate. Maui Land & Pineapple Co., Hawaii, U.S.A. (09/05 – 12/05).

Publications

Chacón, P., H.A. Leblanc and R.O. Russo. 2007. Fijación de carbono en un bosque

secundario de la Región Tropical Húmeda de Costa Rica. Tierra Tropical 3 (1): 1-

11.

Fields of Study

Major Field: Natural Resources

Specialization: Soil Science

vii TABLE OF CONTENTS

ABSTRACT ...... ii

DEDICATION ...... iv

ACKNOWLEDGEMENTS ...... v

VITA ...... vii

TABLE OF CONTENTS ...... viii

LIST OF FIGURES ...... ii

1. INTRODUCTION ...... 1

2. LITERATURE REVIEW ...... 4

The Carbon Cycle and Soil Organic Carbon Sequestration ...... 4

Soil Organic Carbon Sequestration in Tropical Soils ...... 5

Factors for Carbon Sequestration in Tropical Soils ...... 7

Effect of Land Use and Management on SOC ...... 7

Climatic parameters of SOC ...... 8

Effect of Soil Structure on SOC ...... 10

Ecoregions as a Tool for the Study of SOC ...... 15

Study Region: Costa Rica ...... 15

References ...... 17

viii 3. EFFECT OF LAND USE AND CLIMATE ON SOIL ORGANIC CARBON IN

COSTA RICAN ECOREGIONS ...... 25

Abstract ...... 25

Abbreviations ...... 26

Introduction ...... 27

Methodology ...... 28

Study sites ...... 28

Soil Sampling ...... 33

Soil Analyses ...... 33

Statistical Analyses ...... 34

Results and Discussion ...... 35

Conclusions ...... 60

4. PHYSICAL PROTECTION OF SOIL ORGANIC CARBON BY PRIMARY AND

SECONDARY PARTICLES IN COSTA RICAN ECOREGIONS ...... 66

Abstract ...... 66

Abbreviations ...... 67

Introduction ...... 68

Methodology ...... 71

Soil Preparation ...... 72

Aggregate Analyses...... 72

ix Carbon Fractionation ...... 74

Statistical Analysis ...... 76

Results and Discussion ...... 76

Conclusions ...... 105

References ...... 106

5. SUMMARY AND RECOMMENDATIONS FOR FUTURE RESEARCH ...... 113

6. REFERENCES ...... 117

7. APPENDICES ...... 131

Appendix A. Review of soil organic carbon pool (Mg C ha-1) in different ecoregions

and land uses of Central America ...... 132

Appendix B. List of mass conversions under the International System of Units (SI)

used in this study ...... 146

Appendix C. Complete particle size distribution of the soils studied...... 147

x LIST OF TABLES

Table 3.1. Characterization of the land uses studied in three ecoregions of Costa Rica.. . 33

Table 3.2. Soil organic carbon concentration (± C.I.) up to 30 cm in different land uses, soil types and life zones of Central America...... 40

Table 3.3. C:N ratios at 0 – 30 cm depth in different land uses of Costa Rica ...... 41

Table 3.4. Relationship between the absolute vertical distribution of SOC and mean annual temperature (MAT), mean annual precipitation (MAP), altitude and texture ...... 47

Table 3.5. Total Organic Carbon Pool (0 – 100 cm) of three ecoregions in Costa Rica. .. 49

Table 3.6. Correlations between climatic variables and soil organic carbon (SOC) based on the data obtained in this study...... 51

Table 3.7. Correlations relating SOC concentration (0 – 30 cm) to mean annual precipitation (MAP), mean annual temperature (MAT) and altitude in different life zones of Central America...... 53

Table 3.8. Review of soil organic carbon pool (Mg C ha-1) at 0 - 100 cm in different ecoregions and land uses of Central America...... 55

Table 3.9. Estimated C sink capacity for Costa Rican soils up to 1 meter depth under different life zones...... 56

Table 3.10. Estimated SOC pool in soils of Central America forest, croplands and pasturelands ...... 57

ii Table 3.11. Costa Rica’s greenhouse emissions inventory for 1990 (baseline year), 2005 and predicted emissions for 2009...... 58

Table 3.12. Potential for soil organic carbon sequestration under different scenarios in

Costa Rica...... 59

Table 4.1. Soil Organic Carbon concentration in the sand fraction (POM-C) of different land uses in three ecoregions of Costa Rica...... 77

Table 4.2. Organic carbon contents of total soil (TC), particulate organic matter (POM-C) and the silt + clay fraction (S+C –C) of different land uses in three ecoregions of Costa

Rica at 0 – 30 cm...... 78

Table 4.3. Capacity level of the silt plus clay fraction (g C kg-1 soil) up to 30 cm depth..

...... 84

Table 4.4. Carbon concentration in the macro (4,750 – 250 µm) and microaggregates

(<250 µm) of different land uses in three ecoregions of Costa Rica...... 86

Table 4.5. Enrichment factors (E) of C in different fractions of the soil of different land uses at 0 – 10 cm depth...... 91

Table 4.6. Mean Weight Diameter of the water stable aggregates in different land uses of

Costa Rica...... 94

Table 4.7. Correlations among carbon in the whole soil (TC) and different fractions and the physical properties for 12 land uses of Costa Rica ...... 104

iii LIST OF FIGURES

Figure 3.1. Bulk density at 0 – 100 cm depth in three ecoregions of Costa Rica: Atlantic,

Montane and Pacific ...... 36

Figure 3.2. Carbon concentration in different land uses of the Montane Pacific and

Atlantic ecoregions ...... 37

Figure 3.3. Soil organic carbon pool in different land uses of the Montane, Atlantic and

Pacific ecoregions...... 43

Figure 3.4. Vertical distribution of soil organic carbon as a proportion of the total pool in the Pacific, Atlantic and Montane ecoregions ...... 46

Figure 3.5. Relationship between altitude and mean annual temperature in Central

America. Redrawn from original data from studies in Central America ...... 52

Figure 4.1. Dilution effect of C associated to POM in three different ecoregions: Pacific,

Atlantic and Montane at 0 – 10 cm...... 79

Figure 4.2. Relationship between the silt plus clay content of soils and the C associated to these particles in the ecoregions of this study...... 80

Figure 4.3. Relationship between the content of silt and clay and the carbon in the silt plus clay particles of three ecoregions in Costa Rica...... 83

Figure 4.4. Relationship for aggregate C and water stable aggregates (WSA) in the

Pacific, Montane and Atlantic ecoregions...... 87

ii Figure 4.5. Relationship between soil organic carbon in the bulk soil and the carbon associated to the aggregate and particulate organic matter fractions in 12 land uses of

Costa Rica...... 88

Figure 4.6. C:N rations of the macro (>250µm) and microaggregates (>250µm), particulate organic matter (POM) and bulk soil in three different ecoregions: Pacific,

Montane and Atlantic ...... 89

Figure 4.7. Distribution of Water Stable Aggregates (WSA) at 0 – 10 cm depth in different land uses of the (a) Atlantic, (b) Pacific and (c) Montane ecoregions...... 93

Figure 4.8. Frequency distribution of water drop penetration time (WDPT) among aggregates of different land uses in three ecoregions of Costa Rica...... 95

Figure 4.9. Water drop penetration time (WDPT) of different land uses of Costa Rica. .. 97

Figure 4.10. Tensile strength of different land uses of Costa Rica...... 102

Figure 4.11. Moisture retention the (a) Pacific, (b) Montane and (c) Atlantic ecoregions at

0 – 10, 10 – 20 and 20 – 30 cm...... 103

iii 1. INTRODUCTION

Strategies to mitigate abrupt climate change (ACC) include offsetting anthropogenic greenhouse gas (GHG) emissions through terrestrial carbon (C) sequestration (Dilling,

2007; Lal, 2003). Stable soil organic carbon (SOC) pools are a key component for terrestrial C sequestration (Lal, 2003; Craswell and Lefroy, 2001) and constitute a cost- effective alternative to generate terrestrial emissions offsets (Sohngen and Mendelsohn,

2003). Despite its technical mitigation potential, there remain questions concerning the current magnitude of SOC pool in tropical regions (Canadell et al., 2007). The purpose of this research is to determine how C is securely stored and in what approximate amount in some soils of Costa Rica, and to determine the C sink capacity for three neotropical ecoregions under diverse soil management options.

The terrestrial C sink comprises the biotic and the pedologic pools; the latter includes the

SOC pool. Since 1850 more than 80 Pg (1 Pg = 1015 g) of C have been released from the soil due to land use changes such as deforestation and soil cultivation (Lal, 2004).

Around 120 Pg C per year are absorbed from the atmosphere through photosynthesis, but a similar quantity is returned back through soil and plant respiration (Lal, 2008). Yet, if appropriate management practices were introduced to prevent even 6 - 8 % of the photosynthesized C from going back into the atmosphere, this quantity would be sufficient to offset the total global fossil fuel emissions and mitigate the adverse effects of

ACC (Lal, 2008).

1 The soils of the Tropics are relevant to the global C cycle (GCC) due to the magnitude of its area and biomass production (Lal and Kimble, 2000). However, deforestation and land use change have the potential to destabilize vulnerable C pools resulting in the release of carbon dioxide (CO2) into the atmosphere and consequently requiring policymakers to select higher targets of CO2 emission reductions (Canadell et al., 2007). Some studies estimate the cumulative loss of SOC between 50 to 70% of the original pool in world croplands (Lal, 2000). In the tropics, soils may contain 20 – 40 % less SOC than their potential capacity after 5 years of cultivation (Detwiler, 1986). Consequently, the conversion into appropriate land use management practices could increase the SOC pool, providing a series of benefits not only on ACC mitigation, but also on desertification and erosion control, water quality, food security and soil fertility (Lal, 2004).

To properly manage soils as C sinks it is imperative to understand the mechanisms by which C is lost or enhanced in the soil (Rovira and Vallejo, 2003). The rate at which SOC is released to the atmosphere depends on its bioavailability, which is in part governed by climate, land use and management (Goebel et al., 2005; Zehetner et al., 2003; Diaz-

Romeu et al., 1970; Kemper and Koch, 1966). Some studies have addressed the role of land use change on SOC depletion/accretion, especially in forest – pasture sequences

(Veldkamp, 1994; Van Dam et al., 1997; Powers and Schlesinger, 2002). Yet, there are large uncertainties on the available estimates of the current SOC pools of tropical

America and the region’s potential for C sequestration (Canadell et al., 2007).

Historically, more research has been done in some tropical regions compared to others; as a result, there is more available information for the humid tropics than for the dry

(seasonal) regions and highlands. The tropical dry regions are probably among the most

2 threatened areas in Mesoamerica; yet, these ecosystems are probably the least studied

(Arroyo-Mora et al., 2005). Similarly, research in tropical highlands is ideal since these soils can sustain a large SOC pool due to their inherent characteristics and favorable climatic conditions (Jimenez and Lal, 2006).

In order to identify appropriate soils and land use schemes for establishment of terrestrial

C offsets, it is also imperative to calculate the proportion of the total C pool that is physically protected (Six et al., 2002; Hassink, 1997). Soil organic carbon stabilization occurs through physical and chemical mechanisms. Two mechanisms of physical protection are aggregate formation and physical binding with clay and silt particles (Six et al., 2002; Bronick and Lal, 2005). Understanding the physical protection capacity of tropical soils may contribute to the relevant issue of permanence of sequestered C and therefore to efficient GHG offset (Dilling, 2007, Hassink, 1997).

This research aims to characterize the SOC pool up to 1 m depth in 12 land uses, distributed in 3 contrasting ecoregions of Costa Rica. Specific objectives of this research are: (i) to determine the effect of land use on SOC pool; (ii) to establish the role of climatic parameters on SOC pool; and (iii) to assess the role of primary and secondary soil particles on the physical protection of SOC.

The hypothesis tested is that SOC pool is significantly influenced by climate and land use, and its permanence is strongly dependent on the physical protection provided by stable aggregates and primary particles.

3 2. LITERATURE REVIEW

The Carbon Cycle and Soil Organic Carbon Sequestration

Carbon forms the basis of all living organisms when associated with other important elements such as nitrogen, hydrogen and oxygen (N, H and O). Carbon dioxide (CO2) is a byproduct of human and animal respiration and a major atmospheric component which helps maintain a suitable temperature on earth. When combined with hydrogen into hydrocarbons, C forms the basis of the world’s fossil fuels (Kaleita, 2006).

Through photosynthesis, plants absorb CO2 from the atmosphere and convert it into carbohydrates. About half of the carbon consumed in the photosynthesis process is released through plant respiration; the remaining half is stored as biomass. Under natural or undisturbed conditions, this biomass is incorporated into the soils and becomes part of the soil organic carbon (SOC) pool. The SOC pool is comprised of animal and plant residues at various stages of decomposition, chemical and microbiological breakdown products, and the bodies of microorganisms and small animals (Lal, 2008). As organic matter decomposes, CO2 is released back into the atmosphere. Carbon dioxide emissions from agriculture and deforestation are caused by the total or partial removal of biomass from the field, increases in the mineralization rates due to changes in soil temperature and moisture and losses by leaching and erosion (Lal, 2003). However, under appropriate management, the C in the soil’s biomass can be permanently stored (mean residence times may vary from

4 102 to 103 years), becoming a form of C sequestration (Lal, 2008; Pacala and Socolow,

2004; Schimel, 1995).

Terrestrial C sequestration has a series of ancillary benefits besides abrupt climate change

(ACC) mitigation (Lal et al., 2003). Organic C plays a key role in preventing and mitigating soil degradation: the loss of actual or potential productivity and utility due to a decline in soil quality. Degraded soils lose its ability to produce economic goods and perform environmental functions (Lal, 2003). The benefits of SOC on soil structure are multiple: organic matter affects soil structure directly through the stabilization of aggregates and indirectly through increased aeration, increased infiltration rates, increased water holding capacity and decreased surface crusting (Martius et al., 2001;

Craswell and Lefroy, 2001; Swift and Woomer, 1993; Oades et al., 1989). Organic matter also increases the cation exchange capacity (CEC) in soils, a function that is particularly important in developing countries, where the capacity of buying inorganic fertilizers is limited (Lal and Logan, 1995).

Soil Organic Carbon Sequestration in Tropical Soils

The tropical region contains around one quarter of the global SOC pool (Craswell and

Lefroy, 2001). Some studies indicate that the tropic’s rain forests and savannas are the biomes with the largest SOC pool up to 3 m depth (474 and 345 Pg of C, respectively), from which approximately 60% is concentrated in the first meter below the surface

(Jobbágy and Jackson, 2000). General estimations of SOC in Costa Rica are of 8.3, 11.1 and 37.8 kg OC m-2 for dry forests, moist forests and mountain rain forests, respectively

(Alvarado, 2006). The SOC pool in Ultisols and Oxisols of the tropics is estimated in 204

5 Pg, which corresponds to 40% of the tropic’s total SOC pool. Although the Andisols have greater SOC sequestration rates, their C pool only accounts for 9% of the Tropic’s SOC total pool, since they occupy roughly 3% of the area in the Tropics. It is estimated that tropical Entisols and Inceptisols contain SOC pool of around 80 Pg to 1-m depth, around

16% of the total tropical SOC (Eswaran, 1993).

Contrary to the general conception that most tropical soils are too weathered to sustain a significant C pool, studies conducted in Costa Rica indicate otherwise (Powers, 2004;

Powers and Schlesinger, 2002; Van Dam et al., 1997; Veldkamp, 1994). These studies have indicated large stocks of relatively labile C in the dry, seasonal forests and also in deeply weathered soils below tropical wet forests (Veldkamp et al., 2003). The predominance of fine-grained hydrous oxides in the mineral clay fraction explains the accumulation of SOC due to chemical and physical bonding of organic matter to aggregates (Oades et al., 1989). Hence, it has been suggested that predictions on the direction and magnitude of changes in the SOC pool in tropical soils can only be satisfactorily achieved if the stabilizing mechanisms are considered (L pez – Ulloa et al.,

2005; Hassink et al., 1997).

Mostly active volcanoes distribute along the mountain range across the Central American isthmus, an extension of the Andean Cordillera. Here, it is common to find associations of Andisols with other soil groups. The soils of volcanic origin contain amorphous clays

(allophane) as a product of the weathering process of volcanic ashes. The stability of the organic C in these soils is enhanced by the mixture of allophanic aluminum silicates that form complexes with organic matter and provide a physical barrier against mineralization

(Sollins et al., 1988; Blasco, 1971; Diaz-Romeu et al., 1970; Palencia and Martini, 1970).

6 Therefore, the mean residence times of C in allophanic soils may be several thousands years (van Dam et al.,1997). In addition, important properties like water holding capacity are fostered by the combination of allophane and organic matter (Hoyos and Comerford,

2005; Diaz-Romeu et al., 1970; Palencia and Martini, 1970; Gavande, 1968).

Factors for Carbon Sequestration in Tropical Soils

Soil C sequestration is affected by interconnected natural and anthropogenic factors, including: land use (Lal, 2003; Craswell and Lefroy, 2001), climate (Jobbágy and

Jackson, 2000; Post et al., 1982; Diaz-Romeu et al., 1970) and soil structure (Bronick and

Lal, 2005; Six et al., 2002; Hassink et al., 1997).

Effect of Land Use and Management on SOC

The SOC pool is highly reactive and sensitive to natural and anthropogenic perturbations.

Therefore, land use change from natural to agricultural ecosystems depletes the SOC pool over time, generally in the order cropland > grazing land > forest (Lal, 2003). The change from natural ecosystems to croplands induces changes in the balance of C inputs and turnover rates; generally, the annual additions of organic matter are reduced when the forest is cleared for cultivation and when small scale farming transforms into large scale cash crop cultivation (Craswell and Lefroy, 2001; Follett et al., 2005). Additionally, mineralization rates may increase under tillage systems due to the exposure of organic matter (Govaerts et al., 2007). However, while soil structure and organic matter contents may deteriorate under cultivation (Govaerts et al., 2007), the intensity of change may vary with the inherent properties of soil and its management. Some forms of cultivation,

7 like organic farming, agroforestry and no-tillage systems have proved to be sustainable land use options to maintain the SOC stock at safe levels (Johnson et al., 2007; Bronick and Lal, 2005; Albrecht and Kandji, 2003).

Climatic parameters of SOC

Relatively greater SOC pools are reported in tropical regions in comparison to temperate soils (Jobbágy and Jackson, 2000) mainly due to differences in temperature, mineralogy, vegetation and soil fauna (Six et al., 2002). While lower temperatures during winter season limits plant growth and microbial activity in temperate regions, the variation in annual temperature is relatively low in the tropics (Janzen, 1967). Previous studies in

Central America indicate that organic matter increases with altitude, rainfall and lower temperatures (Diaz-Romeu et al., 1970; Palencia and Martini, 1970). In udic tropical soil moisture regimes, both annual temperature and effective precipitation allow greater biomass production; however, the organic matter decomposition rates might be higher in comparison to the temperate region and ustic tropical soil moisture regimes (Oelbermann et al., 2006). In addition, the mechanical properties of soil structure can be severely affected by erosion due to intense precipitation (Thierfelder et al., 2005).

Given the constant and high temperatures, the rates of SOC sequestration are lower when compared to temperate soils (Lal and Kimble, 2000). However, in tropical highlands where the temperature is low, the decomposition rates may be significantly lower and

SOC sequestration rates may be higher (Jimenez and Lal, 2006). The Q10 factor or Vant

Hoff factor is the change in the decomposition rate of organic matter due to variable temperature (all other factors remaining the same) and states that the respiration rate

8 doubles for a temperature increase of 10 °C (Fang and Moncrieff, 2001). Hence, for any particular level of precipitation, SOC increases with decreasing temperature (Post et al.,

1982).

Actual evapotranspiration is considered a reliable climatic control to explain decomposition rates at large scales (e.g. temperate and tropical regions, or between ecoregions), because it reflects both temperature and moisture effects. When the potential evapotranspiration equals annual precipitation, SOC density may reach approximately 10 kg C m-2, with the exception of the warm temperate and subtropical soils (Post et al.,

1982).

Elevation also appears to be a determinant factor for SOC concentrations as soil texture, temperature and mineralogy are related to this feature (Zehetner et al., 2003; Powers and

Schlesinger 2002, Bell and van Keulen 1995). The lapse rate is a generally followed ratio for tropical environments stating that for every 1000 meters of elevation, temperature decreases around 6 °C (Powers and Schlesinger, 2002). As a result of this elevation - temperature relationship, the decomposition of organic matter reduces as elevation increases. Altitude also affects the distribution of primary particles and mineralogy of soils. In residual soils of northeastern Costa Rica, sand concentration in the top 10 cm increased with elevation, clay concentration decreased, and silt concentration remained approximately constant. Similarly, relatively young non-crystalline soils are often found at high elevations, while highly weatherable oxic soils tend to be present at lower elevations. This distribution in soil mineralogy has a significant effect on SOC pools (Powers and Schlesinger, 2002).

9 The relationship among climate and SOC at the subsurface layers is still unclear, but it is believed that the climatic parameters that influence the organic matter dynamics in the surface do not have the same degree of impact on the organic matter in the subsoil

(Jobbágy and Jackson, 2000; van Dam et al., 1997). Vegetation has stronger effects on the vertical distribution of SOC than climate itself. It has been shown that SOC content at

20 cm in relation to the first meter is about 42% in grasslands and 50% in forests

(Jobbágy and Jackson, 2000). Carbon sequestration rates are affected by the amount of plant residues (organic matter) introduced into the soil and the quality of the plant material (C:N ratio, lignin content, and phenolic compound content) (Blanco - Canqui and Lal, 2004). Although vegetation is not a climatic control, it is a component of the

SOC dynamics that is strongly linked to climate; vegetation patterns are a reflection of the climatic conditions that govern a certain ecoregion (e.g. in the humid tropics, rain forests provide a constant litter supply, while the temperature and moisture regimes allow litter decomposition throughout the year).

