Agriculture Is Very Important in Uganda As the Main Source Of

MAKERERE UNIVERSITY

COLLEGE OF AGRICULTURAL AND ENVIRONMENTAL SCIENCES

SCHOOL OF AGRICULTURAL SCIENCES

IMPACTS OF EUCALYPTUS PLANTATION ON SOIL PHYSICO-CHEMICAL

PROPERTIES IN BAMUNANIKA SUB-COUNTY, LUWERO DISTRICT, UGANDA.

MUGAYI EMMANUEL

REG NO 16/U/7248/PS

SUPERVISOR: DR. GODFREY TAULYA(PhD).

A SPECIAL PROJECT REPORT SUBMITTED TO THE SCHOOL OF AGRICULTURAL SCIENCES IN PARTIAL FULL FILLMENT OF THE REQUIREMENT FOR THE AWARD OF BACHELOR OF SCIENCE IN AGRICULTURAL LANDUSE AND MANAGEMENT

SEPTEMBER 2019 i

DECLARATION

DEDICATION This work is dedicated to my Daddy Lawrence Kayinamula, Mum Edith Komushana and my lovely siblings Monica kasande, Scovia Kamatenesi and Late Ronald Tumusiime (may your soul rest in eternal peace). I love you all so much and you are very close to my heart.

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ACKNOWLEDGEMENTS My profound appreciation goes to my parents for love, care above all for sponsoring me to study a degree program in this great institution of Makerere university and also making this academic journey possible. I also register my heartfelt gratitude to my supervisor Dr. Godfrey Taulya for facilitating this research, his support, mentorship role, constructive criticisms and taking time to read through my work. Much appreciation also goes to soil science laboratory technicians of Makerere university, Madam Gorreti and Madam Lydia for their keen interest, guidance and efficient supervision during preparation and lab analysis of soil samples to make it a success. I am also highly grateful to all the lecturers of the Agricultural land use & Management Program and other staff of school of agricultural sciences for their insightful professional work, friendship and mentorship during my academic journey at the university. Thank you also goes to my hardworking field assistant during field data collection: Mr. Ssemanda (bodaboda rider) of nabiteete village who would ride me to the field to meet farmers every day and collecting soil samples from the field.

Finally, I extend my appreciation to all my loving and caring friends who helped me in one way or the other to make this work, my stay and studies in Makerere university a success:

Mr. Okello David, Mr. Tamale Paul, Mr. Mugabe Demnitrous and Mr. Kisomose Paul. God bless you all.

ABSTRACT Eucalyptus is very important to people’s livelihood as source of income and ecosystem services. However, issues surrounding these eucalyptuses on soils and the environment have raised unresolved debates among conservationists and natural resource managers. Therefore, this study was carried out to assess the impact of eucalyptus on selected soil physico- chemical properties in Bamunanika county Luwero district, where cultivation of Eucalyptus has been identified as a priority practice for integrated watershed management. A study area with both natural forest and Eucalyptus plantation was selected in Bamunanika subcounty. A 20-m long transect emanating from each of the two land use types (Eucalyptus vs. natural forest) was set up. Along each transect, a soil sample was taken from the distances 0, 5, 10, 15 and 20 m from the edge of the land use type. Four undisturbed core samples were taken using pF rings and core sampler from each of the sampling points (within 40 cm of each other on either side of the transect) and uniquely labelled prior to transportation to the lab for bulk density determination. Four cores were taken at each sampling point, parallel to the edge of the land use type and composited. The samples were air-dried, ground and sieved to pass through 2-mm mesh and then analyzed for the chemical properties (pH, total N, soil organic matter, available P and exchangeable bases). Soil analyses were done using standard laboratory methods. The data subjected to linear regression with groups in GenStat 15. The soil bulk density under the Eucalyptus spp. plantation (1.6 g cm-3) was significantly higher compared to that of the native forest (1.06 g cm-3). Exchangeable calcium and total nitrogen were also significantly higher in natural forest compared to the Eucalyptus land use.

TABLE OF CONTENTS

DECLARATION ------i DEDICATION ------ii ACKNOWLEDGEMENTS ------iii ABSTRACT ------iv CHAPTER ONE ------1 INTRODUCTION ------1 1.1 Back ground of the study. ------1 1.2 Problem statement ------4 1.3 Justification for the study ------4 1.4. Objectives of the study