Effect of Soil Structure on SOC

Soil structure refers to the size, shape and arrangement of solids and voids, continuity of pores and their capacity to retain and transmit fluids and organic and inorganic substances, and the ability to support root growth (Lal, 1991). Soil structure and aggregate stability are important to improving soil fertility, increasing productivity, enhancing porosity and decreasing erodibility (Bronick and Lal, 2005). The structure of soils is composed by primary and secondary particles. Primary particles are individual

10 units of sand, silt and clay, while the secondary particles result from the arrangement and binding of primary particles into aggregates by the effect of organic compounds and inorganic cementing agents (Blanco–Canqui and Lal, 2004). Land management and climate influence soil aggregation and aggregate stability; however, these factors have a lower impact on the distribution of primary particles (Cerda, 2000).

Soil organic carbon stabilization may occur through physical and chemical mechanisms.

The 2 main physical mechanisms of physical protection are (i) microaggregation (53–250

µm) formation within macroaggregates (> 250 µm) and (ii) physicochemical bonding to clay and silt particles (Bronick and Lal, 2005; Six et al., 2002). Although the adsorption of organic matter to clay and silt particles is recognized as an important determinant of

SOC stability, this physical protection is characterized by a saturation phenomenon (Six et al., 2002; Hassink, 1997; Kemper and Koch, 1966). The amount of C associated with clay and silt particles is mainly affected by soil texture, while C concentration in larger size fractions (i.e. sand and aggregates) is mainly affected by the input of organic matter and not as much by soil texture (Christensen, 2001; Hassink, 1997). Therefore, since the

SOC associated with sand and aggregates is greatly affected by land use change (i.e. biomass inputs modified), it is a good indicator of the impacts of soil management

(Cambardella and Elliot, 1992). The analysis of SOC into different size compartments allows the identification of labile fractions that respond more readily to land use changes and serve as better indicators than SOC as a whole (Hassink et al., 1997).

The SOC encapsulated within aggregates is less vulnerable to physical and microbial degradation (Six et al., 2002). Therefore, soil aggregates provide an important reservoir of stable carbon (Trujillo et al., 1997). Despite its importance, only a few studies on the

11 effect of aggregation on SOC sequestration have been conducted in the Neotropics

(Hoyos and Comerford, 2005; Koutika et al., 1997) and other tropical regions (Adesodun et al., 2007; Spaccini et al., 2004; Atsivor et al., 2001). Hence, relationships between

SOC concentration and structural stability are still unclear. For example, tensile strength of aggregates may in some soils correlate positively with SOC and have the opposite correlation in others (Perfect et al., 1995).

The basic structure of aggregates is composed of domains of clay colloids bonded to polyvalent cations and organic matter. The hierarchical theory of aggregation states that:

(i) microaggregates usually bind together by effect of young organic matter into macroaggregates (Six et al., 2000; Jastrow et al., 1996) and (ii) that the bonds within microaggregates are stronger than the bonds between microaggregates (Edwards and

Bremner, 1967). The POM that does not bind within aggregates is more susceptible to microbial decomposition and therefore decomposes very rapidly (Six et al., 2002). While soils with 2:1 clay dominance show a clear aggregate hierarchy with greater SOC amounts in macroaggregates, soils with 1:1 clays (most tropical soils) generally do not exhibit this pattern (Zech et al., 1997; Six et al., 2000; Hoyos and Comerford, 2005).

Plant roots also play an important role on aggregate formation and stabilization (Bronick and Lal, 2005; Six et al., 2002). As a result, grasslands may present higher aggregate stability over rainforest soils (Adesodun et al., 2007; Spaccini et al., 2004; Tisdall and

Oades, 1982).

Aggregates protect SOC by forming a barrier against microorganisms, controlling food web interactions, and influencing microbial turnover. A significant decrease of oxygen concentration within aggregates also inhibits microbial accessibility to SOC (Six et al.,

12 2002). Soil biota modifies organic matter through the processes of mineralization and immobilization of nutrients, making them available to plants and other organisms (Zech et al. 1997). Under poor aggregate stability, the microbial activity increases and depletes

SOC, which eventually leads to lower microbial biomass and activity and consequently a lower production of microbial-derived binding agents and loss of aggregation (Jastrow,

1996; Six et al., 1998). Aggregate properties like mean weight diameter (MWD), water stable aggregates (WSA), water repellency (WDPT), moisture retention (MR) and tensile strength (TS) are direct indicators of soil structure and, for that matter, indirect indicators of SOC stability.

Aggregate stability is a function of whether the cohesive forces between particles withstand an applied disruptive force (Kemper and Rossenau, 1986). This physical attribute varies not only with management, but is also influenced by climate and soil type, i.e. soils from humid regions tend to have higher aggregate stabilities (Kemper and Koch,

1966). When the purpose of a given study is to simulate erosion by water, aggregate stability is determined by the wet sieving test. Results on this test are generally presented in the forms of percentage of water stable aggregates (WSA) and mean weight diameter

(MWD). Aggregate stability does not indicate proportional SOC distributions according to the abundance of a certain aggregate size. For example, the aggregates of a Mexican

Acrisol distributed mainly (65%) in the 2-0.2 mm size; however, around 79 % of the aggregate associated C was concentrated in the finer (< 0.05 mm) fraction (Covaleda et al., 2006).

Several authors have found a positive correlation between SOC and WSA (Chenu et al.,

2000; Koutika et al. 1997); and MWD (Atsivor et al., 2001) in tropical ecoregions. In

13 contrast, Spaccini et al. (2004) did not find any significant correlations in Nigerian soils, and Lehmann et al. (2001) failed to find any correlation in Oxisols in Brazil, respectively.

Apparently, the correlation among WSA and SOC is less significant in tropical soils, where other cementing agents and mechanisms play a role in stabilizing macroaggegates

(Six et al., 2000; Oades and Waters, 1991).

Water repellency indicates the degree of hydrophobicity of soil particles. The hydrophobic substances that cause repellency are beneficial for conserving water by reducing its capillarity and evaporation, as well as the leaching of nutrients. Water repellency is associated with SOC since the hydrophobicity of organic matter beneficiates the formation and protection of stable aggregates (Chenu et al., 2000) and hydrophobic organic matter itself is more stable against microbial decomposition (Goebel et al., 2005).

Tensile strength measures the strength of individual soil aggregates when a force (F) is applied across the aggregate unit, causing an elastic deformation. When F is increased gradually, the internal stress reaches the tensile strength (TS) of the aggregate, which results in a failure or deformation of the particle. Tensile strength may be influenced by moisture content (Utomo and Dexter, 1981), clay content, SOM and aggregate size

(Imhoff et al., 2002). The TS of aggregates generally decreases with increasing moisture and/or aggregate size, but could also be affected by the time of storage after sampling.

The heterogeneity in structure and aggregate size distribution produces differences in the moisture retention properties between soils (Sanchez, 1976). Soil organic carbon also affects soil moisture characteristics, e.g. soil wetness at field capacity (FC) increases linearly with increases in C (Franzluebbers, 2002).

14 Ecoregions as a Tool for the Study of SOC

Ecoregions are units of land containing a distinct assemblage of natural communities and species, with boundaries that approximate the original extent of natural communities prior to major land-use change (Olson et al., 2001). World ecoregions were established by the World Bank and the World Wildlife Fund (Olson et al., 2001; Olson and

Dinerstein, 1998) and were conceived as a tool for the conservation and study of biodiversity and could be also used for soil carbon mapping. Under this classification system, Latin America belongs to the Neotropic biome, which is comprised of 210 ecoregions.

Holdridge's life zones (1979) were used as a base for the classification of the Central

American ecoregions. These zones are based on three climatic parameters: mean annual biotemperature; mean annual precipitation; and potential evapotranspiration (PET) ratio.

Biotemperature defines the latitudinal or altitudinal belts of life zones, while the PET ratio defines the humidity provinces (Ray et al., 2006). These life zones correlate directly to soil patterns and C concetration, since this classification system takes into account factors such as climate, topography, soil type, geology and drainage (Diaz-Romeu et al.,

1970; Holdridge, 1967).

Study Region: Costa Rica

Due to its geographic position as the confluence center of North and South America and between the Pacific and Atlantic Oceans, Costa Rica contains several of the representative habitats of Latin America. In a relatively small area (52,000 km2), it is possible to find all soil orders but Aridisols and Gelisols, elevations from 0 to 3,800

15 m.a.s.l., perudic to ustic soil moisture regimes, and soil temperature regimes from isofrigid to isohyperthermic. The soils of Costa Rica present highly variable parent materials and heterogeneous relief. Most soils are relatively young by world standards; for example, some volcanic soils in the mountainous regions are only 400 years old, and the alluvial plains are less than 3,400 years old; thus, the predominant soils of this region are the Inceptisols with andic and oxic tendencies (Bertsch et al., 2000).

Soil organic matter has been a subject of study in Costa Rica since the 1930’s (Jenny,

1930). In the 1970’s, soil scientists studied the SOC pool in the plow layer for agronomic purposes and established that the magnitude of SOC pool was impacted by precipitation, temperature and altitude variability (Müller et al., 1968; Palencia and Martini, 1970;

Diaz-Romeu et al., 1970). In the mid 1980’s – 2000’s, research on SOC was oriented to C sequestration under different land use and soil type schemes (Andrade et al., 2008;

Buurman et al., 2007; Jimenez et al., 2007; Oelbermann et al., 2006; Powers, 2006, 2004;

Cleveland et al., 2003; Powers and Schlesinger, 2002a,b; Guggenberger and Zech, 1999;

Montagnini and Porras, 1998; Reiners et al., 1994; Veldkamp, 1994; Van Dam et al.,

1997).

The potential for C sequestration in Costa Rica and the neotropical region is limited by the aggressive agricultural expansion to zones vulnerable to land degradation, contributing to the nearly 75% of the Central American lands that face severe soil degradation (Greenland, 1995). It is estimated that 47 % of the soils in Costa Rica are highly prone to degradation and that by 1989 approximately 20 % of the national territory was severely eroded (CADETI, 2000).

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24 3. EFFECT OF LAND USE AND CLIMATE ON SOIL ORGANIC CARBON

IN COSTA RICAN ECOREGIONS

Abstract

Given the feedback of the soil organic carbon (SOC) pool in tropical regions to atmospheric composition and climate change, it is important to understand the patterns and mechanisms of SOC sequestration, and capitalize on the land use and management practices that act as net sinks, rather than sources of GHG emissions. However, the size and dynamics of the carbon pools in tropical soils are still poorly known.

The general objective of the present study was to estimate the SOC pool in soils of Costa

Rica and to assess their carbon (C) sink capacity to offset anthropogenic emissions by different management scenarios. The specific objectives were: (i) to determine the effects of land use on the SOC concentration and pool, (ii) to determine the relationship of climatic parameters on SOC concentration, vertical distribution and sink capacity, and

(iii) to estimate the total SOC pool and potential for SOC sequestration in the Central

American region. A field study was established in 3 ecoregions of Costa Rica: the

Atlantic Moist ecoregion, the Pacific Dry ecoregion and the Montane ecoregion. Within each ecoregion, 3 agricultural land uses were sampled for SOC at a depth of 0 – 100 cm.

In addition, a mature forest was also sampled within each ecoregion to estimate the SOC sink capacity of the crops.

25 The SOC pool at 1-m depth was estimated at 114 – 150 Mg C ha-1 in the Atlantic Moist ecoregion, 76 – 165 Mg C ha-1 in the Pacific Dry ecoregion and 166 – 246 Mg C ha-1 in the Montane ecoregion of Costa Rica. The C sink capacity was of 18.1 - 36.7 Mg C ha-1,

14.1 – 88.6 Mg C ha-1 and 9.4 – 80.7 Mg C ha-1 in the Atlantic, Pacific and Montane ecoregions, respectively. The effect of land use on SOC pool was specific to the ecoregion: there were more significant differences on SOC in the Pacific ecoregion than in the Atlantic or Montane ecoregions. Within ecoregions, the main mechanism by which land use directly influenced the SOC pool was by changes in bulk density; however, SOC was also strongly affected by texture. Climate also had a significant effect on SOC concentration and SOC pool, as well as its vertical distribution. Altitude impacted SOC; however, this may be an indirect indicator of the effect of soil type distribution in Costa

Rica, since Andisols predominate at high altitudes. In agricultural soils, the effect of climate was not as significant as it is in soils under natural vegetation, because the effect of management was predominant.

A review of the available literature allowed to estimate that the total SOC pool in Central

America is of 6,454.6 Tg of C, from which 3,075 Tg C are stored in forest soils, 1,779 Tg

C are in pasture soils and 992 Tg C are in croplands. The SOC estimated for Costa Rica is of 823 - 872 Tg of C, with a potential for SOC sequestration of around 826 – 2,251 Gg

C yr-1. Under these estimations, around 26 – 71% of the country’s total emissions may be offset by agricultural and forest soils.

Abbreviations

BD, soil bulk density; C, carbon; C:N, Carbon to Nitrogen ratio; MAP, mean annual precipitation; MAT, mean annual temperature; SOC: soil organic carbon.

26 Introduction

The soils of the Neotropics constitute one of the largest vulnerable carbon (C) pools, which are on the verge to becoming net sources of greenhouse gases (GHG) emissions under the current trends of deforestation and land use management (Canadell et al.,

2007). Given the feedback of the soil C pool to atmospheric composition and climate change, it is important to understand the patterns and mechanisms of soil organic carbon

(SOC) sequestration (Jobággy and Jackson, 2000), and capitalize on the land use and management practices that act as net sinks, rather than sources of GHG emissions (Lal,

2007). Despite its importance, the size and dynamics of the carbon pools in tropical soils are still poorly known (Canadell et al., 2007; Batjes, 1996).

About 1,550 Pg of C are stored in the first meter of the world’s soils (Batjes and

Sombroek, 1997) from which 403 - 506 Pg are concentrated in the tropics (Batjes, 1996;

Eswaran et al., 1993). Sombroek et al. (1993) estimated that about 174 Pg of SOC are stored in the soils (0 – 100 cm) of Latin America, from which 26 Pg are in Central

American soils.

One of the relevant research questions in SOC sequestration and commoditization is the role of land use and climate on the soil C sink capacity (Lal, 2007). The SOC pool can be significantly modified by the impact of anthropogenic activities such as deforestation and land use change (Batjes, 1996). Generally, the SOC pool tends to be lower in agricultural soils in comparison to natural ecosystems because of lower inputs of biomass, higher decomposition rates, changes in the soil moisture and temperature regimes, and soil erosion and leaching (Lal, 2007). Under tropical conditions, soil cultivation may reduce the SOC pool up to 40% just 5 years after forest clearing (Detwiler, 1986). However,

27 SOC depleted soils have also a potential C sink capacity, which under the adoption of recommended management practices (RMPs) may provide services for C sequestration

(Lal, 2007).

Precipitation and temperature are climatic features that have been identified as factors of soil organic matter (SOM) accrual in the Central American region (Diaz-Romeu et al.,

1970). Generally, soil organic C correlates positively with mean annual precipitation

(MAP) and correlates negatively with mean annual temperature (MAT)(Jobággy and

Jackson, 2000). Since the climate of Costa Rica varies according to elevation, soil texture, temperature and mineralogy are also intrinsically related to this feature (Powers and Schlesinger, 2002).

The general objective of this study was to estimate the SOC pool in soils of Costa Rica and to assess their C sink capacity to offset anthropogenic emissions by different management scenarios. The specific objectives were: (i) to determine the effects of land use on the SOC concentration and pool, (ii) to determine the relationship of climatic parameters on SOC concentration, vertical distribution and sink capacity, and (iii) to estimate the total SOC pool and potential for SOC sequestration in the Central American region.

Methodology

Study sites

This study was conducted on an ecoregional basis. Three contrasting ecoregions were selected in Costa Rica: Isthmian – Atlantic Moist Forests, Central American Pacific Dry

Forests, and Talamancan Montane Forests. Information about climate and soil texture of

28 the sites studied is available on Table 3.1. Each ecoregion included 3 agricultural land uses and one undisturbed forest. Unfortunately, there is limited information regarding the previous history and management of the agricultural sites, and the time since forest clearing is unknown (all land owners estimated that land was cleared more than 50 years ago).

Isthmian – Atlantic moist forests

The sampling sites of this ecoregion were located on the main campus of EARTH

University (10° 10’ N and 83° 37’ W), in the Atlantic lowlands (50 meters above sea level) of Costa Rica. The Holdridge’s life zone classification for this site is premontane wet forest basal belt transition (Bolaños and Watson, 1993). This region has a mean relative humidity of 89 %, a mean annual temperature (MAT) of 24.5 °C, and a mean annual precipitation (MAP) of 3,227 mm. The rainfall distribution is bimodal (Solano-

Quintero and Villalobos-Flores, 2001). Oil palm (PA) (Elaeis guineensis), pineapple (PI)

(Ananas comosus), banana (BA) (Musa acuminata) and a mature rain forest (RF) were the land uses selected for soil sampling. These soils classify as Andic Dystropepts from the Neguev series (Sancho et al., 1989) and the textural class is clay loam (34.4 % sand,

33.3 % clay).

The PA has been grown for 14 years and was previously used as a pasture for over 60 years. This soil has not received any form of land preparation since the establishment of the plantation in 1994, and receives periodical fertilizer applications that consist of 18-5-

15-6-2 (N-P-K-Mg-B at 143 kg ha-1) as well as glyphosate for weed control. Every two weeks, approximately 2 leaves per plant are cut and left on the soil for decomposition.

29 The BA plantation was established 29 years ago, but it was renovated 7 years ago. The management of this plantation is considered “sustainable”, including applications of organic amendments such as bokashi (fermented compost used to inoculate the soil with effective microorganisms) and pest control with effective microorganisms; however, conventional pesticides are also applied periodically. The PI is a 12 year old cash crop plantation that receives intensive land use preparation annually as well as intense application of fertilizers (10-30-10, potassium nitrate, iron sulfate, zinc sulfate, calcium and boron, among others), herbicides and pesticides and growth regulators.

Central American Pacific dry forests

Sampling for this ecoregion was based at the EARTH University’s La Flor Campus in

Guanacaste (10º 35’ N; 85º 32’ O) at 40 - 100 meters above sea level (masl). This region has a mean relative humidity of 74 %, MAT of 28 °C, MAP of 1,800 mm and it has a dry period of 5 months. The Holdridge’s life zone classification for this area is tropical dry forest (Solano-Quintero and Villalobos-Flores, 2001). Soils under sugar cane (CA)

(Saccharum officinarum), pasture (PAS) (various native species), and mango (MA)

(Mangifera indica) were selected for sampling. Samples taken at La Flor are from soils classified as Typic Dystrustepts (Sancho, 2006). Since La Flor lacks a mature dry forest, samples were collected at the Santa Rosa National Park (10°48’ N, 85°36’ O), where the soils are classified as Typic Ustropepts (Oficina de Planificación Sectorial Agropecuaria,

1999). This dry forest (DF) has been subject to several fires since pre-colonial times. The climate at Santa Rosa has a MAT of 25 °C, a MAP (for the period 1994 - 2006) of 1,698

30 mm and a dry period of 6 months (Investigadores ACG, 2007). The texture of the soils in this ecoregion is loamy (48.7 % sand, 19.8 % clay).

At La Flor, it is estimated that the forest was removed to establish pastures about 60 years ago. The CA plantation was grown continuously for 25 years, and was abandoned for 10 years, during which there were no fertilizer applications or tillage. In 2005, this farm was donated to EARTH University, which renovated the plantation and re-started the commercial production. Similarly, the MA site was grown for 38 years and abandoned for a period of 10, until the farm was donated. At the time of sampling, the CA was in its growing stage, undergoing fertilization and pest control. The MA plantation was also in its growing stage and received fertilizations, hormone control and pruning. Fertilizer applications consist mainly of 5.5-9.7-6.5-0.6 at 300 kg ha-1 in the MA site and 360 kg ha-1 yr-1 of urea in the CA plantation. After harvest, it is estimated that about 7 Mg ha-1 of residues are incorporated into the soil in the CA plantation. The PAS site corresponds to a degraded pasture that was also abandoned for 10 years and that currently does not receive any management.

Talamancan montane forests

This ecoregion comprises the volcanic highlands of Central America (10° 11' N, 84° 24'

W). This region is characterized by mean relative humidity of 90 %, MAT of 15 °C, and

MAP of 2,016 mm. The rainfall distribution is unimodal (Solano-Quintero and

Villalobos-Flores, 2001). The altitude at the sites sampled ranges from 1,200 to 1,800 masl and the relief is extremely steep (most slopes are of 48%). In the Talamancan highlands, soil samples were taken at an organic farm (OR) (various vegetables in a

31 rotation system), a conventional farm (CO) (various vegetables in a rotation system), a coffee plantation (CF) (Coffea arabica) and a mature cloud forest (BP). These soils are classified as Pachic and Typic Hapludands (CEDECO, 2006), with a clay loam texture

(29.8 % sand, 36.1 % clay).

The OR farm has been certified for 7 years and consists of a rotation of lettuce, cilantro, onions and tomatoes, among other crops. The soil receives periodical applications of compost. The vegetables are grown under a crop rotation system and bed preparation and weed control are done manually. The CO farm is adjacent to the organic and it is also under crop rotation. The bed preparation is also done manually and most of the fertilizers and pesticides used in this farm are often inorganic.

The coffee plantation has been grown continuously for 52 years, previous to that, the site

-1 was a pasture. This crop receives several applications of lime (700 kg CaCO3 ha ), fertilizers (18-5-15-6-2 at 800 kg ha-1 yr-1) and herbicides (mostly glyphosate at 4000 cm3 ha-1 yr-1). Two years ago, the area was planted with Dracaena sp. on an alley crop system.

32 Table 3.1. Characterization of the land uses studied in three ecoregions of Costa Rica. Particle size distribution corresponds to the average of the soil profile (0 – 100 cm).

Ecoregion Land use Abbreviation Sand Clay Silt Texture MAP MAT Altitude % (USDA) (mm) (°C) (masl) Montane Coffee CF 8.9 54.7 36.4 Clay 1950 22 1200 Organic farming OR 38.5 37.5 24.0 Clay loam 2820 15 1800 Conventional farming CO 36.6 35.6 27.8 Clay loam 2820 15 1800 Lower Montane Wet forest BP 35.3 16.7 48.0 Loam 2016 15 1500 Atlantic Moist Banana BA 46.0 29.5 24.4 Sandy clay l. 3600 25 64 Oil palm PA 30.4 39.4 30.2 Clay loam 3600 25 64 Pineapple PI 30.6 45.0 24.5 Clay 3600 25 64 Tropical Wet (rain) forest RF 30.8 19.3 50.0 Loam 3600 25 64 Pacific Dry Sugarcane CA 36.9 16.8 46.4 Loam 1620 27 60 Mango MA 58.5 17.3 24.2 Sandy loam 1620 27 60 Pasture PAS 59.2 16.8 24.0 Sandy loam 1620 27 60 Tropical Dry forest DF 40.3 28.5 31.2 Clay loam 1698 25 300

Soil Sampling

Soils of these three ecoregions and diverse land uses were sampled in August of 2008.

Within each land use, four pseudo-replicate units (subsamples) were established at least

25 m apart located on the same landscape position along a transect. One soil profile of 50 x 50 cm was dug in each sampling site. Bulk samples of about 1 kg were obtained for 0 –

10, 10 – 20, 20 – 30, 30 – 40, 40 – 50, 50 – 70 and 70 – 100 cm depths. In addition, soil cores were obtained at each depth interval to determine bulk density (BD) with the core method (Blake and Hartge, 1986), and field moisture content with the thermogravimetric method (Gardner, 1986). Soils from the Atlantic and Pacific ecoregions were air-dried for two weeks, but the soils from the Montane ecoregion were maintained at field moisture in consideration for its hysteretic properties. These bulk soil samples were carefully packed and shipped for physical analysis at the laboratories of the Carbon Management and

Sequestration Center at The Ohio State University.

Soil Analyses

33 Total carbon (TC), nitrogen (N) and C:N ratio were determined for the complete soil profile (0 – 100 cm depth) by the dry combustion method (Nelson and Sommers, 1996) using a C:N analyzer (Vario Max, Elementar Americas Inc., Germany). The HCl test

(Nelson, 1982) was conducted to detect the presence of carbonates in the soils and all samples tested negatively. Therefore, TC is assumed as SOC. The BD was used to compute the total C pool on a mass per unit area basis (Batjes, 1996). The C sink capacity for the agricultural land uses within each ecoregion was estimated by the difference in TC pool in the reference forest and the TC pool in the crop soil.

Climatic data for each ecoregion was obtained from the National Institute of

Meteorology, and databases at the Santa Rosa National Park and EARTH University.

Climatic records come from the meteorological stations closest to the study sites. The climatic parameters evaluated were MAP and MAT. Altitude was also studied given the direct relationship between this feature and climate.

Statistical Analyses

Statistical analyses were conducted to establish significant differences on C concentration and C pool (Tukey-Kramer’s HSD) among treatments (land use) at a = 0.05. Simple regression analyses were conducted to establish relationships between the climatic parameters and SOC concentration, TC pool and vertical distribution of SOC. Altitude was also included in these correlations (Powers and Schlesinger, 2002). All statistical analyses were conducted on JMP 7 Statistical Software (SAS Inc., NC).

34 Results and Discussion

Bulk density, carbon concentration and C:N ratio

Results on bulk density (BD) are shown on Figure 3.1. This property tended to increase with depth, and was significantly affected by land use (P < 0.05). At the soil surface (0 –

10 cm), BD ranged from 0.35 (BP) – 0.74 (CF) Mg m-3 in the Montane ecoregion, from

0.71 (CA) – 1.1 (PAS) Mg m-3 in the Pacific ecoregion and from 0.64 (BA) – 0.80 (PI)

Mg m-3 in the Atlantic ecoregion. In specific, BD was lower in the Montane and Atlantic ecoregions due to the andic nature of those soils (Hoyos and Comerford, 2005; Lopez-

Ulloa et al., 2005). The PI soils had significantly greater BD than the BA and RF soils.

Similarly, in the Montane ecoregion the CF soil was significantly greater at all depths in comparison to the other treatments. In the Pacific ecoregion, PAS and DF exhibited larger DB at 0 – 70 cm, but the differences were significant only at specific depth intervals. The assessment of bulk density is necessary in order to assess the mass of C per unit area for inventories of C storage (Powers, 2004; Lal, 2002). However, there is a considerable lack of data on BD, which compromises the estimations of C pools at a regional and global basis (Carter et al., 1998).

Soil compaction is a common effect of land use change from forest to agriculture and pasture, thus increasing BD in comparison to adjacent soils under natural vegetation.

Some studies standardize the values on C pool to a common soil mass by multiplying the ratio of BD in current to previous land uses by the soil C pool under the current land use

(Veldkamp, 1994). However, in the case of this study, it was not clear what the appropriate reference value should be, since the sites had undergone a series of land use

35 transitions that include pasture and natural regeneration. In a study of the effect of land use change on SOC in Northeastern Costa Rica, Powers (2004) indicated that correction for compaction did not create a significant difference on the results or interpretation of data on C pool.

Bulk Density (Mg m-3) Bulk Density (Mg m-3) 0.60 0.80 1.00 1.20 0.00 0.20 0.40 0.60 0.80 1.00 1.20 10 10 20 20

) 30 30 m

c 40 ( 40 h t 50 50 p e

d 60 60 l i

o 70 70 S 80 80 90 90 100 100 BA PA PI RF BP CF CO OR Bulk Density (Mg m-3) 0.60 0.80 1.00 1.20 1.40 1.60 10 20

) 30 m

c 40 ( h

t 50 p

e 60 d l

i 70 o

S 80 90 100 CA MA PAS DF

Figure 3.1. Bulk density at 0 – 100 cm depth in three ecoregions of Costa Rica: Atlantic, Montane and Pacific (in clockwise order). The horizontal lines indicate Tukey’s HSD when P< 0.05 (the absence of lines indicate that there was no significant difference between land uses).

Results of SOC concentration are presented in Figure 3.2. At 0 – 30 cm, SOC concentration varied from 9.8 (PAS) – 33.9 (CA) g C kg-1 in the Pacific, from 43.7(CF) –

36 78.7(BP) g C kg-1 in the Montane and from 15.5(PI) – 34.3(BA) g C kg-1 in the Atlantic ecoregion.

Soil Organic Carbon (g C kg-1) 0 20 40 60 80 100 120 10 20

) 30 m

c 40 ( h

t 50 p

e 60 D l

i 70 o

S 80 90 100 BP CF CO OR 0 10 20 30 40 50 10 20 30 ) m

c 40 ( h

t 50 p

e 60 D l i 70 o S 80 90 100 DF MA CA PAS 0 10 20 30 40 50 10 20

) 30 m

c 40 ( h

t 50 p

e 60 D l

i 70 o

S 80 90 100 BA RF PA PI

Figure 3.2. Carbon concentration in different land uses of the Montane Pacific and Atlantic ecoregions (in order from top to bottom). Horizontal bars indicate the Tukey’s HSD when P<0.05.

37 A review of 165 studies on SOC sequestration in different land uses and ecoregions of

Central America (Table 3.2) indicates an average concentration of 47.3 ± 12 g C kg-1 at

0 – 30 cm depth. The average C concentration for crop soils (n = 86), was of 35.1 ± 5 g C kg-1, and for forest soils 71± 11 g C kg-1 (n = 82). This study presented on average greater SOC concentrations because only Andisols and Inceptisols were sampled and other soil types which typically have lower SOC concentrations (e.g. Ultisols and

Vertisols) were excluded.

Other soils in this study were considerably poor in C concentration (i.e. PAS and CF and

PI) in reference to the other agricultural and forest sites from the same ecoregion and also in reference to the Central American average. Specific reasons for these results may be the sandier texture, low CEC and poor vegetation in the PAS site and the soil erosion and lack of biomass inputs in the CF and PI plantations. It is estimated that 46 x 106 ha are under severe land degradation in Central America, of which 28% is attributed to agriculture and 9% to overgrazing (Oldeman, 1994). Erosion is one of the causes of land degradation given the large proportion of steep land, for example, it is estimated that 70% of Costa Rica’s area has slopes > 8% (Posner and McPherson, 1981), in which coffee, sugar cane and vegetables are usually grown.

The SOC concentration of the land uses in this study are comparable to the results found by other studies in the region (Table 3.2). Carbon concentration in the BA soil is similar to that found in other banana and cash crop plantations in the area by Powers (2004). The

SOC concentration in the PI soils is considerably low; however, it is not possible to establish comparisons due to the lack of other studies. Information of SOM on pineapple plantations deserves special attention in Costa Rica, since it is estimated that the

38 harvested area has been expanded by 300 % in the last 10 years (Bach, 2007). The SOC concentration observed in the CF soil is comparable to other values reported by the literature. Mendez et al. (2009) estimated a C concentration of 29.7 g C kg-1 in shade grown coffee of El Salvador at 900 masl, while Bertsch et al. (1991) reported 29.6 –

43.5 g C kg -1 in conventional coffee plantations at 650 – 1350 masl in Costa Rica at 0 –

30 cm depth. The results on C concentration in PAS were significantly lower than other values reported in Tropical wet life zones by Powers (2004) (37.9 – 45.7 g C kg-1 at 0 –

30 cm in Northeastern Costa Rica), Hughes et al. (2000) (45.3 g C kg-1 at 0 – 30 cm in

Southeastern Mexico) and Geissen and Morales-Guzman (2006) (19.8 g C kg-1 at 0 – 30 cm in Southern Mexico). Possible reasons for this dramatic difference are pasture age

(the pasture in this study is > 60 years), poor grass quality and poor management. Thus,

Amézquita et al. (2005) found that SOC sequestration is greater on pasture sites under improved grasses (e.g. Brachiaria) and silvopasture.

Carbon concentrations from CO and OR did not differ to those of BP, and the organic management did not seem to have an effect on SOC in comparison to the conventional.

This may be due to the high resilience of tropical Andisols to land disturbance, as has been discussed by Lopez-Ulloa et al. (2005), Powers and Schlesinger (2002) and

Veldkamp (1994).

39 Table 3.2. Soil organic carbon concentration (± C.I.) up to 30 cm in different land uses, soil types and life zones of Central America. The life zone estimations include cultivated soils.

Carbon concentration (g kg-1) Land use Mean n Forest 71.1 ± 11.0 82 Secondary Forest 48.3 ± 9.5 16 Fallow 66.7 ± 25.4 13 Agroforestry 32.8 ± 10.5 9 Pasture 39.6 ± 9.8 27 Crop 35.1 ± 5.3 86 Tree plantation 37.4 ± 11.0 12 Soil type Alfisol 45.8 ± 2.0 34 Andisol 72.7 ± 14.0 44 Entisol 26.7 ± 32.0 2 Inceptisol 47.5 ± 5.8 80 Mollisol 38.4 ± 19.2 14 Oxisol 41.0 ± 9.6 8 Ultisol 45.1 ± 7.9 28 Vertisol 17.4 ± 5.1 13 Life Zone Tropical dry forest 31.1 ± 7.9 24 Tropical moist forest 76.2 ± 26.4 21 Tropical wet forest 45.4 ± 6.9 29 Tropical wet premontane belt transition 56.2 ± 5.1 47 Premontane wet basal belt transition 37.1 ± 8.0 13 Lower montane moist forest 47.5 ± 12.1 7 Lower montane wet forest 65.3 ± 33.6 8 Montane wet forest 75.7 ± 31.0 11

Carbon to Nitrogen ratios (C:N) are presented in Table 3.3. The distribution of N concentration in these soils followed the same pattern as SOC concentration (not shown).

In general, the range of C:N values observed in this study (8.53 – 12.1) are similar to the range reported by Diaz Romeu et al. (1970) in their survey of Central American soils and

40 the values found by Powers and Schlesinger (2002) in Northeastern Cost Rica (8.8 to

13.3). Soils from the Pacific Dry ecoregion had a greater range of C:N. There was no significant effect of land use on this property in any ecoregion, with exception of DF and

CA at 10 – 20 cm depth. Similarly, Trujillo et al. (1997) found no effect of land use on the C:N ratio of a clay loam in the Colombian savannas.

Table 3.3. C:N ratios at 0 – 30 cm depth in different land uses of Costa Rica

CN ratios Atlantic Moist Ecoregion

BA RF PA PI LSD(0.05) 10 cm 9.74 9.35 9.28 10.38 1.66 20 cm 9.94 9.02 9.15 10.96 2.07 30 cm 9.57 9.44 9.34 8.67 1.77 Pacific Dry Ecoregion

DF MA CA PAS LSD(0.05) 10 cm 11.46 11.52 11.84 9.89 2.54 20 cm 11.42 11.72 9.66 10.40 1.92 30 cm 12.10 12.04 10.21 8.53 4.09 Montane Ecoregion

CF OR BP CO LSD(0.05) 10 cm 10.96 10.53 10.52 9.96 1.47 20 cm 10.96 9.70 11.08 9.94 1.67 30 cm 10.78 10.88 10.95 10.13 1.83

Soil organic carbon pool and carbon sink capacity

The SOC pool on mass per unit area (Mg ha-1) is presented on Figure 3.3. It was observed that the

C decreased with depth from 0 – 50 cm, and then increased slightly from 50 – 100 cm, probably due the artifact of higher BD at deeper layers. Soil organic carbon was not significantly affected

41 by land use in the Montane ecoregion at any depth interval. Similarly, SOC differed significantly only at 0 – 10 cm depth in the Atlantic ecoregion. These results may be in part due to the effect of BD and soil compaction, which results in greater soil masses in certain soils

(Schuur et al., 2001).

The differences on SOC pool associated to land use were more evident in the Pacific ecoregion. Here, SOC in the CA soil was significantly higher than MA and PAS. In general, SOC decreased in the order CA >DF > MA > PAS. Jimenez et al., (2008) reported higher SOC values for sugarcane in the same region (EARTH Campus at La

Flor). This higher pool in CA might be due to the addition of the crop’s residues, estimated at 7 Mg ha-1, even higher than the estimated additions of litter in the dry forest of Santa Rosa of 2 Mg ha-1 (Burnham, 1997). In addition, Jimenez et al., (2008) observed that SOC content in the litter of the sugarcane plantation was greater (7.6 Mg C ha-1) than that of the SOC content in adjacent mango and savanna areas (4.5 and 4.9 Mg C ha-1 in the mango and savanna litter, respectively).

42 Total C (Mg ha-1) 0 10 20 30 40 50 60 70

10

20 ) m c

( 30 h t

p 40 e D l

i 50 o S 70

100

BP CF CO OR

0 10 20 30 40

10

20 ) m

c 30 ( h t

p 40 e D l

i 50 o S 70

100

PI PA BA RF

0 10 20 30 40 50

10

20 ) m c

( 30 h t

p 40 e D l

i 50 o S 70

100

PAS CA MA DF

Figure 3.3. Soil organic carbon pool in different land uses of the Montane, Atlantic and Pacific ecoregions (in order from top to bottom). Horizontal bars indicate the Tukey’s HSD when P<0.05.

43 The relative vertical distribution of SOC is presented in Figure 3.4. The results indicate that while CA and DF had almost identical distributions, the PAS and MA behaved differently in the Pacific ecoregion. Around 65 % of the total C pool was located in the first 40 cm of depth in the soils of CA and DF, while in the soils of PAS and MA it was around 50 % and 70% of the total SOC, respectively. Carbon distribution was also somewhat consistent in the Atlantic ecoregion. Here, around 50 – 60% of the total SOC was located in the first 40 cm. At the superficial layer (0 – 20 cm), PI had lower SOC proportions (25%) in comparison to all other land uses, which may be due to the effect of land preparation, because PI was the only land use under periodical tillage (PA and BA are semi-perennials). In the Montane ecoregion, CO, OR and BP showed identical distributions (around 28 % at 0 – 20 cm depth) in comparison to CF (around 40% in the

0 – 20 cm layer).

The results differed greatly between ecoregions, which may be attributed to contrasting climatic and vegetation patterns. In a global study of the vertical distribution of C in relation to climate and vegetation, Jobbágy and Jackson (2000) indicated that world grasslands and croplands contain around 40 % of the total SOC in the first 20 cm (as was observed in PAS, CA and CF). In forests, the proportion of total SOC in the first 20 cm is about 45 % and 30 % in the tropical rain and tropical dry forests, respectively. The results on vertical distribution found in the forests in this study are in accordance to the ones reported by Jobbágy and Jackson (2000); however, the proportions in the crop soils were more irregular.

Most of the studies conducted in Central America have analyzed the SOC up to of 30 cm depth on average. This might result in an underestimation of the total SOC pool of up to

44 50 %. Alvarado (2006) established regression models to estimate the total SOC pool to a depth of 1 meter using SOC data from 0 – 30 cm on a life zone basis. In his estimations, the SOC in the first 30 cm corresponds to 60 % on the total C pool in the Tropical Dry

(DF) and Tropical Wet (RF) forests and to 55 % in the Lower Montane Rain forests (BP), which were somewhat similar to the proportions found in the mature forests of this study.

Jobbágy and Jackson (2000) found a significant positive correlation between the relative amount of SOC with mean annual precipitation (MAP), and a negative correlation with mean annual temperature (MAT), but the strength of these correlations was higher at the surface (0 – 20 cm) and was subsequently lower at deeper layers. When establishing the relationships between the relative distribution of SOC and climatic variables and texture, the results in this study (Table 3.4) were opposite to those reported by the literature. This might be due to the higher proportion on SOC at 0 – 20 cm in the Pacific Dry ecoregion

(therefore obtaining negative correlations with MAP), and the confounding effects of land use. Finally, the dry forest may have had a greater SOC concentration at 0 – 20 cm due to the additions of charcoal from forest fires (Lorenz et al., unpublished).

45 DF

PAS

MA

CA

0% 20% 40% 60% 80% 100%

RF

PI

PA

BA

0% 20% 40% 60% 80% 100%

OR

CO

CF

BP

0% 20% 40% 60% 80% 100%

0 - 20 cm 20 - 40 cm 40 - 70 cm 70 - 100 cm

Figure 3.4. Vertical distribution of soil organic carbon as a proportion of the total pool in the Pacific, Atlantic and Montane ecoregions (in order from top to bottom)

46 Table 3.4. Relationship between the absolute vertical distribution of SOC and mean annual temperature (MAT), mean annual precipitation (MAP), altitude and texture

Soil Depth MAT MAP Altitude Clay Silt Sand 0 - 20 cm 0.49 -0.69 -0.30 -0.07 -0.19 0.27 20 - 40 cm -0.22 0.23 0.22 0.21 0.23 -0.45 40 - 70 cm -0.84** 0.84* 0.00 0.59 -0.28 -0.47 70 - 100 cm 0.33 -0.16 -0.25 -0.26 -0.12 0.41 Numbers in bold indicate significance at P <0.01, *P <0.05, **P < 0.1

The cumulative (total) SOC pool and estimated C sinks are presented in Table 3.5. Soil organic C pool ranged from 113.6 – 150.3 Mg ha-1 in the Atlantic ecoregion, from 75.9 –

164.5 Mg ha-1 in the Pacific ecoregion and from 165.7 – 246.4 in the Montane ecoregion.

SOC pool decreased in the order DF = CA = MA = PAS and BP = OR = CO = CF in the

Pacific and Montane ecoregions, respectively. No significant differences on SOC pool were reported in the Atlantic ecoregion.

Sombroek et al. (1993) estimated that the soils of Central America have a SOC pool of

99 Mg C ha-2 up to 1 m depth. In this study, only MA and PAS from the Pacific ecoregion were below this value and the remaining land uses were considerably higher. Alvarado

(2006) estimated the total SOC pools (0 – 100 cm) of tropical dry, moist and montane wet forests at 82.9 Mg C ha-2, 111.4 Mg C ha-2 and 243.6 Mg C ha-2, respectively. On a global basis, tropical evergreen forest may have a SOC pool of 186 Mg C ha-2, while the tropical deciduous forests are estimated to have 158 Mg C ha-2 at 0 – 100 cm depth

(Jobbágy and Jackson, 2000). The rain forest in this study presented an intermediate value in relation to the one provided by the literature and in the case of BP the values were approximated. The SOC values for DF are significantly greater than the ones

47 reported by Alvarado (2006). This may be in relation to the charcoal, which may account for up to 20 % of the total SOC, as was previously estimated in the same forest of this study (Lorenz et al., unpublished data).

The C sink capacity of the soils in this study was of 18 - 37 Mg ha-1 in the Atlantic Moist ecoregion, 14 - 89 Mg ha-1 in the Pacific Dry ecoregion and 9 - 81 Mg ha-1 in the

Montane ecoregion. In the case of BA, CA and OR, the SOC pools were not significantly different from those in the forests that were used as references. When assessing the C sink capacity under agricultural sites in comparison to the soils under natural vegetation, paired forests are the ideal scenario. However, due to the land fractionation and lack of land use history, this ideal could not be achieved at the time this field study was designed.

In the case of the Pacific ecoregion, for example, the dry forest used in this study corresponds to the last remnant of this ecosystem in Costa Rica (Santa Rosa National

Park). On the other hand, it is possible that the management that these agricultural sites receive is such that SOC values are maintained and C depletion is hindered (Sá et al.,

2001). It appears that the soils under agricultural management in the Atlantic ecoregion lost less SOC in comparison to the crops in the Pacific and Montane ecoregions. In the case of the Pacific ecoregion, this may be due to the sandier texture of the MA and PAS sites. In the Montane ecoregion, soil erosion and management (lack of contour lines and terraces, lack of cover crops, etc) in the CF may have contributed to the SOC depletion in relation to the BP forest.

48

Table 3.5. Total Organic Carbon Pool (0 – 100 cm) of three ecoregions in Costa Rica. Land uses with different letters are statistically different (a=0.05)

Total Organic Potential C Land use C Sink Capacity Mg ha-1 Atlantic Moist BA 150.3 a - RF 150.2 a - PA 132.2 a 18.1 PI 113.6 a 36.7 Pacific Dry DF 164.5 a - CA 150.4 ab 14.1 MA 85.6 bc 78.9 PAS 75.9 c 88.6 Montane BP 246.4 a - OR 237 ab 9.4 CO 179.5 bc 66.9 CF 165.7 c 80.7

Effects of land use and climate on soil organic carbon

The correlations between SOC concentration (g C kg-1), SOC density (Mg ha-1) and total

SOC pool (0 -100 cm) and climatic and soil physical variables are presented in Table 3.6.

It was observed that SOC had a negative relationship with sand (r = -0.20, P< 0.05), while in the case of silt and clay, the correlation was positive (r = 0.20, P<0.05). This is in line with the lower SOC values in MA and PAS, soils with sandier textures. Bulk density also had a strong, negative effect on SOC concentration (r = -0.72, P<0.01), SOC density (r = -0.20, P<0.01) and SOC pool (r = -0.60, P<0.01). The inverse relationship between BD and elevation (due to the predominance of Andisols in the Montane

49 ecoregion) may counteract the influence of increased SOC concentrations as was observed by Powers and Schlesinger (2002). Therefore, comparisons of SOC among long altitudinal ranges should be based on SOC concentration rather than SOC density. In a study on volcanic soils from the Colombian savannas, Hoyos and Comerford (2005) also found a negative effect of BD not only on SOC and moisture content, but also on C:N ratio. In this study, C:N did not seem to be related to this property, but it did show a positive relationship with SOC and a negative relationship with MAP (r = -0.56, P<

0.01). Diaz Romeu et al. (1970) also found a tendency of lower C:N values at higher

MAP, which may be attributable to the effects of moisture and vegetation on the dynamics of C and N.

Climatic variables also had strong effects on SOC. The SOC concentration and the total

C pool had a positive relationship with altitude and a negative relationship with MAT.

Particle size distribution and soil mineralogy differ with elevation. For example, in residual soils of northeastern Costa Rica, sand concentration in the top 10 cm increased with elevation, clay concentration decreased, and silt concentration remained approximately constant. Similarly, relatively young non-crystalline soils are often found at high elevations, while highly weathered oxic soils tend to locate at lower elevations.

This distribution in soil mineralogy has a significant effect on SOC pool (Powers and

Schlesinger, 2002).

The effect of MAP was not strong, in contradiction to what has been reported before in other regional and local studies (Powers and Schlesinger, 2002; Diaz-Romeu et al.,

1970). This may be related to the confounding effects of charcoal in the DF and soil

50 management in the CA soil from the Pacific Dry ecoregion, which has higher SOC concentrations and SOC pools than other land uses in the Atlantic Moist ecoregion.

Table 3.6. Correlations between climatic variables and soil organic carbon (SOC) based on the data obtained in this study. Numbers in bold indicate significance at P < 0.01.

2 3 4 5 6 7 8 9 10 11 12 13 1. Sand -0.64 -0.33 -1.00 - 0.20* 0.19 -0.25 -0.03 0.32 -0.15 -0.37 0.11 -0.16** 2. Clay -0.52 0.64 0.15* 0.22* -0.06 -0.28 -0.13 0.04 0.40 0.19* -0.15** 3. Silt 0.33 0.42 -0.29 0.12 0.38 -0.20 0.13 0.30 -0.31 0.32 4. Silt + clay 0.20* -0.19 0.26 0.03 -0.32 0.15 0.37 -0.20 0.23

5. C (g kg-1) -0.72 0.61 0.68 0.23* 0.58 -0.19 -0.86 0.87 6. Bulk density -0.20 -0.74 0.19 -0.60 -0.28 0.81 -0.70

7. Carbon (Mg ha-1) 0.26* 0.31 0.44 -0.22 -0.29 0.27 8. Field moisture content -0.05 0.43 -0.06 -0.67 0.58 9. CN ratio 0.01 -0.56 0.17 -0.06 10. C pool (0 - 100 cm) 0.06 -0.69 0.66 11. MAP -0.08 -0.09 12. MAT -0.96 13. Altitude -

The results in this study also indicate a strong negative relationship between MAT and altitude (r = -0.96, P< 0.01).The lapse rate for Costa Rica was estimated at 5.2 – 6.5 °C km-1 by Coen (1983). A linear regression using 132 studies from Central America (Figure

3.5) estimated a lapse rate of 5.5°C km-1, in accordance to what Powers and Schlesinger

(2002) estimated for Northeastern Costa Rica (y = -0.0059x + 25.6, R2 = 0.85).

51 35 e r u

t 30 a

r y = -0.0055x + 26.21

e 25

p R² = 0.93

m 20 e ) T C l 15 ° a ( u

n 10 n

A 5 n a

e 0

M 0 1000 2000 3000 4000 Altitude (masl)

Figure 3.5. Relationship between altitude and mean annual temperature in Central America. Redrawn from original data from studies in Central America (n=132). For a complete list of studies see Appendix A.

Since there were contradictory relationships between climate and SOC based on the results of this study, a second correlation was conducted using several soil profiles from the literature of SOC in Central America (Table 3.7). In addition, the samples were divided and studied according to the main life zone. In this case the correlations were more in line to the expected relationships between SOC and MAP and MAT. The MAP was correlated positively (r = 0.35 – 0.94, depending on the life zone) with SOC, while

MAT had a negative effect (r = - 0.37 to -0.75). In general, these correlations were stronger in the Montane ecoregion, intermediate in the Pacific (tropical dry life zone) and lower in the Atlantic ecoregion (tropical wet and rain life zones).

52 Table 3.7. Correlations relating SOC concentration (0 – 30 cm) to mean annual precipitation (MAP), mean annual temperature (MAT) and altitude in different life zones of Central America.

Life Zone Tropical Dry Tropical Wet/Rain Montane a 2 3 4 2 3 4 2 3 4 1. MAP -0.51** -0.67* 0.56* 0.24** -0.29 0.35 -0.68* 0.32 0.94 2. MAT -0.48* -0.66* -0.87 -0.37 -0.82 -0.75 3. Altitude 0.21 0.35 0.39 4. SOC - - - a Data from the Lower Montane and Montane life zones were combined. Numbers are in bold when P< 0.05, *P <0.05, **P <0.01.

Soil organic carbon inventory in soils of Costa Rica and Central America

A partial review of the cumulative SOC pool from 0 – 100 cm depth is shown on Table

3.8 (the complete review is available on Appendix A). Based on these studies, the average SOC pool for Central America is estimated at 148.7 Mg C ha-1, which is significantly greater than the pool estimated by Sombroek et al. (1993).

After conducting a survey of C and N concentrations on several soils of Central America,

Diaz Romeu et al. (1970) concluded that the conditions of temperature and precipitation in which Holdridge’s Life Zones are based have a direct influence on SOC concentration.

In Table 3.9, data on SOC (in Mg C ha-1) obtained from the literature were grouped by life zones. On average, it is estimated that Central American croplands may have a C pool of 123 Mg C ha-1, while pasturelands have on average 152 Mg C ha-1. Forest soils may store 170 Mg C ha-1. It is estimated that the C sink capacity for these soils is 120 Mg

C ha-1 in the Lower Montane Wet forests, 31 Mg C ha-1 in the Premontane Wet forest, 5.7

Mg C ha-1 in the Tropical Dry forest and 32 Mg C ha-1 in the Tropical Wet forest. In some

53 cases, pasturelands had a higher C pool than natural vegetation, which was also reported by Cerri et al. (2003) in Brazil.

54

Table 3.8. Review of soil organic carbon pool (Mg C ha-1) at 0 - 100 cm in different ecoregions and land uses of Central America (complete review in Appendix A).

Study Area Holdridge's Life Zone Soil¥ Land use SOC Reference Mg ha-1 Central Panama Moist tropical forest Tropoudalf 45 yr. Pasture 85.0 Potvin et al., 2004 20 yr. Teak 183.0 Northeastern Costa Rica Tropical wet Haploperox (residual) Forest 330.0 ± Veldkamp et al., 2003 Haploperox (alluvial) Forest 213.1± Central Costa Rica NA Dystrustept Organic vegetables 94.0 CEDECO, 2006 Hapludand Organic vegetables 259.5 Dystrustepts Organic coffee 149.3 Haplustepts Conventional coffee 88.1 Haplustults Organic sugar cane 140.9 Haplustults Conventional sugar cane 106.4 5 6 Hapludand Sugar cane in transition to 314.7 organic Northeastern Costa Rica Premontane, wet forest NA Forest 128.0 Amézquita et al., 2005 basal belt transition Silvopasture Acacia + 168.0 Arachis Improved pasture 194.0 Native grass 208.0 Degraded pasture 94.0 Brachiaria pasture 134.0 Central-western Costa Subhumid tropical NA Forest 185.0 Rica Silvopasture Brachiaria+ 130.0 Cordia+ Guazuma Native grass 169.0 Forage bank 130.0 Degraded pasture 129.0 Southern Mexico Tropical lower montane, NA Forest (Oak/Cloud) 242.8 De Jong et al., 1999 premontane moist, Forest (Pine) 172.6 subtropical lower montane, Forest (Degraded) 184.2

55 Table 3.9. Estimated C sink capacity for Costa Rican soils up to 1 meter depth under different life zones. The Montane ecoregion was divided into Lower Montane and Premontane, while the Tropical Dry and Tropical Wet life zones correspond to the Pacific Dry and Atlantic Moist ecoregions, respectively.

Carbon Pool Carbon Sink Mg C ha-1 Lower Montane Wet Forest Natural vegetationa 293.2 - Croplands 172.6 120.6 Premontane Wet Forest Natural vegetationa 162.7 - Croplands 132.0 30.7 Pasture 166.7 -4.0 Tropical Dry Forest Natural vegetationa 123.7 - Croplands 118.0 5.7 Tropical Wet Forest Natural vegetationa 101.2 - Croplands 69.3 31.9 Pasture 147.7 -46.5 a Values on natural vegetation according to Alvarado, 2006. The estimations for pasture included soils under native, degraded, improved and silvopasture management

Based on the data obtained from the literature review on SOC pools in soils of Central

America, the C total pool and C sink up to 1 m depth were estimated for this region

(Table 3.10). Country area statistics were retrieved from FAO’s Statistical Database

(FAOSTAT, 2009). These estimations were based on references only from studies on mature forest, crops and pasture soils; therefore forestry plantations, agroforestry systems, organic agriculture and other land uses were excluded. Mexico was excluded from these calculations because only the Central and Southern regions of this country are considered to be part of Central America.

The total SOC pool for the region is estimated at 6,454.6 Tg of Carbon, from which 3,075

Tg C are stored in forest soils, 1,779 Tg C are in pasture soils and 992 Tg C are in

56 croplands. These values are similar to the estimations by Bernoux and Volkoff (2006) of

6,214 Tg C from 0 – 100 cm using digital soil maps.

Table 3.10. Estimated SOC pool in soils of Central America forest, croplands and pasturelands (data on area estimated for 2007; FAO Statistical Database, 2009)

Area (1000 ha) Soil Organic Carbon (Tg) Total Crop Permanent Forest Crop Pasture Forest Total Country (land) area1 pasture cover Country Belize 2,281.0 152.0 136.9 1,653.0 18.7 21.5 281.3 300.0 Costa Rica 5,106.0 500.0 2,250.0 2,397.0 61.5 353.7 407.9 823.1 El Salvador 2,072.0 597.0 637.0 287.6 73.4 100.1 48.9 222.5 Guatemala 10,716.0 2,514.0 1,950.0 3,830.0 309.2 306.5 651.8 1,267.5 Honduras 11,189.0 1,428.0 1,700.0 4,335.0 175.6 267.2 737.8 1,180.6

Nicaragua 11,999.0 2,184.0 3,106.0 4,979.0 268.6 488.3 847.4 1,604.2 Panama 7,434.0 695.0 1,535.0 4,288.8 85.5 241.3 729.9 1,056.7

Total 50,797.0 8,070.0 11,314.9 21,770.4 992.4 1,778.7 3705.1 6,454.6

1 Crop area includes land under the category of permanent (perennial) crops and temporary (annual) crops by FAO, whenever information about temporary cropland area was not available, the "arable land and permanent crops" area was used instead. Data on pasture area for Belize was estimated as 6% of the total land area (World Resources Institute, 2006).

Based on the data retrieved from the whole Central American region, Costa Rica has a total C pool of 823 Tg C from which 408 Tg C are from forest soils, 62 Tg C in agricultural soils and 354 Tg C in pasture soils (Table 3.10). Based on estimations using only data from studies conducted within the country, the total SOC pool is estimated at

872 Tg of C (not shown), which is slightly higher than the estimations made by Alvarado

(2006) of 782 Tg C based on 52 samples from diverse life zones.

57

Potential for SOC sequestration and offseting GHG emissions in Costa Rica

In 1998, Costa Rica released the First National Communication on GHG emissions as part of its commitment to the United Nations Framework Convention on Climate Change

(UNFCCC). The inventory estimated that in 1990, around 4,404.4 Gg CO2 eq were emitted to the atmosphere, from which 139.8 Gg CO2 eq were emitted by the agricultural sector (IMN, 2005). It is estimated that 4% of the emissions from agriculture are derived from soils. By 2005, the country’s emissions increased by 128%, at a rate of 103 Gg C eq per year (Table 3.11).

Table 3.11. Costa Rica’s greenhouse emissions inventory for 1990 (baseline year), 2005 and predicted emissions for 2009.

Emissions a b (Gg CO2 eq) (Gg C eq) Sector 1990 2005 1990 2005 2009d

Agriculture 139.8 4,970.0 38.1 1,355.5 1,706.7

- Crop soils 54.2c 68.3 Land use change 1,210.6 -2,356.7 330.2 -642.7 Country total 4,404.4 10,077.3 1,201.2 2,748.4 3,160.9 Annual rate (2005-1990) 103.1 a Source: IMN, 2005 b C eq = CO2 eq x (12/44) c According to IMN estimations, crop soils emissions correspond to 4% of the agricultural sector emissions d Predicted, based on annual rate

The data on the country’s area under cropland, pastureland and forestry plantations were used to estimate the potential for SOC sequestration. The estimations of different regional studies on SOC sequestration rates were also used (Table 3.12). Based on these data, it is

58 estimated that around 826 – 2,251 Gg C are sequestered per year. These amounts correspond to 26 – 71% of the country’s total emissions. The SOC sequestration can offset around 48 – 132 % of the net emissions from agriculture. In addition, the aboveground biomass that is part of the Payment for Environmental Services (PES) system is offsetting 23 % of the country’s emissions. This is a partial value, since estimations on agroforestry were not possible because there is no available data on area extension; however, about 2.6 million trees are planted (FONAFIFO, 2009).

Table 3.12. Potential for soil organic carbon sequestration under different scenarios in Costa Rica.

Carbon sequestration Land use Area Carbon sequestration potential

(1,000 ha) Mg C ha-1 yr-1 Gg C yr-1

Cropland 500 1.8a - 1.2b 600 - 900

Pasture 2,250 0.1a - 0.6c,d 225 - 1,350

Plantations 1.25e 0.4a - 0.9c 0.5 - 1.12

Natural regeneration 1.66e 0.3f 0.5 Total 826 - 2,251 Percentage of emissions from agriculture 48 - 132 % Percentage of country emissions 26 - 71 % a Jimenez and Lal, 2006 b Follet et al., 2005 c Potvin et al., 2004 d Montenegro and Abarca, 2002 e FONAFIFO, 2009 f Reiners et al., 1994

Despite the importance of estimating the SOC potential to offset GHG emissions, it is also imperative to address the issue of permanence of the sequestered SOC, since it

59 constitutes a relevant step for C commoditization (Dilling, 2007). Hence, it is necessary to calculate the proportion of the total C pool that is physically protected (Six et al., 2002;

Hassink, 1997). The SOC associated to the mineral fraction (silt + clay) may be securely stored in tropical soils for decades (Schwendenmann and Pendall, 2006). Therefore, the physical protection capacity of the soils in this study was also determined, as will be discussed in the following chapter.

Conclusions

The results obtained in this field study provided evidence for the following conclusions:

1. The SOC pool (0 – 100 cm) is estimated at 114 – 150 Mg C ha-1 in the Atlantic

Moist ecoregion, 76 – 165 Mg C ha-1 in the Pacific Dry ecoregion and 166 – 246

Mg C ha-1 in the Montane ecoregion of Costa Rica. The C sink capacity was of

18.1 - 36.7 Mg C ha-1, 14.1 – 88.6 Mg C ha-1 and 9.4 – 80.7 Mg C ha-1 in the

Atlantic, Pacific and Montane ecoregions, respectively. The country’s total C pool

is estimated at 823 – 872 Tg C, with a potential for SOC sequestration of 826 –

2,251 Gg C per year.

2. The effect of land use on SOC pool was specific to the ecoregion: there were

more significant differences on SOC in the Pacific ecoregion than in the Atlantic

or Montane ecoregions. Within ecoregions, the main mechanism by which land

use directly influenced the SOC pool was by changes in bulk density; however,

SOC was also strongly affected by texture. The SOC in some crop soils did not

differ significantly from the SOC pool of the mature forests that were used as

references, which may be due to: (a) the lack of a paired system of croplands and

60 forests, or (b) an indication that the type of management is not depleting the SOC

pool at significant amounts.

3. Climate had an evident effect on SOC concentration, SOC density and SOC pool.

The vertical distribution of SOC was also affected by climatic parameters.

Altitude impacted SOC by its influence on bulk density, field moisture content

and soil texture; however, this may be an indirect indicator of the effect of soil

type distribution in Costa Rica, since Andisols predominate at high altitudes.

Land use and management have confounding effects on the relationship between

climate and SOC. In agricultural soils, the effect of climate is not as significant as

it is in soils under natural vegetation, because management may modify the soil’s

moisture and temperature characteristics.

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Veldkamp, E. 1994. Organic-carbon turnover in 3 tropical soils under pasture after deforestation. Soil Science Society of America Journal 58: 175-180.

World Resources Institute (WRI). 2006. EarthTrends: the environmental information portal. Forests, grasslands and drylands, country profile – Belize. Available online at: http://earthtrends.wri.org/text/forests-grasslands-drylands/country-profile-19.html

65 4. PHYSICAL PROTECTION OF SOIL ORGANIC CARBON BY PRIMARY

AND SECONDARY PARTICLES IN COSTA RICAN ECOREGIONS

Abstract

The study of soil organic carbon (SOC) dynamics in tropical soils requires the separate assessment of the soils fractions which provide different levels of SOC protection.

Conceptual models of SOC protection define 3 mechanisms: (i) silt plus clay protected

SOC (S+C – C, mineral particles <53 µm), (ii) microaggregate protected SOC (WSA – C,

53 – 250 µm), and (iii) biochemically protected SOM. In addition, a fourth SOC pool is defined as the unprotected carbon (C), which is comprised in part of particulate organic matter (POM). Recent studies provide evidence for the existence of a C saturation level based on the inherent physical characteristics of the silt plus clay particles and soil aggregates. According to these models, once this saturation level is reached, the additional organic C is stored in the unprotected POM.

Therefore, this study assessed the role of primary (sand, silt and clay) and secondary

(micro and macroaggregates) particles on the physical protection of SOC at the surface (0

– 30 cm) of 12 land uses, distributed in 3 contrasting ecoregions of Costa Rica. Specific objectives of this research were to: (i) determine the effect of land use on the C content in

POM and WSA; (ii) determine if the current physical protection capacity models are useful indicators of SOC sequestration and, (iii) establish the relationships among aggregate properties and SOC.

66 The SOC was separated into different size fractions according to the conceptual models on SOC protection. In addition, aggregate stability was determined by studying the water stable aggregates (WSA), mean weight diameter (MWD), tensile strength (TS), water drop penetration time (WDPT) and moisture retention (MR).

The POM was significantly lower in soils of the land uses under more intensive management in two ecoregions. In addition, there was a physical protection capacity of 6

– 60% in the silt plus clay fraction, which corresponds to a C sink capacity of 4 – 44 Mg

C ha-1. However, certain soils were significantly deviated from their expected capacity level, which may indicate that the models suggested by the literature are not reflective of the real attainable C storage capacity, and should be modified to reflect higher SOC concentrations and silt plus clay contents of some tropical soils. There was no evidence that the aggregates from these soils follow an aggregate hierarchy model, and the relationship between WSA and WSA-C was less apparent in these soils than the usually strong linear regression reported for soils from temperate regions. Finally, the magnitude and direction of the effect of SOC on WSA, MWD, TS, WDPT and MR was specific to certain soils in the specific ecoregions.

Abbreviations

TC: total carbon, POM-C: carbon in the particulate organic matter or sand size fraction,

WSA-C: carbon in the water stable aggregates, S+C-C: carbon in the silt plus clay or mineral fraction, SOM: soil organic matter, SOC: soil organic carbon, WSA: water stable aggregates, MWD: mean weight diameter, TS: tensile strength, MR: moisture retention,

WDPT: water drop penetration time.

67 Introduction

Soil organic matter (SOM) is associated to soil particles through different mechanisms and levels of protection. The study of SOC dynamics in tropical soils requires the separate assessment of these fractions, which can be achieved through carbon (C) fractionation techniques (Christensen, 1992). Six et al. (2002) proposed a conceptual

SOM model based on three physicochemically defined SOM pools: (i) silt plus clay protected SOC (S+C – C, mineral particles <53 µm), (ii) microaggregate protected SOC

(WSA – C, 53 – 250 µm), and (iii) biochemically protected SOM (due to its own chemical composition and through chemical complexing processes). In addition, there is a fourth SOC pool defined as unprotected carbon, composed of the sand-size, particulate organic matter (POM). The POM is considered to be greatly affected by land use change and management and therefore is generally used as an indicator of the effect of anthropogenic practices on the SOC pool and C dynamics (Carter, 2002; Christensen,

2001; Amelung et al., 1998; Cambardella and Elliot, 1992).

Previous models on SOM dynamics assumed a linear increase of SOC with increasing organic matter inputs, predicting that the soil C pool can in theory be increased without limits (Paustian et al., 1997). Soils under natural vegetation reflect the natural balance between C inputs and outputs, and usually serve as reference sites for agricultural soils towards what is physically attainable under the common conditions of soil type and climate. However, SOC stocks under natural vegetation are not necessarily representing an upper limit in SOC pool. In fact, there are studies demonstrating higher SOC pools for managed soils over naturally vegetated due to increased inputs in organic matter under tropical conditions (Cerri et al., 2003; Sá et al., 2001).

68 Recent studies provide evidence for the existence of a C saturation level based on the inherent physical characteristics of the silt plus clay (e.g. surface area, clay mineralogy) particles and soil aggregates (e.g. aggregate stability) (Stewart et al., 2008; Six et al.,

2002). Hassink (1997) postulated a model for the physical protection capacity or

“capacity factor” of SOC, related to a maximum amount of organic C associated with clay and silt particles. This physicochemical mechanism of protection depends not only on the clay content, but also on the clay mineralogy, where the latter seems to be more determining in tropical soils (Lopez – Ulloa et al., 2005; van Dam et al., 1997;

Veldkamp, 1994). Once this maximum capacity is reached, the additional organic C is stored in the unprotected pools (POM) (Carter, 2002). Under this scenario, the C associated to the silt plus clay fraction is considered to indicate the capacity of a soil to protect SOC (Hassink, 1997).

Soil aggregates protect C by forming a barrier against microorganisms, controlling food web interactions, and influencing microbial turnover (Six et al., 2002). The basic structure of aggregates is composed of domains of clay colloids bonded to polyvalent cations and organic matter. The hierarchical theory of aggregation states that: (i) microaggregates usually bind together by effect of young organic matter into macroaggregates (Six et al., 2000; Jastrow et al., 1996); and (ii), that the bonds within microaggregates are stronger than the bonds between microaggregates (Edwards and

Bremner, 1967). While soils with 2:1 clay dominance show a clear aggregate hierarchy with greater SOC amounts in macroaggregates, soils with 1:1 clays (most tropical soils) generally do not exhibit this pattern (Zech et al., 1997; Six et al., 2000; Hoyos and

Comerford, 2005).

69 Under poor aggregate stability, the microbial activity increases and depletes SOC, which eventually leads to a loss of aggregation (Jastrow, 1996; Six et al., 1998). Aggregate properties like mean weight diameter (MWD), water stable aggregates (WSA), water repellency (WDPT), moisture retention (MR) and tensile strength (TS) are direct indicators of soil structure and SOC protection. Aggregate stability is a function of whether the cohesive forces between particles withstand an applied disruptive force

(Kemper and Rossenau, 1986). Water repellency indicates the degree of hydrophobicity of soil particles. This property is associated with SOC since the hydrophobicity of organic matter beneficiates the formation and protection of stable aggregates (Chenu et al., 2000) and hydrophobic organic matter itself is more stable against microbial decomposition (Goebel et al., 2005). Tensile strength measures the strength of individual soil aggregates when a force (F) is applied across the aggregate unit, causing an elastic deformation. Finally, the heterogeneity in structure and aggregate size distribution also influences the moisture retention properties between soils, which also influences SOC dynamics (Blasco, 1971).

Based on the factors considered above, SOC sequestration in tropical soils may be a function of the accumulation of C into stable aggregates, POM and silt plus clay particles.

Yet, given the contrast in soil type, clay mineralogy and climate, it is necessary to determine whether the protective capacity models that were established mainly for temperate and subtropical soils are effective in explaining the dynamics of SOC protection in tropical soils. This study assessed the role of primary (sand, silt and clay) and secondary (micro and macroaggregates) particles on the physical protection of SOC at the surface (0 – 30 cm) of 12 land uses, distributed in 3 contrasting ecoregions of Costa

70 Rica. Specific objectives of this research were to: (i) determine the effect of land use on the C content in POM and WSA; (ii) determine if the current physical protection capacity models are useful indicators of SOC sequestration and, (iii) establish the relationships among aggregate properties and SOC.

Methodology

This study was conducted on three contrasting ecoregions of Costa Rica. The Isthmian –

Atlantic Moist ecoregion was studied on the main campus of EARTH University (10° 10’

N and 83° 37’ W), in the Atlantic lowlands (50 masl) of Costa Rica. This region has a mean relative humidity of 89 %, MAT of 24.5 °C, and a MAP of 3,227 mm. Oil palm

(PA) (Elaeis guineensis), pineapple (PI) (Ananas comosus), banana (BA) (Musa acuminata) and a tropical rain forest (RF) were the land uses selected for soil sampling.

At the Central American Pacific Dry ecoregion, soils from EARTH University’s La Flor

Campus in Guanacaste were studied. This region has a mean relative humidity of 74 %,

MAT of 28 °C and MAP of 1,800 mm. Soils under sugarcane (CA) (Saccharum officinarum), pasture (PAS) (various native species), and mango (MA) (Mangifera indica) were selected for sampling. In addition, samples of a mature dry forest were collected at the Santa Rosa National Park (10°48’ N, 85°36’ O). Finally, from the

Talamancan montane ecoregions (mean relative humidity of 90 %, MAT of 15 °C, and

MAP of 2,016 mm) soil samples were collected from an organic farm (OR) (various vegetables in a rotation system), a conventional farm (CO) (various vegetables in a rotation system), a coffee plantation (CF) (Coffea arabica) and a mature cloud forest

(BP).

71 The ecoregions and land uses selected for this study as well as the sampling procedures are described in Chapter 3.

Soil Preparation

Subsamples of air-dried bulk soil from 0 – 100 cm depth were gently pressed and passed through a 2 mm sieve and was used for particle size determination by the hydrometer method (Gee and Or, 2002). Pretreatment for organic matter removal with hydrogen peroxide was used on samples with SOC concentration above 2.5%. Approximately 40 g of soil was dispersed in 250 ml of distilled water and 100 ml of a 5 g L-1 solution of sodium hexametaphosphate (Na HMP) and Na2CO3. The Na2CO3 was used to raise the pH of the solution to 12, in order to achieve a better dispersion. The samples were placed in a reciprocal shaker and allowed to react overnight. An electric mixer was then used to stir the samples for 20 minutes previous to conducting the readings with the hydrometer.

Observations were taken at 40 seconds after the hydrometer was inserted, and at 3 hours after the first reading.

For those samples with andic properties (Montane ecoregion), the particle size assessment was conducted on soils at field moisture content. A subsample was dried at

105 °C for moisture content determination. Pre-treatment with hydrogen peroxide was also used to oxidize organic matter (Gee and Or, 2002).

Aggregate Analyses

A sub sample of soil from the surface layers (0 – 10, 10 – 20 and 20 – 30 cm depth) was manually sieved through a nest of sieves with 8.0, 4.75, and 2.0 mm openings for obtaining aggregates of different size fractions.

72

The dry aggregates retained on the 4.75 mm sieve were used for assessment of the aggregate stability by wet sieving by the Yoder (1936) method. Aggregates were wetted through capillarity and sieved under water for 30 minutes. Six aggregate size – fractions obtained were: > 4.75 mm, 4.75 – 2.0 mm, 2.0 – 1.0 mm, 1.0 – 0.5 mm, 0.5 – 0.25 mm and < 0.25 mm. This data on aggregate size distribution was computed to calculate the

% WSA and the MWD. Aggregates were then classified into two fractions: macroaggregates (> 250 µm) and microaggregates (< 250 µm). The data of aggregate size distribution was used to calculate the MWD (Kemper and Rosenau, 1986):

Where, n is the number of aggregate size ranges; i is the mean diameter of any particular size range of aggregates separated by sieving; and, mi is the weight of the aggregates of that size range as a fraction of the total dry weight of the sample.

Upon drying at 45°C, a subsample of the micro and macroaggregate fractions from the wet sieving test was ground to a < 250 µm size for the determination of aggregate – associated SOC, N and C:N ratio by the dry combustion method (Nelson and Sommers,

1996) using a C:N analyzer (Vario Max, Elementar Americas Inc., Germany).

Crushing forces were applied for determining the TS of aggregates according to the method by Dexter and Kroesbergen (1985). A pre-treatment consisted of placing aggregates of 4.75 – 8.0 mm size in a sealed chamber with a saturated salt solution of sodium chloride for two weeks in order to equilibrate to a relative humidity of 75%

(Rockland, 1960). After equilibration, ten aggregates per replicate were crushed in an apparatus based on the Horn and Dexter design (1989). The force (F) applied to crush each aggregate was recorded and used as per Equation 2 (Dexter, 1975): 73

Where, F is the polar force at crushing point and d is the diameter of the aggregate.

The diameter of aggregates was determined by the “Method 1” by Dexter and

Kroesbergen (1985), where the longest, intermediate and smallest diameter of each aggregate was measured with a caliper and the average of these was used in Equation 2.

Ten aggregates of the 4.75 – 8.0 mm fraction per replicate were also equilibrated by the saturated sodium chloride solution and were then used to determine the water drop penetration time (WDPT) using the method developed by Letey (1969). A drop of deionized water was placed on top of every aggregate to record the time (in seconds) it takes to be absorbed.

The moisture retention (MR) of soil at 0 – 30 cm depth was assessed for aggregates of approximately 8 mm (Blanco–Canqui et al., 2005; Dorel et al., 2000). A series of equilibria between the water in the soil sample and moisture retention potentials of 0, -1,

-3, -6, -33, -60, -300 and -1,500 kPa were established using a combination of tension table and pressure plate apparatus (Klute, 1986). At each equilibria, the water content (on mass basis) was determined and paired with a value in the matric pressure head. The data pairs were then plotted on a retention function to develop a soil moisture characteristic curve (SMCC).

Carbon Fractionation

The sand-size organic matter (POM) for the first 30 cm depth was determined through the physical fractionation method by Cambardella and Elliot (1992). A sample of 10 g of soil

-1 was dispersed in 30 ml of 5 g L solution of Na HMP and Na2CO3 and was then placed

74

on a reciprocal shaker overnight. The dispersed samples were then passed through a 53

µm sieve and rinsed several times with water. The soil retained on the sieve contained the cPOM and was dried at 45°C for C analysis (Nelson and Sommers, 1996).

In order to express POM-C on a mass basis, the following calculation was made (Sollins et al., 1999):

-1 Where POM-C is the coarse particulate organic matter in g kg , Cs is the % C in the sand

-1 fraction and Ws is the dry mass of sand fraction in the soil in g g . The silt + clay associated C was assumed to be the difference between total soil organic C (TC) and

POM-C (Carter et al., 2003; Cambardella and Elliot, 1992).

The enrichment factor E (Equation 4) was calculated to estimate the content of C in a particular size fraction in relation to that of the whole soil (Amelung et al., 1998;

Christensen, 1992):

[4]

The enrichment factor was calculated for POM, mineral – C and WSA – C. Wherever E >

1, the size fraction is being enriched, whereas if E < 1, the C in the size fraction is being depleted.

The physical protection capacity of soils at 0 – 30 cm was also estimated following the models by Hassink (1997) (Equation 5) and Elustondo et al. (1990) (Equation 6) consisting of a simple linear regression between the silt + clay associated C and the dry mass of silt and clay fraction in the soil:

75

Statistical Analysis

Simple descriptive statistics were calculated for all properties. When the data were not normally distributed, results were log-transformed to account for the individual aggregate variability. Tukey’s HSD was calculated to determine significance between treatments, soil depths and aggregate fractions (a = 0.05), using JMP 7 Statistical Software. Simple regression and correlation coefficients between mechanical properties and C concentration were also obtained with JMP 7.

Results and Discussion

Carbon associated to silt plus clay and sand (POM) fractions

Carbon fractionation divided soil particles into sand-size POM (> 53 µm) and the silt plus clay or mineral fraction (< 53 µm). Results on POM-C (on sand fraction basis) are shown in Table 4.1. It was observed that POM reflected the effects of long term soil cultivation on SOC in the Atlantic and Pacific ecoregions, and the results were consistent with the differences in TC found among land uses (Six et al., 2002; Amelung et al., 1998).

However, the Montane ecoregion showed no effect of land use on POM content

(P < 0.05). In the Pacific ecoregions, soils under pasture (PAS) and mango (MA) contained lower POM-C concentration than those under dry forest (DF) and sugar cane

(CA). In the Atlantic, soils under pineapple (PI) contained the lowest POM-C concentration. In general, POM concentration tended to decrease with depth. 76

Table 4.1. Soil Organic Carbon concentration in the sand fraction (POM-C) of different land uses in three ecoregions of Costa Rica. Land uses with same letters are not statistically significant (a = 0.05).

Carbon concentration (g C kg-1 sand) 0 - 10 cm 10 - 20 cm 20 - 30 cm Pacific Dry DF MA PAS CA DF MA PAS CA DF MA PAS CA 25.8 a 15.3 ab 3.9 b 18.0 ab 8.7 ab 6.5 b 1.1 b 17.5 a 7.2 ab 5.4 ab 0.8 b 11.6 a Montane BP CF CO OR BP CF CO OR BP CF CO OR 24.9 a 31.7 a 25.3 a 27.4 a 17.9 a 20.5 a 27.2 a 24.6 a 11.0 a 19.3 a 24.3 a 21.2 a Atlantic Humid BA PA RF PI BA PA RF PI BA PA RF PI 19.5 ab 19.9 ab 35.6 a 7.8 b 28.1 a 13.8 ab 12.9 ab 8.3 b 12.0 a 12.1 a 18.9 a 6.7 a

In general, the proportion of POM-C in reference to the C concentration in the bulk soil

(TC) ranged from 3% (CF) to 27% (BA), which was in direct relation to the sand content in the soil (Table 4.2). The upper boundary of this range is similar to the average POM-

C/TC proportion observed in arable temperate soils (Carter et al., 2003). Thus, larger proportions of SOC were associated to clay and silt: from 73 to 97% of the total C

(Amelung et al., 1998). The values from TC and C fractionation for the three different surface layers were pooled into a composite layer from 0 – 30 cm depth. The soils from the Montane ecoregion were characterized by significant differences in POM-C concentration among coffee (CF) and the rest of land uses, but no significant differences in the S+C–C were observed (P < 0.05). Angers (1998), also reported that the clay fraction is less affected by land use than the POM fraction. However, in the Atlantic and

Pacific ecoregions, land use had a significant effect on S+C–C, following a similar trend to POM-C and TC. Generally, soils from native vegetation and pasture have higher POM than cultivated soils (Carter et al., 2003). In this study, it was observed that soils from the

77

land uses under more intensive management contained significantly lower POM-C than those supporting extensive land uses.

Table 4.2. Organic carbon contents of total soil (TC), particulate organic matter (POM-C) and the silt + clay fraction (S+C –C) of different land uses in three ecoregions of Costa Rica at 0 – 30 cm. Land uses with same letters are not statistically significant (a=0.05).

POM Total soil Silt + Clay Land use POM-C C/Total C C C g C kg soil-1 % Pacific Dry DF 30.24 a 6.17 a 24.07 a 20% CA 33.86 a 7.64 a 26.22 a 23% MA 21.90 ab 4.51 ab 17.39 ab 21% PAS 9.80 b 1.19 b 8.60 b 12% Montane BP 78.67 a 7.16 a 71.50 a 9% CF 43.73 b 1.33 b 42.41 a 3% CO 60.84 ab 9.54 a 51.30 a 16% OR 56.02 ab 9.47 a 46.55 a 17% Atlantic Moist BA 34.34 a 9.27 a 25.07 a 27% PA 24.06 bc 3.76 b 20.30 ab 16% PI 15.50 c 2.50 b 13.00 b 16% RF 27.77 ab 5.65 ab 22.12 ab 20%

In general, POM-C concentration decreased exponentially with the increase in sand content (Figure 4.1). This relationship is known as the dilution effect, indicating that the more the sand is present in the soil, the more the mineral matter dilutes the SOC pool

(Amelung et al., 1998). Similar effects can be also seen on clay and silt fractions (Zinn et al., 2007).

78 40 y = 424.7e-0.006x 50 ) -0.003x d R² = 0.84 y = 52.8e n

a 40 R² = 0.62 s 30 1 - g

k 30

C 20 g ( 20

C y = -7.1ln(x) + 47.3 - 10 M R² = 0.83 10 O P 0 0 0 200 400 600 800 0 200 400 600 Sand (g kg-1 soil) Sand (g kg-1 soil) MA, CA, DF PAS 50 ) d n

a 40 s 1 - g

k 30 C g ( 20 C - y = 37.9e-0.001x M 10

O R² = 0.30 P 0 0 200 400 600 Sand (g kg-1 soil)

Figure 4.1. Dilution effect of C associated to POM in three different ecoregions: Pacific, Atlantic and Montane (in clockwise order) at 0 – 10 cm. The values from pasture and pineapple were excluded when computing the regressions for the Pacific and Atlantic ecoregions, respectively.

The soils used in this study have contrasting characteristics with regards to soil type, texture and C content. The silt plus clay content in soils ranged from 269 – 980 g kg-1 soil, while the silt plus clay carbon concentration (S+C– C) ranged from 2.6 – 102 g C kg-1 soil at 0 – 30 cm. Relationships between silt plus clay content and S+C– C were better explained when soils were divided into two groups: (1) soils with a silt + clay content of 200 - 700 g kg-1 soil and 0 – 100 g C kg-1 soil (contained most of the soils from

79 the three ecoregions); and (2) soils from PA, CF, PI and PAS. Soils from the second group were higher in silt + clay content (600 – 980 g kg-1 soil), but generally lower in C content (less than 60 g C kg-1 soil, Figure 4.2). For both groups, the relationship was explained better with exponential functions (Sparrow et al., 2006). Furthermore, relationships were not as linear as has been previously reported (Six et al., 2002; Hassink,

1997). It has been suggested that the constant C content of the silt plus clay fraction might be an artifact of the dispersion methodology and it is not related to a finite saturation capacity (Baldock and Skjemstad, 2000). Due to the characteristics of high

SOM content and andic properties in this study, soil dispersion was difficult to attain and could represent an important source of methodological error.

100 ) l i o

s Group 1 Group 2

1 80 - y = 1.7e0.0051x y = 0.811e0.0042x g k R² = 0.52 R² = 0.71 C g (

t 60 l i s + y a l

c 40 n i n o b r

a 20 C

0 200 400 600 800 1000 Clay + silt content (g kg-1 soil)

Figure 4.2. Relationship between the silt plus clay content of soils and the C associated to these particles in the ecoregions of this study.

80 The physical protection capacity models (Hassink, 1997; Elustondo et al., 1991) were applied to the soils in this study (Figure 4.3). The data show that most of the soils from the Montane ecoregions were above the capacity lines of the two models, along with several soils from the Pacific Dry ecoregions. However, as explained by the distribution in Figure 4.2, soils under more intensive management (or degraded soils) from the three ecoregions (PI, MA and PAS) were in general below the capacity protection lines.

Review of the literature indicates contradictory reports with regards to the effect of land use on the S+C-C (Six et al., 2002, Hassink, 1997). However, the results of this study indicate a consistent depletion of SOC in the POM and silt plus clay fractions affected by land use intensity (Table 4.2). It was also observed that the soils with higher proportion of sand (Group 1) are saturated and exceed the capacity factor, while the soils with heavier textures are generally below Hassink’s or Elustondo’s regression lines, as has been also reported in other temperate and subtropical soils (Sparrow et al., 2006; Carter et al., 2003). Most of the soils exceeded the regression line that Six et al. (2002) presented in their review of tropical soils.

The data presented support the conclusion that the soils in these ecoregions surpassed the protection capacity of the silt plus clay fraction, and C may either be adsorbed to micro and macroaggregates or incorporated into the POM fraction. However, the applicability of these models to these soils is still under discussion. While the model by Elustondo et al. (1990) and the fractionation methodology used in this study defined the division between sand – size fraction and the mineral fraction at 53 µm, the model by Hassink

(1997) used a size division of 20 µm. Nonetheless, a study on temperate grasslands found very similar pools under the 20 µm and 53 µm divisions (Amelung et al., 1998). Some of

81 the soils in this study exhibited clay plus silt contents over 70%, but very few soils with similar proportions were used in the construction of these models. Finally, the C contents in some soils in this study (i.e. Andisols) significantly exceeded those used to develop the models (< 30 g C kg-1 soil). Another assumption under Hassink’s model is that when the protective capacity of the soil is exceeded, C starts to accumulate only in the POM fraction (Stewart et al., 2008; Carter et al., 2003). In this study however, the proportion of

POM-C in reference to the TC was not significantly greater under those soils in which the protective capacity was already reached (e.g., BP was 184% over its estimated capacity, but POM was only 9% of the TC).

A similar deviation from Hassink’s model was presented in soils from Tasmania by

Sparrow et al. (2006), who concluded that soils may have an extra C capacity provided by the higher proportion of silt plus clay and higher SOC concentrations. Finally, in tropical soils, the type of clay seemingly plays an important role in C stabilization than the quantity itself (Lopez-Ulloa et al., 2005; Six et al., 2002). In soils with 1:1 clays, the

Al and Fe oxides are flocculants that may reduce the available surface for adsorption of

SOM (Six et al., 2002). On the other hand, allophane and Al-humus complexes in andic soils may significantly increase the capacity level, as was observed in the soils from the

Montane ecoregion. In previous studies conducted on tropical Inceptisols and Andisols

(Lopez-Ulloa et al., 2005; Powers and Schlesinger, 2002; Veldkamp, 1994) it has been shown that C stabilization was occurring due to allophane and Al-humus complexes in volcanic soils, while in the Inceptisols, the stabilization was more related to silt + clay content. However, after studying the effect of land use change on SOC, the C stabilized by Al-humus complexes in Andisols was more resistant to land use change (a larger

82 proportion of the C derived from the original forest remained in the soil after a period of

> 20 yrs) than the C stabilized by silt and clay in Inceptisols of Ecuador and Costa Rica

(Lopez-Ulloa et al., 2005; Veldkamp, 1994). Consequently, the physical protection capacity in silt + clay particles on tropical soils with contrasting mineralogy needs additional research.

100 ) l i 90 o s 1 - 80 g k

C 70 g (

y 60 a l

C 50 d n a 40 t l i S 30 n i n

o 20 b r a 10 C 0 200 300 400 500 600 700 800 900 1000 Silt and Clay content (g kg-1 soil)

Pacific Dry Montane Atlantic Moist

Figure 4.3. Relationship between the content of silt and clay and the carbon in the silt plus clay particles of three ecoregions in Costa Rica. The solid line represents Elustondo et al. (1990) capacity factor, the dashed line represents Hassink’s (1997) capacity model and the fine dot line represents Six et al. (2002) regression for several tropical and subtropical soils

The capacity level of the silt plus clay fraction was determined using Elustondo et al.

(1990) model, since the particle size break used in this study was in accordance to the one

83 used in the model (53 µm). The results are shown in Table 4.3, where it can be observed that the soils from BA, BP, CF, CO, OR and CA (Group 1 in Figure 4.2) had a higher C content than the capacity level, from 4 to 184% over the estimated capacity. On the other hand, the soils from PA, PI, RF, DF, MA and PAS contained SOC from 6 to 60% below the estimated capacity. Assuming the tested models reflect the real capacity of these soils, there remains a substantial potential for SOM storage. The trends in results presented here may be due to a combination of factors: land management (as was mentioned before), soil type (all land uses from the Montane ecoregion exceeded the capacity level) and total C content (BA and CA had the highest TC on their ecoregional basis).

Table 4.3. Capacity level of the silt plus clay fraction (g C kg-1 soil) up to 30 cm depth. Capacity estimated by: (g C kg-1 soil) = 9.04 + 0.27(% silt + clay), based on Elustondo et al. (1990).

Atlantic Moist Montane Pacific Dry BA PA PI RF BP CF CO OR CA DF MA PAS Silt + Clay C (g C kg-1 soil) 25.1 20.3 13.0 22.1 71.5 42.4 51.3 46.6 26.2 24.1 17.4 8.6 Capacity level 24.1 30.3 27.2 29.6 25.1 34.4 25.1 25.1 23.1 25.4 22.5 21.5

According to the model by Hassink (1997), once the capacity level of clay plus silt is filled, the additional C is stored in the POM fraction (Carter et al., 2003; Carter, 2002). In a study that compared the effect of crop residues additions under different types of management in Brazilian soils, Sá et al. (2001), reported a significant C increase in no till soils over naturally vegetated soils. However, a greater percentage of the C derived from crop residues was accumulating in the POM fraction than in the silt plus clay fraction.

Under this scenario, the maximum physical protection capacity for SOM is ultimately

84 determined by the maximum microaggregation that may protect the POM-C that otherwise will be accumulating in the unprotected C pool (Six et al., 2002).

Aggregate associated carbon The SOC concentration was determined in the WSA, which were separated according to their size into macroaggregates (4,750 – 250 µm) and microaggregates (<250 µm, Table

4.4). In general, the SOC concentration in aggregates decreased with increase in depth for all land uses. In the Pacific ecoregions, WSA-C was significantly more in the DF and CA than other land uses. In the Montane ecoregion, there were no significant differences among land uses for the 0 – 10 cm layer, but BP contained significantly more SOC in the

10 – 20 cm depth and OR in 20 – 30 cm depth than other land uses. In the Atlantic Moist ecoregions, SOC in PI was significantly lower than that in other land uses for all depths

(P <0.005). Carbon concentration in different aggregate fractions differed significantly level (a = 0.05) among land uses only in few cases (DF, PAS, CO and PI) for specific depths. Although there was a tendency of higher SOC concentration in the macroaggregate fraction, following the aggregate hierarchy theory (Six et al., 2002); soils with 1:1 clays and andic characteristics generally do not exhibit this pattern (Zech et al.,

1997; Six et al., 2000; Hoyos and Comerford, 2005). Spaccini et al., (2004), found an even distribution of C concentration among macro and microaggregates in a well structured Inceptisol of Nigeria. Similarly, Jimenez et al. (2008, 2007) found no significant differences in SOC concentration among aggregate sizes in studies conducted in soils from secondary forests and on tropical timber plantations located on EARTH

University in the Atlantic ecoregion. However, since macroaggregates are comprised of

85 POM (and other labile fractions) and microaggregates, only the C contained in the microaggregates is generally considered to be protected (Carter et al., 2003).

Table 4.4. Carbon concentration in the macro (4,750 – 250 µm) and microaggregates (<250 µm) of different land uses in three ecoregions of Costa Rica. Land uses with same letters are not statistically significant (a=0.05). The asterisks indicate significant differences between aggregate sizes within each land use.

Carbon concentration (g kg-1) 0 - 10 cm 10 - 20 cm 20 - 30 cm Pacific Dry DF MA PAS CA DF MA PAS CA DF MA PAS CA 4,750 - 250 µm 43.9 a* 28.5 b 19.4 b 46.0 a 30.5 b* 16.8 c 11.3 c* 41.8 a 20.6 ab 10.5 bc 4.2 c 29.7 a < 250 µm 29.8 b 24.3 bc 16.1 c 39.7 a 23.4 b 15.4 bc 7.0 c 37.7 a 17.7 ab 9.0 bc 3.2 c 27.9 a Montane BP CF CO OR BP CF CO OR BP CF CO OR 4,750 - 250 µm 99.2 a 52.5 a 44.6 a 73.2 a 84.0 a 44.3 b 43.7 b* 34.4 b 25.8 b 33.0 ab 43.7 ab* 54.0 a < 250 µm 84.2 a 56.2 a 46.1 a 55.7 a 69.3 a 42.9 b 39.6 b 35.2 b 52.8 a 50.9 a 40.6 a 53.9 a Atlantic Humid BA PA RF PI BA PA RF PI BA PA RF PI 4,750 - 250 µm 39.7 a 32.1 a 35.7 a 12.3 b 37.3 a 24.3 ab 24.9 ab 15.7 b* 33.0 a 20.1 a 19.8 a 16.2 a < 250 µm 31.2 a 27.5 ab 31.0 a 12.3 b 33.9 a 24.2 ab 22.7 ab 12.5 b 36.6 a 20.1 b 22.0 b 14.9 b

There was a relationship between WSA-C and the percentage of water stable aggregates in the soil (Figure 4.4). However, this relationship was not as strong as the one found in other soils of the tropics and temperate regions (Six et al., 2002), specially in the

Montane ecoregion, where other factors such as clay mineralogy may play a major role on aggregation. Six et al. (2002) indicated that the correlation among WSA and SOC may be less significant than in temperate soils since other agents of aggregate stabilization

(e.g. sesquioxides, allophane) may predominate (Six et al., 2000; Oades and Waters,

1991). For example, Fe and Al oxides have been identified as important means of organic matter stabilization in other Inceptisols of the Atlantic ecoregion (Power and Schlesinger,

2002; Veldkamp, 1994).

86 100 100 ) 90 % 90 ( y

t 80 i l

i 80 b

a 70 t

S 70

e 60 t a

g 60 e 50 r y = 8.2ln(x) + 43.7 g g 50 y = 14.0ln(x) + 37.6 40 R² = 0.07 A R² = 0.37 40 30 0 20 40 60 0 50 100 150 Aggregate Carbon (g C kg-1) Aggregate Carbon (g C kg-1)

100 ) % ( y t i l i b a t

S 90 e t a g

e y = 4.2ln(x) + 84.6 r

g R² = 0.34 g A 80 0 20 40 60 Aggregate Carbon (g C kg-1)

Figure 4.4. Relationship for aggregate C and water stable aggregates (WSA) in the Pacific, Montane and Atlantic ecoregions (in clockwise order).

Stewart et al. (2008) stated that soils which exhibit a linear relationship between TC and

C associated to all or some soil fractions may not be characterized by C saturation. In contrast, soils which exhibit an exponential relationship between TC and C associated to all or some soil fractions are in fact demonstrating a finite behavior of C saturation. The data in Figure 4.5 show a linear relationship between total C concentration in the soil and

87 the concentration of C associated to aggregates and POM. These mathematical relationships were stronger for the WSA-C than for the POM-C, but in general are all of the linear type, providing evidence that these soils may not be influenced by C saturation in any of these fractions. Stewart et al. (2008) also reported that the unprotected C (POM-

C) had a better fit under the linear model. Inference on populations based on these data must be interpreted with caution, since smaller sections of an asymptotic curve can appear linear when the range being observed is small (Stewart et al., 2008). Thus, Carter et al. (2003) also reported that the WSA-C content was proportional to the TC in

Canadian soils, but the finite capacity model was appropriate to the characteristics of their soils as well.

160 WSA (macro) C = 0.75x + 7.2 WSA (micro) C = 0.66x + 8.0 140 R² = 0.64 R² = 0.56 ) n

o 120 i t c

a 100 r f 1 - 80 g k

C 60 g ( 40 n

o POM C = 0.29x + 6.1 b

r 20 R² = 0.38 a

C 0 0 20 40 60 80 100 120 140 160 Total Soil Carbon (g kg-1)

POM-C WSA (macro)-C WSA (micro)-C Figure 4.5. Relationship between soil organic carbon in the bulk soil and the carbon associated to the aggregate and particulate organic matter fractions in 12 land uses of Costa Rica.

88 The C:N ratios for TC and the different size fractions are shown in Figure 4.6. The N concentration followed a pattern similar to that of SOC (Adesodun et al., 2007), and also tended to decrease with increase in soil depth. The POM presented higher C:N ratios than the aggregate particles, since there is a greater microbial alteration of SOM in finer fractions (Amelung et al. 1998; Christensen, 1992). Generally, uncultivated soils have higher C and N concentrations than cultivated soils (Adesodun et al., 2007; Sá et al.,

2001). However, (Trujillo et al., 1997) reported no significant effect of land use on C:N ratios of the whole soil. In this study, statistical significance was observed only in the aggregate fractions in soils from the Pacific ecoregion, in the POM and aggregate fractions in the Montane ecoregion and in the POM fraction of the Atlantic region. While in the Pacific and Montane ecoregions the mature forests had narrower C:N ratios than cultivated soils, in the Atlantic ecoregion the mature forest was lower than the agricultural sites.

40 20 20 35 30 15 15

o 25 i t

a 20 10 10 R

N 15 C 10 5 5 5 0 0 0 > 250 µm < 250 µm POM Whole > 250 µm < 250 µm POM Whole > 250 µm < 250 µm POM Whole Soil Soil Soil DF CA MA PAS BP CF CO OR PI PA BA RF

Figure 4.6. C:N rations of the macro (>250µm) and microaggregates (>250µm), particulate organic matter (POM) and bulk soil in three different ecoregions: Pacific, Montane and Atlantic (in order from left to right)

89 The enrichment factor (E) indicates the degree of accrual or depletion of C in a certain soil particles (Amelung et al., 1998). Soil fractions with E >1 indicate enrichment of C, while E < 1 indicate depletion with respect to the C in the whole soil. Enrichment factors for the soils in this study are shown in Table 4.5. The E values tended to decrease as the particle size content in the soil increased (Zinn et al., 2007; Amelung et al., 1998). While some soils indicated enrichment in at least some of the fractions, the DF, MA, CO and PI were in general depleted. Soils from other land uses also showed depletion or neutrality

(E=1) in some of their fractions, which was the case of PAS, BP, BA, RF and PA. Only

CA and CF presented a higher accrual in the aggregate fractions. CF was the only treatment to exhibit a C accrual in the POM fraction. Higher enrichment factors in the

POM were also inversely proportional to the sand size content of the soil, as was explained by the dilution effect in Figure 4.1.

Although Amelung et al. (1998) reported an inverse relationship between the E of POM and mean annual temperature, in this study there was no apparent relationship between these variables, possibly due to confounding effects of soil type and texture.

90 Table 4.5. Enrichment factors (E) of C in different fractions of the soil of different land uses at 0 – 10 cm depth. No significant differences were reported among land uses (Tukey’s HSD, a =0.05)

Enrichment factor Pacific Dry CA PAS DF MA > 250 µm 3.0 1.0 0.9 0.8 < 250 µm 2.5 0.9 0.6 0.7 POM 0.9 0.2 0.6 0.4 Silt + Clay 0.7 0.9 0.7 0.8 Montane CF OR BP CO > 250 µm 4.5 1.2 1.0 0.9 < 250 µm 5.0 1.0 0.9 0.9 POM 2.8 0.5 0.2 0.5 Silt + Clay 0.9 0.8 0.9 0.8 Atlantic Humid BA RF PA PI > 250 µm 1.0 1.0 1.0 0.9 < 250 µm 0.8 0.9 0.8 0.9 POM 0.5 1.0 0.6 0.5 Silt + Clay 0.8 0.8 0.8 0.8

Aggregate physical properties

Commonly used indices of measuring the stability of aggregates against disruptive forces include WSA, MWD, TS and WDPT. Thus, the results from wet sieving were computed to obtain values of WSA and MWD. The proportion of the largest aggregate size (4.75 mm) tended to decrease with depth (Koutika et al., 1997). The WSA was significantly affected by land use in all the ecoregions studied (Figure 4.7). In the Atlantic ecoregion,

PI had significantly lower WSA in the 4.75 mm size, and higher in the = 2.0 mm fraction.

In the Pacific ecoregions, soil under CA exhibited a lower macroaggregation than those under other land uses. In the Montane ecoregions, results were more dissimilar and

91 significant differences were only observed at the 1 mm size. Aggregate stability may be negatively affected by machinery disturbance (Blanco–Canqui and Lal, 2004) and positively influenced by vegetation that protects soil against high intensity rainfalls (Beer et al., 1998) and increases organic matter (Atsivor et al., 2001). Further, tillage also affected the stability of aggregates in PI and CA, since PI is a labor intensive cash crop and CA is a recently renovated plantation. When WSA data were pooled into micro and macroaggregates, the proportion of the latter fraction was higher in soils from all land uses and depths. The results presented herein are similar to those of Rodriguez et al.

(2002) who also reported higher stability of macro than microaggregates in Andisols of the Canary Islands. There were no significant differences in macrooagregate proportion among land uses, as was also reported by Wood (1977) for some Oxisols of Hawai’i. In the Pacific, the proportion of macro WSA ranged from 77.4 – 97.4 % in CA and MA, respectively. In the Montane ecoregion, macro WSA ranged from 74.8 - 85.5% (CO and

CF, respectively). The proportion of macroaggregates was highest in the Atlantic ecoregion (95 – 99% in PI and BA, respectively), in accordance to Kemper and Koch

(1966) who indicated that soils from the humid regions tend to have high aggregate stabilities. It was only in this last ecoregion that the WSA was significantly correlated with TC (r = 0.65, P <0.05 at 0 – 10 cm, data not shown).

92 140 140 120 120 ) 100 (a) ) 100 (b) % % ( 80 ( 80 A A

S 60 S 60 W 40 W 40 20 20 0 0 0.1 0.2 0.5 1.0 2.0 4.75 0.1 0.2 0.5 1.0 2.0 4.75 Aggregate Size (mm) Aggregate Size (mm) PI BA PA RF CA MA PAS DF 80 (c) 60 ) % (

A 40 S

W 20

0 0.1 0.2 0.5 1 2 4.75 Aggregate Size (mm) BP CF CO OR

Figure 4.7. Distribution of Water Stable Aggregates (WSA) at 0 – 10 cm depth in different land uses of the (a) Atlantic, (b) Pacific and (c) Montane ecoregions. Horizontal bars indicate the Tukey’s HSD when P < 0.05.

The data on MWD are shown in Table 4.6. Aggregate diameter tended to decrease with depth and was significantly impacted by land use (Wood, 1977). Spaccini et al. (2004) reported a higher MWD for forest soils than cultivated soils in a Nigerian Inceptisol. In the present study, there were no differences among soils under forests and some of the agricultural sites. In the Pacific ecoregions, the MWD ranged from 1.1 mm (PAS, 20 - 30 cm) to 4.8 mm (MA, 0 – 10 cm). In the Montane ecoregion, MWD ranged from 1.0 – 4.2 mm (for CF at 20 – 30 cm and BP at 0 – 10 cm, respectively). The MWD was larger in soils of Atlantic ecoregion (2.8 – 5.4 mm) than those of other ecoregions.

93

Table 4.6. Mean Weight Diameter of the water stable aggregates in different land uses of Costa Rica. Land uses with same letters are not statistically significant (a=0.05).

Mean Weight Diameter (mm) 0 - 10 cm 10 - 20 cm 20 - 30 cm Pacific Dry MA DF Pas CA MA DF Pas CA MA DF Pas CA 4.8 a 4.6 a 3.8 ab 2.6 b 3.7 a 3.8 a 2.4 a 3.1 a 1.5 ab 2.8 a 1.1 b 2.4 ab Atlantic Humid RF PA BA PI RF PA BA PI RF PA BA PI 5.4 a 5.4 a 5.3 a 3.9 b 5.3 a 5.3 a 5.2 a 2.8 b 5.4 a 5.1 a 5.1 a 3.3 b Montane BP OR CF CO BP OR CF CO BP OR CF CO 4.2 a 3.5 a 3.3 a 3.1 a 3.6 a 3.3 a 1.4 b 2.5 ab 3.3 a 2.8 a 1.0 b 2.8 a

The water drop penetration time was determined for individual aggregates of 4.75 – 8.0 mm. The data in Figure 4.8 presents the frequency distribution of WDPT for these individual aggregates. Aggregates from soils of the Montane ecoregion showed a larger

WDPT distribution and higher hydrophobicity, than those from other ecoregions. Over

90% of the aggregates from soils under CA and MA in the Pacific ecoregion had WDPT lower than 1 second, while from those under DF and PAS it was 75% and 60%, respectively. Similar WDPT distributions were observed in the soils from the Montane ecoregions for OR, CO and CF land uses, but for the BP, only 50% of the aggregates had time of < 1 second, and for the rest it ranged between 1 and 10 seconds. For the soils from the Atlantic ecoregion, those under PA and RF land uses had over 90 % aggregates with WDPT lower than 1 second, but PI showed more hydrophobic aggregates at 1 (50%) and 2 seconds (10%).

94

100% 100% Montane 90% Pacific 90%

s 80%

s 80% e e t

t 70%

70% a a g

g 60%

60% e e r

r 50% 50% g g g g 40% 40% A A 30% 30% % % 20% 20% 10% 10% 0% 0% 0 1 2 3 4 5 6 7 8 0 1 5 10 20 30 40 50 60 70 80 WDPT (s) WDPT (s) MA CA DF PAS BP CF CO OR 100% 90% Atlantic 80% 70%

s 60% e

t 50% a

g 40% e r 30% g

g 20% A 10%

% 0% 0 1 2 3 4 5 6 7 8

WDPT (s) BA PA PI RF

Figure 4.8. Frequency distribution of water drop penetration time (WDPT) among aggregates of different land uses in three ecoregions of Costa Rica.

The data in Figure 4.9 shows that WDPT tended to decrease with increase in depth for soils from all ecoregions and was significantly affected by land use (Piccolo and

Mbagwu, 1999). All land uses classified as non-water repellent according to Dekker and

Ritsema (2003), with the exception of BP for 0 – 10 cm depth which was classified as slightly repellent. Water repellency enhances SOC stabilization through the formation and protection of stable aggregates due to a reduction in liquid absorption rates, and also because hydrophobic organic matter itself is more resistant to microbial decomposition

95 (Goebel et al., 2005; Chenu et al., 2000). While the soils under mature forests tended to be more hydrophobic in the Pacific and Montane ecoregions, for those in the Atlantic ecoregion the WDPT was significantly greater in the soil under PI than other land uses.

There were contrasting relationships between WDPT and SOC according to the ecoregion. Water drop penetration test showed a positive relationship (r = 0.61, P< 0.05, not shown) with TC in the Montane ecoregion. Similar trends were reported in soils from the Chilean Hapludands (Ellies et al., 2005). In contrast, the soils from the Pacific ecoregion did not seem to show any particular relationship with any other variable. Soils from the Atlantic ecoregion showed an opposite trend: it correlated negatively with TC (r

= -0.63, P< 0.05). The reasons for these contrasting results might be related to the chemical composition of organic matter, which may have differed among climatic, soil and vegetation patterns (Jasinska, 2006). For example, in the Atlantic ecoregion, soils under pineapple had significantly higher WDPT and the lowest TC, but the organic residues are also extremely hydrophobic due to the wax that coats pineapple leaves.

96 WDPT (s) 0 0.2 0.4 0.6 0.8 1 1.2 1.4

AB A Pacific 10 cm B B h t

p A e B

D 20 cm B l

i B o

S A AB 30 cm B B

DF PAS MA CA

0 0.5 1 1.5 2

10 cm Atlantic h t

p A e B D 20 cm B l i B o S A B 30 cm B B

PI PA BA RF

0 2 4 6 8 10

A B 10 cm B B Montane h t

p A

e AB

D 20 cm B l

i B o S

30 cm

BP CF CO OR

Figure 4.9. Water drop penetration time (WDPT) of different land uses of Costa Rica. Land uses with same letters are not statistically significant (a=0.05, no letters are shown when differences are not significant).

97 Data on aggregate tensile strength was log-transformed due to the variability of results

(Figure 4.10). The TS tended to decrease with increase in soil depth (Blanco-Canqui and

Lal, 2004; Perfect et al., 1995). For soils from the Pacific ecoregion, the median TS of

DF was 10 times greater than the median TS of CA (since the distribution was approximately normal, the median values are a good approximation to the mean). For soils from the Montane ecoregion, log of TS was significantly different for OR and CO land uses for 0 – 10 cm depth and between BP and CO for 10 – 20 cm depth , but no significant differences was observed for the deepest layer (20 – 30 cm). In the Atlantic ecoregion, log TS was significantly more in the PA and PI land uses and the lowest in BA for all depths.

The data of TS did not show any strong relationship with soil moisture content or TC when pooled data from all land uses and soil depths were used to compute correlations, but it exhibited a slight relationship with clay content (r = 0.27, P < 0.01, not shown).

However, when data were analyzed separately for each ecoregion, the results were contrasting among ecoregions: there were strong positive correlations between TC (r =

0.61, P < 0.05, data not shown) and TS in the Montane ecoregion, but no relationship between TS and clay or moisture content. In the Pacific Dry ecoregion, TS was not affected by TC, but it correlated with clay content (r = 0.60, P< 0.05, data not shown) and field moisture content (r = -0.73, P< 0.01). In the Atlantic ecoregion, TS did not have strong relationships with any variable. Positive correlations among SOC and TS have been reported for Spanish Inceptisols by Barral et al. (1998), Brazilian Oxisols by Imhoff et al. (2002), and other soils (Causarano 1993). In contrast, several other studies have reported that TS decreased with SOC (Blanco–Canqui and Lal, 2004; Watts and Dexter,

98 1998); while others have reported no effects of SOC (Blanco–Canqui et al., 2005; Rahimi et al., 2000; Guerif, 1988). Consequently, whether the correlation (if any) among SOC, texture and moisture and TS is positive or negative may be site specific.

Soil moisture characteristic curves (SMCC) for 0 – 30 cm depth are shown in

Figure 4.11. The SMCC shows differences in the air entry point between land uses and a greater effect of land use on MR at lower suctions (Blanco-Canqui et al., 2005; Subbian et al., 2000). The MR was significantly affected by land use in all ecoregions and depths

(only the Atlantic ecoregion at 10 – 20 cm did not exhibit differences). In the Pacific ecoregion, MR was higher in the MA and CA and lowest in PAS. Soils under DF had intermediate values of MR. In soils of the Montane ecoregion, MR was significantly higher in BP for 0 – 10 and 20 – 30 cm depth, but for 10 – 20 cm, MR was higher in soils under BP and OR. Less significant differences were observed in MR for soils of the

Atlantic ecoregion. Soils under greater disturbance tended to have lower MR, especially at lower suctions, which may indicate a combined effect of soil management and low TC

(Blanco-Canqui et al., 2005). High moisture retention at high tensions is characteristic in soils with high clay content and also in soils of andic nature. Andisols present unique water retention characteristics, since they hold more water at lower tensions than other soils due to their high porosity/low bulk density and the size of water stable aggregates.

In study, the soils from the Montane ecoregion retained from 40 – 60 % of water (on mass basis) at 15 bar. High moisture retention (on weight basis) has been previously reported in Andisols of the Montane ecoregion in Costa Rica by several authors

(Cooperband and Logan, 1993; González and Gavande, 1969; Gavande, 1968).

99 When correlations were computed for all land uses and depths combined, there were strong relationships among moisture retention and TC, S+C-C and POM-C (Table 4.7) but the correlations were stronger for 3 bar than 15 bar, as has been previously reported in the literature (Kay et al., 1997). Soil organic carbon affects soil moisture characteristics and in return, moisture retention also influences SOC dynamics

(Franzluebbers, 2002). For example, higher productions of CO2 may occur at 0.33 bar in volcanic soils of Costa Rica (Blasco, 1971).

The available water capacity (AWC) correlated more strongly with MR at 3 bar than MR at 15 bar, indicating that this property was more related to field capacity, but also to silt plus clay content (r = 0.32, P<0.01)(Kay et al., 1997). Nonetheless, the AWC was not significantly different among soils from different land uses (Subbian et al, 2000).

When data were analyzed on an ecoregional basis, soils from both the Pacific and

Atlantic ecoregions showed a strong, positive relationship between MR at 3 bar with clay content (r = 0.62 and 0.74 in the Pacific and Atlantic ecoregions respectively, P < 0.01), but no effect of texture was observed for soils from the Montane ecoregion.

Simple correlations among the different C fractions and other aggregate and physical variables are shown in Table 4.7. The POM-C may (Needelman et al., 1999) or may not increase with increase in silt + clay content (Plante et al., 2006). In this study, there was a significant, but low correlation among POM-C and silt + clay contents (r = 0.27, P <

0.01). A substantial part of the POM may be physically protected within stable aggregates (Six et al, 2002). In this study, there was a correlation (r = 0.59, P < 0.01) between POM-C and WSA-C in both micro and the macroaggregate fractions. The C in the microaggregates is also considered to approximate the C associated to the silt and

100 clay (Carter et al., 2003). In this study, the S+C-C had a positive correlation with WSA-C in both micro and the macro size fractions (r = 0.74 and 0.78, P < 0.01). There was consistency on the proportions of C protective fractions among land uses; thus, land uses with higher TC, were also higher in WSA-C and S+C-C. Total C was positively correlated to the C in all other fractions, and was significantly affected by bulk density.

The C:N ratio in both the POM and WSA did not exhibit any strong correlations with any of the other variables. In contrast to results from some Canadian agricultural soils (Carter et al., 2003), WSA was not correlated with silt or clay content, which may indicate other stronger mechanisms of bonding between soil particles. Similarly, WSA was correlated with TC only in the soils of the Atlantic ecoregion (r = 0.65, P < 0.05, not shown).

There is a positive effect of POM-C on mechanical properties of bulk soil, as has been observed by other authors (Carter, 2002). In this case, only MR showed a relationship.

The MR had positive and strong correlations at both 3 bar and 15 bar with POM-C, TC,

S+C – C and WSA-C. Almost all aggregate and bulk soil variables correlated negatively with bulk density (some correlations on ecoregional basis are not shown in Table 4.7).

Total C had a negative correlation with bulk density (R = 0.74, P < 0.01), which might be an indirect indication of the positive effect of SOM on soil structure.

101

Tensile strength (log kPa) 0 1 2 3 4 5 6

A AB Pacific 10 cm BC C h t

p A e B

D 20 cm B l

i B o

S A A 30 cm B B

DF PAS MA CA

Tensile strength (log kPa) 0 1 2 3 4 5 6

A AB Montane 10 cm AB B h t

p AB e A

D 20 cm AB l

i B o

S A A 30 cm A A

OR BP CF CO

Tensile Strength (log kPa) 0 1 2 3 4 5 6 7

A AB Atlantic 10 cm B B h t

p AB e A

D 20 cm AB l

i B o

S AB A 30 cm B C

PA PI RF BA

Figure 4.10. Tensile strength of different land uses of Costa Rica. Land uses with same letters are not statistically significant (a = 0.05).

102

0.7 1.2 ) 0.9 DF MA CA PAS 1

- (a)

g 0.8 0.6 1.0 k 0.7 g 0.5 k

( 0.6 0.8 t

n 0.4 0.5 e 0.6 t

n 0.3 0.4 o 0.4

C 0.3 0.2 e r 0.2 0.2 u

t 0.1

s 0.1 i

o 0.0 0.0 0.0 M 0 1 2 3 0 1 2 3 0.0 1.0 2.0 3.0 Moisture Retention Potential Moisture Retention Potential Moisture Retention Potential (log kPa) (log kPa) (log kPa) 1.6 ) 1.6 1 1.4 BP CO OR CF -

g 1.4

k 1.2 1.4 (b)

g 1.2

k 1.2 ( 1.0 t 1.0

n 1.0 e 0.8 t 0.8 1 n 0.8 0 o 0.6 0.6 4 C 0.6 e

r 0.4 0.4

u 0.4 t

s 0.2 i 0.2

o 0.2 0.0 M 0.0 0.0 0 1 2 3 0 1 2 3 0 1 2 3 Moisture Retention Potential (log kPa) Moisture Retention Potential (log kPa) Moisture Retention Potential (log kPa)

) 0.7 1 - 0.9 BA RF PI PA 0.8 g 0.6 k 0.8 0.7 (c) g

k 0.5 0.7 0.6 (

t 0.6 n 0.4 0.5 e

t 0.5

n 0.3 0.4 o 0.4

C 0.3 0.3

e 0.2 r 0.2 0.2 u

t 0.1 s 0.1 i 0.1 o 0.0 0.0 0.0 M 0 1 2 3 0 1 2 3 0 1 2 3 Moisture Retention Potential (log kPa) Moisture Retention Potential (log kPa) Moisture Retention Potential (log kPa)

Figure 4.11. Moisture retention the (a) Pacific, (b) Montane and (c) Atlantic ecoregions at 0 – 10, 10 – 20 and 20 – 30 cm (in order from left to right). Horizontal bars indicate the Tukey’s HSD when P<0.05.

103

Table 4.7. Correlations among carbon in the whole soil (TC) and different fractions and the physical properties for 12 land uses of Costa Rica

2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 1. Sand content -0.99 -0.29 0.28 -0.26 -0.19* -0.16** 0.12 -0.19* 0.13 0.00 0.00 0.15 -0.20* -0.04 -0.05 -0.09 -0.01 -0.02 2. Silt plus clay content 0.27 -0.26 0.26 0.18** 0.15** -0.11 0.19* -0.12 0.01 0.02 -0.13 0.21* 0.05 0.04 0.07 0.00 0.01 3. POM-C -0.08 0.52 0.62 0.59 -0.08 0.55 -0.10 0.10 0.10 -0.56 -0.06 0.05 0.32 0.45 0.12 0.30 4. POM CN -0.2* -0.18** -0.16 0.11 -0.25 0.17** 0.02 0.10 0.27 0.20* 0.02 -0.20* -0.23* -0.12 -0.21* 5. Silt+clay - C 0.99 0.78 0.09 0.74 -0.07 -0.08 -0.03 -0.70 -0.02 0.37 0.61 0.74 0.32 0.67 6. Total Carbon 0.80 0.09 0.75 -0.06 -0.03 0.00 -0.74 -0.04 0.35 0.61 0.76 0.32 0.67 7. WSA - C (>250 µm) 0.12 0.84 0.01 -0.03 0.01 -0.63 -0.08 0.12 0.44 0.73 0.09 0.61 8. WSA - CN (>250 µm) 0.08 0.75 0.19* -0.29 0.23* -0.17 0.09 0.00 -0.03 0.02 -0.06

1 9. WSA - C (<250 µm) 0.00 -0.15 -0.09 -0.64 -0.11 0.07 0.48 0.72 0.16 0.58 0 5 10. WSA - CN (<250 µm) 0.00 -0.09 0.30 -0.13 -0.05 -0.22* -0.24* -0.14 -0.25 11. WSA 0.89 -0.12 0.45 0.10 -0.14 -0.10 -0.13 -0.06 12. MWD -0.13 0.42 0.09 -0.14 -0.08 -0.14 -0.01 13. Bulk Density 0.03 -0.18** -0.63 -0.79 -0.31 -0.75 14. log Tensile Strength 0.27 -0.14 -0.12 -0.12 -0.14 15. Water Drop Penetration Time 0.22* 0.21* 0.16 0.15 16. Moisture retention (3 bar) 0.73 0.87 0.61 17. Moisture retention (15 bar) 0.31 0.78 18. Available Water Capacity 0.30 19. Field Moisture content - Correlations in bold are significant at P < 0.01, * indicates significance at P < 0.05, ** indicates significance at P < 0.1

104 Conclusions

Results presented support the following conclusions:

1. The POM is significantly lower in soils of the land uses under more intensive

management in the Pacific and Atlantic ecoregions. Therefore, POM may be a good

indicator of the effect of soil disturbance on SOC dynamics.

2. Under the assumption that the current capacity models are adaptable to the soils of Costa

Rica in this study, there was a physical protection capacity of 6 – 60% in the silt plus clay

fraction of some of the land uses studied. Generally, the land uses with soils of heavier

textural classes had a larger C storage capacity than those with lighter texture, which had

already attained their estimated capacity. Soils which significantly deviated from their

capacity level, indicated that given the higher C contents and proportion of silt plus clay

in these soils, the models used here are not reflective of the real attainable C storage

capacity, especially under andic conditions.

3. There was no evidence that the aggregates from these soils follow an aggregate hierarchy

model. Also, the relationship between WSA and WSA-C was less apparent in these soils

than the usually strong linear regression reported for soils from temperate regions. These

findings may reflect the influence of other agents of aggregate formation and

stabilization. Thus, further research is needed to characterize the role of clay mineralogy

on the stability and protection capacity of these tropical soils.

4. The magnitude and direction of the effect of SOC on tensile strength, water drop

penetration time and moisture retention are specific to soils in the specific ecoregions.

105 References

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112 5. SUMMARY AND RECOMMENDATIONS FOR FUTURE RESEARCH

A field study was designed in order to estimate the SOC pool in soils of Costa Rica and to assess their C sink capacity to offset anthropogenic emissions by different management scenarios. The specific objectives were (i) to determine the effects of land use on SOC pool, (ii) to determine the role of climatic parameters on SOC pool (Chapter 3), and (iii) to determine the physical protection capacity of C associated to the mineral fraction of soils (Chapter 4). In addition, other aspects addressed in this study were the review of literature on SOM in Central America, the estimation of the SOC sequestration potential of Costa Rica under different land use scenarios, and the relationships among aggregate mechanical properties and SOC.

Based on the results presented in Chapter 3, it was estimated that the SOC pool (0 – 100 cm) was of 114 – 150 Mg C ha-1 in the Atlantic Moist ecoregion, 76 – 165 Mg C ha-1 in the Pacific Dry ecoregion and 166 – 246 Mg C ha-1 in the Montane ecoregion of Costa

Rica. The C sink capacity was of 18.1 - 36.7 Mg C ha-1, 14.1 – 88.6 Mg C ha-1 and 9.4 –

80.7 Mg C ha-1 in the Atlantic, Pacific and Montane ecoregions, respectively. The country’s total C pool is estimated at 823 – 872 Tg C, with a potential for SOC sequestration of 826 – 2,251 Gg C per year.

In Chapter 4, the results provided evidence – under the assumption that the current capacity models are adaptable to the soils in this study – that there was a physical protection capacity of 6 to 60% in the silt plus clay fraction of some of the land uses

113 studied. Generally, the land uses with soils of heavier textural classes had a larger C storage capacity than those with lighter texture, which had already attained their estimated capacity. Soils which significantly deviated from their capacity level, indicated that given the higher C contents and proportion of silt plus clay in these soils, the models used here are not reflective of the real attainable C storage capacity, especially under andic conditions.

Thus, the SOC pool and C sink capacity for the soils in this study were discussed in

Chapter 3. However, it was also discussed the importance of determining what portion of the total SOC pool is securely protected by its association to the mineral fraction (silt and clay particles).

In summary, the total pool and C sink capacities for the total SOC content as well as the

C associated with silt + clay was estimated as follows:

Total Soil Organic Carbona Silt + Clay Carbona Pool Sink Capacityb Pool C sink capacityc Mg C ha-1 BA PA PI RF BA PA PI RF BA PA PI RF BA PA PI RF 72.1 59.0 40.3 59.2 - 0.19 18.82 - 53.4 44.5 33.1 47.8 - 21.9 36.2 16.2 BP CF CO OR CF CO OR BP BP CF CO OR BP CF CO OR 95.0 101.0 78.7 87.8 - 16.32 7.16 - 81.5 100.5 75.4 68.4 - - - - CA DF MA PAS CA MA PAS DF CA DF MA PAS CA DF MA PAS 87.6 94.5 55.9 33.9 6.8 38.5 60.5 - 62.9 69.3 44.4 29.2 - 3.8 13.0 43.7 aEstimated by the average bulk density at 0 - 30 cm depth and average SOC concentration at 0 - 30 cm bBased on the model by Elustondo et al., (1990). The soils with blank spaces are those that exceed the estimated capacity (including all soils from the Montane ecoregion) c In relation to the mature forest

114

It was observed that soils that had a low sink capacity in comparison to the naturally vegetated soils still had the capacity to retain more SOC in their mineral fraction (i.e. PA,

PI, and the three forests used as reference). Soils under natural vegetation reflect the natural balance between C inputs and outputs, and serve as a reference for agricultural soils towards what is physically attainable under the common conditions of soil type and climate. However, SOC stocks under natural vegetation are do not necessarily represent an upper limit in SOC pool. Several studies have observed higher SOC pools for managed soils over naturally vegetated due to increased inputs in organic matter under tropical conditions, which may indicate the ultimate C sink capacity is determined by the protective capacity in the silt and clay fraction. Thus, the C sink capacity of the soils in this study varies from 4 to 44 Mg C ha-1.

Finally, as suggested in Chapter 4, a relevant research question is to determine specific models of physical protection capacity for tropical soils, according to their clay mineralogy. Other research questions that need to be addressed and prioritized in further studies on SOC in the region include:

Effect of SOM in the adsorption capacity of pesticides in tropical cash crop soils:

In Costa Rica, it is estimated that 49.3 kg a.i.ha-1 of pesticides are applied every

year in pineapple fields (Bravo Durán et al., 2007). Put in perspective, coffee

plantations receive on average 6.5 kg a.i.ha-1 yr-1, sugar cane 10 kg a.i.ha-1 yr-1,

and only melons exceed pineapple with 257 kg a.i.ha-1 yr-1 (De la Cruz et al.,

2004). Therefore, it is necessary to assess the effect of fertilizer and pesticide use

on the SOM dynamics of these tropical cash crops, as well as the role that higher

additions of organic residues may have on the adsorption capacity of pesticides.

115

This research is opportune not only for C sequestration but also for human health

and food security.

Effect of soil erosion on SOC dynamics and SOC relocation

In Costa Rica, coffee is grown in hillsides with over 60 % inclination. About 20% of the country’s area is estimated to be severely eroded, and 47% of the soils nationwide were highly prone to degradation (Bach, 2007). Whether soil erosion is a source or a sink of C emissions to the atmosphere is still under debate (Van Oost et al., 2007; Lal,

2005). Therefore, the research of this topic in the tropical region may provide information to elucidate the role of erosion on the SOC dynamics.

Life cycle assessment of cash crop agriculture in the tropics

In Costa Rica, there has been a change in the agricultural model (which is also observed in other countries of Latin America) in terms of land ownership distribution from small scale agriculture to large scale cash crop operations, which poses a threat to the nation’s food security. The implications of this new tendency to SOC are the reduction of soil organic matter input and permanence into the soil, and a consequent reduction in soil quality. The high productivity of these cash crops derives directly from the intensity in crop density, and fertilizer and pesticides input. It is therefore necessary to conduct the life cycle analysis of these crops and determine their C intensity in relation to other types of management such as agroforestry, organic agriculture and others. This research may provide light to policy makers as to the importance of creating incentives based on the contribution of environmental services rather than on crop productivity. This shift in policy has been occurring in European nations in recent years.

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130 7. APPENDICES

131 Appendix A. Review of soil organic carbon pool (Mg C ha-1) in different ecoregions and land uses of Central America

132 Study Area Holdridge's Life Zone Soil¥ Land use Depth SOC Reference Mg ha-1 Central Mexico Semi-arid, subtropical Haplustoll Wheat - maize 20 24.4* Govaerts et al., 2007 highlands§ rotation, permanent beds + straw

Wheat - maize 19.7* rotation, conventional beds + straw

Wheat - maize 22.6* rotation, permanent beds + partial straw removal Wheat - maize 20.3*

1 rotation, permanent 3

4 beds without straw Wheat 22.6* Maize 22.0* Northeastern Costa Rica Tropical wet forest Tropohumults, Forest 30 81.9 Powers, 2004 transitioning to tropical wet- Dystropepts, 5 - 30 yr Banana 80.6 cool transition forest Dystrandepts 8 - 30 yr Pasture 89.9 1.5 - 12 yr crop 73.8 30 yr Pasture 100.2 4 - 10 yr plantation 85.3 Northeastern Costa Rica Tropical wet forest Dystropept Forest 30 78.7 Powers and Schelesinger, 2002 Dystropept Forest 95.7 Dystropept Forest 68.9 Dystropept Forest 76.8 Dystropept Forest 30 90.8 Tropohumult Forest 115.0

133 Study Area Holdridge's Life Zone Soil¥ Land use Depth SOC Reference Mg ha-1 Tropohumult Forest 76.7 Tropohumult Forest 92.5 Dystropept Forest 97.0 Tropohumult Forest 93.1 Tropohumult Forest 89.8 Tropohumult Forest 52.4 Tropohumult Forest 86.2 Tropohumult Forest 81.5 Dystropept Forest 63.0 Dystrandept Forest 81.1 Dystrandept Forest 84.4 Dystropept Forest 84.0 Dystropept Forest 88.3 Dystrandept Forest 75.6 1

3 Tropohumult Forest 84.9 5 Tropohumult Forest 60.4 Tropohumult Forest 51.1 Dystrandept Forest 85.9 Troporthent Forest 59.5 Dystrandept Forest 78.6 Tropohumult Forest 65.1 Dystropept Forest 75.7 Dystropept Forest 83.7 Dystrandept Forest 81.2 Dystrandept Forest 94.8 Dystrandept Forest 80.8 Dystropept Forest 77.2 Dystrandept Forest 138.6 Dystrandept Forest 88.0

134 Study Area Holdridge's Life Zone Soil¥ Land use Depth SOC Reference Mg ha-1 Los Tuxtlas, Mexico Tall evergreen forest§ Andisols Forest 100 210.3 Hughes et al., 2000 Pasture 166.6 Corn 200.3 Barra Colorado Is. Tropical Moist Forest Oxisols Forest 50 83.8 Schwendenmann and Pendall, Panama Pasture 63.9 2006 Central Mexico NA Pellustert Wheat–corn, burning 30 49.5 Follett et al., 2005 residues Wheat–corn, CT 43.9 Wheat–corn, NT 58.3 Wheat–bean, CT 45.2 Wheat–bean, NT 45.3 Northeastern Costa Rica Premontane wet forest Cambisol 19 yr alley crop with 40 162.0 Oelbermann et al., 2006 Erithryna p. Central Mexico NA Vertisol Vegetable crop 20 0.6* Oleschko et al., 1996 Vegetable crop 0.8* rotation, NT § 1 Southern Mexico Humid tropical Acrisols 6 yr Secondary forest 20 51.9 Geissen and Guzman, 2006 3

6 16 yr pasture (B. 47.5 decumbens ) Northeastern Costa Rica Humid tropical§ (possibly Humitropept Forest 60 57.8 Van Dam et al., 1997; tropical wet forest) 18 yr. Pasture 66.3 Veldkamp, 1994 Hapludand Forest 116.1 25 yr. Pasture 105.3 Central America Pacific Several different Andisols Various land uses 30 24.2* Diaz-Romeu et al., 1970 and Atlantic Litosols 22.0* Fluvisols 31.0* Inceptisols 28.0* Pluvisols 24.0* Brunizems 23.0* Luvisols 31.0* Regosols 33.0* Rodosols 24.0* Vertisols 18.0* 135 Study Area Holdridge's Life Zone Soil¥ Land use Depth SOC Reference Mg ha-1 Central Mexico NA Alfisol Plowed corn 20 17.1* Bell and Van Heulen, 1995 Entisol 10.4* Vertisol 16.6* Alfisol 31.8* Northeastern Costa Rica NA (possibbly tropical wet Hapludand Hyeronima a . 20 38.1 Russell et at., 2004 forest transitioning to plantation tropical wet-cool transition Cedrela o. 30.6 forest) plantation Cordia a. plantation 32.0 Northeastern Costa Rica Tropical wet forest Humitropept Forest 20 62.2* Powers, 2006 transitioning to tropical wet- 34 yr. Pasture 46.9*

1 cool transition forest 3

7 Northeastern Costa Rica NA (possibly tropical wet See Powers and Forest 30 80.5 Powers and Veldkamp, 2005 forest transitioning to lower Schlesinger, montane and montane rain 2002 3 - 60 yr. Pasture 76.7 forests) Central Panama Moist tropical forest Tropoudalf 45 yr. Pasture 100 85.0 Potvin et al., 2004 20 yr. Teak 183.0 Northern Costa Rica NA (possibly tropical wet) Tropofluvent 50 yr. Pasture 50 233.0 Lopez et al., 1999 Silvopasture + 190.0 Cordia a. Northwestern Belize Subtropical moist Shallow calcareous Forest (rain) 20 63.0* Eaton, 2001 Forest (palm) 55.0* Forest (swamp) 51.0* Central Costa Rica NA Udivitrand Various crops 20 59.9* Henriquez et al., 1992 Southwestern Costa Rica Hapludand 66.8* Southwestern Costa Rica Haplohumult 34.0*

136 Study Area Holdridge's Life Zone Soil¥ Land use Depth SOC Reference Mg ha-1 Northern Costa Rica Haplohumult 15.1* Southeastern Costa Rica Humitropept 22.6* Southwestern Costa Rica Eutropept 20.6* Northwestern Costa Rica Pellustert 25.5* Northeastern Costa Rica Pellustert 34.0* Different regions, Costa Low montane Andisol Silvopasture 10 41.2 Montenegro y Abarca, 2002 Rica Inceptisol Forest 35.2 Wet premontane Inceptisol Silvopasture 20.9 Forest 15.7 Tropical wet Silvopasture 42.8 Forest 24.8 Western Costa Rica Subhumid tropical Rhodustalf 3 yr Brachiaria 20 64.4 Ramos Ventimilla, 2003

1 silvopasture 3

8 15 yr Degraded pasture 63.1 25 yr Secondary forest 41.4 Southern Nicaragua Very humid premontane NA Forest (managed) 60 132.6 Lagos-Real and Venegas tropical Forest (undisturbed) 104.7 Berrios, 2003 Southern Mexico Tropical lower montane Redzina Young fallow 100 59.2* Mendoza-Vega and Messing, and premontane moist and 5yr fallow 55.1* 2005 subtropical lower montane 20 yr fallow 74.2* and montane wet zones Phaozem Young fallow 33.6* Forest 47.0* Northern Costa Rica Tropical wet Humitropept Forest 10 72.0* Guggenberger and Zech, 1999 Pasture 58.0* 3 yr. Secondary 71.0* forest 12 yr. Secondary 75.0* forest 18 yr. Secondary 81.0* forest 137 Study Area Holdridge's Life Zone Soil¥ Land use Depth SOC Reference Mg ha-1 Northeastern Costa Rica NA (possibly tropical wet Acrisol Agroforestry 20 49.9 Tornquist et al, 1999 forest) Pasture 57.2 Northeastern Costa Rica Premontane, wet forest Hapludand Tree Plantation 30 67.2 Alvarez, 2005 basal belt transition Northeastern Costa Rica NA Inceptisol 9 yr. Cacao- 45 127.5 Fassbender, 1998 Erhythrina AFS Northeastern Costa Rica NA Inceptisol 6 yr Erhythrina AFS 20 18.5* Haggar, 1990 6 yr G. sepium AFS 14.8* Northeastern Costa Rica NA Inceptisol 10 yr Erhythrina 10 27.8* Mazzarino et al., 1993 AFS 10 yr G. sepium 27.5* AFS

1 Northeastern Costa Rica Premontane, wet forest Hapludand 25 yr Secondary 30 96.9 Daqui, 2006 3

9 basal belt transition forest Northeastern Costa Rica Premontane wet forest Dystrandept 5 yr Succession 80 46.1* Ewel et al., 1991 forest 5 yr Enriched forest 41.3* Monocrop 42.1* Northeastern Costa Rica NA (possibly tropical wet Dystropept 4 yr Tree plantation 30 29.7* Montagnini and Porras 1998; forest) 4 yr Fallow 30.3* Montagnini, 2000 Northeastern Costa Rica Tropical Wet Forest Tropohumult 10 - 36 yr Pasture 10 16.8 Reiners et al., 1994 4 - 10 yr abandoned 16.6 pasture 10 - 20 yr Secondary 22.4 forest Forest 20.6 Southwestern Costa Rica Tropical Wet Forest Ultisol Active Pasture 50 47.9* Penuela and Drew, 2004 4 yr abandoned 78.0* pasture 10 yr abandoned 39.4* pasture

138 Study Area Holdridge's Life Zone Soil¥ Land use Depth SOC Reference Mg ha-1 20 yr abandoned 37.1* pasture Forest 53.5* Southwestern Costa Rica Tropical wet lowland Oxisol 20 yr pasture 10 37.0 Cleveland et al., 2003 Mollisol 20 yr pasture 55.0 Oxisol Forest 34.0 Mollisol Forest 45.0 Northeastern Costa Rica Humid tropical premontane, Hapludults, Conventional AFS 50 111.1 Porras Venegas, 2006 rain tropical premontane, Dystrudepts, Organic AFS 87.7 very humid premontane Eutrudepts Crop 190.7 Forest 123.9

Southern Mexico NA Acrisol Forest 20 94.5 Leon - Najera et al., 2006 1 4

0 Acrisol Corn 71.9 Phaozem Forest 124.1 Phaozem Corn 75.4 Northeastern Nicaragua Humid tropical NA Mucuna + Corn 20 31.1* Phillip and Gamboa, 2003 Nicaragua, various Dry tropical Mollisol Sorghum 22.0* Garcia et al., 2007 locations Andisol 8.1* Vertisol 23.2* 27.3* Central Mexico NA Pinus plantation 11 10 42.9* Maycotte et al., 2002 mo. after fire Pinus plantation 20 92.6* mo. after fire Southern Mexico NA Alfisol 4 yr crop 30 22.4* Uribe y Petit, 2007 4 yr Mucuna fallow 25.5* 4 yr Sucession 32.8*

139 Study Area Holdridge's Life Zone Soil¥ Land use Depth SOC Reference Mg ha-1 Southern Mexico Dry deciduous lowland Regosols and Disk ploughing Agave 20 32.5 Bautista-Cruz et al., 2007 forest* Leptosols crop Animal ploughing 39.0 Agave Minimum tillage 95.6 Agave Northeastern Costa Rica NA Udivitrand Forest 30 100.0* Buurman et al., 2007 Udivitrand Pasture 29.5* Fulvudand Pasture 79.7* Placudand Forest 88.0* Fulvudand Forest 75.5* Hydrudand Forest 109.5* Hydrudand Forest 136.3*

1 Hydrudand Forest 156.5* 4

1 Hydrudand Pasture 60.5* Southern Mexico Humid subtropical Melanudand Pinus forest 20 226.4 Campos et al., 2006 Abandoned crop 227.8 Corn 174.1 Hapludand Forest 91.2 Corn 68.4 Pasture 141.4 Kandiustult Forest 83.6 Sugar cane 81.4 Northwest Nicaragua NA Andisol Conventional crop 30 62.0 Castillo and Joergensen, 2001 Crop rotation + 54.0 organic amendments Southern Mexico NA Regosol 4 yr slash and burn 30 108.9 Roncal-García et al., 2008 Corn + trees 7 yr. Taungya AFS 89.6 24 yr Fallow 109.9 7 yr Enriched Fallow 105.2

140 Study Area Holdridge's Life Zone Soil¥ Land use Depth SOC Reference Mg ha-1 Southeastern Mexico Tropical wet Alfisol/Ultisol 2-6 yr Eucalyptus 30 48.7 Delgado-Caballero et al., 2009 Southeastern Mexico NA Different soils Pasture 20 19.5* Geissen et al., 2009 Temporal agriculture 22.3* Fruit plantation 11.6* Sugar cane 20 18.5* Forest (both primary 27.0* and secondary) Barra Colorado Is. Tropical Wet Forest Mostly Inceptisols Forest 30 69.5 Grimm et al., 2008 Panama Northeastern Costa Rica NA NA Prioria forested 20 61.0* Groffman et al., 2001 wetland Disturbed Prioria 117.0*

1 Raphia forested 329.0* 4

2 wetland Disturbed Raphia 270.0* Belize NA NA 20 yr Sugar cane 20 37.0* Hsieh, 1996 40 yr Citrus orchard 27.3* 50 yr Secondary 70.1* forest Guatemala NA Haprendolls, Mayan 30 111.5 Jensen et al., 2007 Argiudolls, Archaeological Zone Endoaquolls Poas Volcano, Costa NA Andisol Forest (cloud) 20 135.7* Herre et al., 2007 Rica Masaya Volcano, Forest 99.3* Nicaragua Western El Salvador Humid subtropical Andisol Shade grown Coffee 10 29.7 Mendez et al., 2009

Western Mexico NA Regosol Forest (Dry) 10 26.5* Montaño et al., 2009 Belize NA Acrisol Citrus orchard 30 79.81 Morgan et al., 1995

141 Study Area Holdridge's Life Zone Soil¥ Land use Depth SOC Reference Mg ha-1 Honduras NA NA 8 - 15 yr Shrimp pond 10 3.93 Munsiri et al., 1996 Northeastern Costa Rica Tropical wet Eutropept Crop 20 30.15 Nobre et al., 2001 Central Mexico NA Andisol Crop 30 81.6 Ordoñez et al., 2008 Pasture 89.9 Avocado orchards 84.9 Plantation 76.2 Forest (degraded) 72.8 Forest (pine) 93.1 Forest (oak) 116.4 Central Mexico NA Ultisol 11 yr Pasture + 30 9.0* Valles de la Mora et al., 2008 Arachis p. 11 yr Pasture 10.7*

1 Northeastern Costa Rica Tropical wet 3 yr Sucession 30 34.9* Schedlbauer and Kavanagh, 4

3 8 yr Secondary forest 34.7* 2008

15 yr Secondary 37.0* forest 33 yr Secondary 37.6* forest Southeast Mexico NA Haplustoll, Mayan 30 151* Sweetwood et al., 2009 Calciustoll Archaeological Zone (Black C) 0.8* Petraquept, Mayan 8.8* Endoaquept Archaeological Zone (Black C) 0.58* Southwestern Costa Rica NA Paleustalf Pasture 40 28.1* Monge and Alvarado, 1979 Eutropept Oil palm 18.6* Central Costa Rica Very humid low montane Udivitrand 2 yr Plantation 20 41.4* Segura et al., 2005 4 yr Plantation 37.7* 7 yr Plantation 25.8*

142 Study Area Holdridge's Life Zone Soil¥ Land use Depth SOC Reference Mg ha-1 Northwestern Costa Rica NA Humitropept Mango 20 44.7* Ponchner et al., 1993 Dystropept Mango 36.5* Ustropept Mango 31.9* Central Costa Rica NA Andisol Coffee 43.5* Bertsch et al., 1991 Northwestern Costa Rica Inceptisol Coffee 35.4* Northwestern Costa Rica Inceptisol Coffee 29.6* Northeastern Costa Rica Tropical wet Haploperox Forest 300 330.0 Veldkamp et al., 2003 (residual) 25 yr Pasture 30 104.6 Haploperox Forest 300 213.1 (alluvial) 25 yr Pasture 30 97.9 Northwestern Guatemala NA Haplustand 3-6 yr Fallow 30 59.2* Wittman and Johnson, 2008 Crop 74.5* Northern Guatemala NA Ancient Mayan maize 30 55.5* Wright et al., 2009 1

4 cultivation 4 Argiudoll, Control (no Mayan 34.2* Haprendoll cultivation) Northeastern Costa Rica Premontane, wet forest Andisol Degraded Pasture 30 97.4 Jimenez et al., 2007 basal belt transition 15 yr Plantation 100.8 Southeastern Mexico NA Red Karst soil 1-12 yr Fallow 10 61* Weisbach et al., 2002 Black Karst soil 1-25 yr Fallow 133* Northeastern Costa Rica Premontane, wet forest Dystropept 15 yr Secondary 50 204.1 Jimenez et al., 2008a basal belt transition forest 60 yr Secondary 216 forest Northwestern Costa Rica Dry tropical Ustropept Pasture (savanna) 50 74.8 Jimenez et al., 2008b 35 yr Mago 65.9 28 yr Citrus 66.5 25 yr Sugar cane 152.4 Northwestern Costa Rica NA Hydrandept NA 30 100.4 Alvarado et al., 2001: Hydrandept 115.2 compilation of several other Dystrandept 176.6 local studies Dystrandept 46.3

143 Study Area Holdridge's Life Zone Soil¥ Land use Depth SOC Reference Mg ha-1 Central Costa Rica Dystrandept 308.8 Hapludand 222.5 Haplustand 202.8 Northeastern Costa Rica Hapludand 182.2 Southereastern Costa Dystrandept 90.3 Rica Central Costa Rica NA Dystrustept Organic vegetables 100 94.0 CEDECO, 2006 Central Costa Rica Hapludand Organic vegetables 259.5 Central Costa Rica NA Organic vegetables 143.5 Central Costa Rica Dystrustepts Organic coffee 149.3 Central Costa Rica Haplustepts Conventional coffee 88.1 Central Costa Rica Haplustepts Organic coffee 107.5

1 Central Costa Rica Haplustults Organic sugar cane 140.9 4

5 Central Costa Rica Haplustults Conventional sugar 106.4 cane Central Costa Rica Hapluhumult Sugar cane in 123.6 transition to organic Central Costa Rica Hapludand 314.7 Northeastern Costa Rica Premontane, wet forest NA Forest 100 128.0 Amézquita et al., 2005 basal belt transition Silvopasture Acacia 168.0 + Arachis Improved pasture 194.0 Native grass 208.0 Degraded pasture 94.0 Brachiaria pasture 134.0 Central-western Costa Subhumid tropical NA Forest 100 185.0 Rica Silvopasture 130.0 Brachiaria+ Cordia+ Guazuma Native grass 169.0 Forage bank 130.0 Secondary forest 116.0 Degraded pasture 129.0

144 Study Area Holdridge's Life Zone Soil¥ Land use Depth SOC Reference Mg ha-1 Central Mexico NA Regosol Unburned pasture 10 35.9 Cotler and Ortega-Larrocea, Burned pasture 52.3 2006 Forest (Dry) 94.7 Southern Mexico Tropical lower montane, NA Forest (Oak/Cloud) 100 242.8 De Jong et al., 1999 premontane moist, Forest (Pine) 172.6 subtropical lower montane, Forest (Degraded/ 184.2 montane wet zones fragmented) Crop 153.3 Pasture 124.8 Southern Mexico Tropical lower montane, NA Forest 100 176.3 De Jong et al., 2000 premontane moist, Secondary forest 135.9 subtropical lower montane, Pasture 86.0 1 4

6 montane wet zones Crop 122.0 Central Costa Rica Very humid low montane Hapludand Conventional coffee 100 165.7 This study Organic vegetables 237.0 Conv. vegetables 179.5 Forest 246.4 Northeastern Costa Rica Premontane, wet forest Dystropept Banana 150.3 basal belt transition 15 yr. Oil palm 132.2 8 yr Pineapple 113.6 Forest 150.2 Northwestern Costa Rica Dry tropical Dystrustept 15 yr Sugar cane 100 150.4 28 yr Mango 85.6 Pasture 75.9 Ustropept Forest 164.5

¥ Either USDA or WRF, whichever classification system is provided by the reference § Not a Holdridge Life Zone *SOC concentration (g C kg-1), since no data on bulk density was provided AFS: Agroforestry system, CT: conventional tillage, NT: no tillage

145

Appendix B. List of mass conversions under the International System of Units (SI) used in this study

SI SI Factor Prefix Symbol Mass Length 1015 g peta- Pg 1012 g tera- Tg 109 g giga- Gg 106 g mega- Mg 10-3 g milli- mm 10-6 g micro- µm

146 Appendix C. Complete particle size distribution of the soils studied.

Depth Sand Clay Silt Sand Clay Silt Sand Clay Silt cm % % % Pacific Dry Ecoregion Atlantic Moist Ecoregion Montane Ecoregion Dry forest Banana Conventional farming 0 - 10 40.2 30.7 29.1 37.4 29.4 33.2 40.0 29.7 30.3 10 - 20 36.6 37.5 25.8 42.2 32.7 25.1 33.8 31.9 34.3 20 - 30 33.5 38.3 28.3 41.1 24.9 34.0 41.0 30.3 28.7 30 - 40 43.1 28.3 28.6 51.6 28.8 19.5 34.4 35.6 29.9 40 - 50 46.0 23.4 30.6 57.4 26.5 16.1 35.8 39.2 25.0 50 - 70 42.2 21.5 36.3 43.1 39.3 17.6 30.6 42.3 27.1 70 - 100 40.6 19.3 40.1 49.4 25.0 25.6 40.3 40.5 19.2 Sugarcane Oil palm Organic farming 0 - 10 50.4 17.2 32.4 29.7 51.1 19.2 38.7 32.3 29.0 10 - 20 46.7 17.4 35.9 17.7 58.4 23.9 37.5 31.9 30.6 20 - 30 43.8 18.9 37.3 24.1 53.9 21.9 33.1 37.2 29.7 30 - 40 37.7 17.7 44.6 33.0 32.0 35.0 42.1 35.6 22.3 40 - 50 27.1 16.6 56.3 38.1 27.0 34.9 38.5 41.2 20.3 50 - 70 25.4 15.0 59.7 37.3 25.3 37.4 40.1 43.5 16.4 70 - 100 27.0 14.6 58.4 32.7 28.4 38.9 39.2 40.8 20.0 Mango Pineapple Cloud forest 0 - 10 58.0 18.4 23.6 38.0 44.5 17.5 32.0 12.7 55.3 10 - 20 50.0 20.6 29.4 37.4 42.1 20.6 34.9 13.8 51.3 20 - 30 45.8 24.3 29.9 27.1 45.2 27.7 35.5 15.7 48.8 30 - 40 44.8 22.5 32.8 30.2 47.0 22.9 37.9 17.3 44.8 40 - 50 70.1 16.0 13.9 25.8 44.5 29.8 35.6 18.4 46.0 50 - 70 78.6 8.6 12.9 33.1 41.9 25.0 38.7 20.0 41.3 70 - 100 62.4 11.0 26.6 22.4 49.7 27.9 32.8 19.1 48.1 Pasture Rain forest Coffee 0 - 10 49.7 17.2 33.1 21.6 17.6 60.8 7.40 48.90 43.70 10 - 20 57.7 19.6 22.6 29.4 10.5 60.1 6.00 58.30 35.70 20 - 30 56.3 20.2 23.4 27.7 18.1 54.2 9.33 51.67 39.00 30 - 40 67.2 14.0 18.8 34.0 21.6 44.4 10.00 56.10 33.90 40 - 50 65.1 10.6 24.4 43.7 19.2 37.1 5.00 57.60 37.40 50 - 70 61.7 11.9 26.4 29.9 22.0 48.1 9.80 54.40 35.80 70 - 100 56.8 24.1 19.2 29.1 25.8 45.0 14.90 56.20 28.90

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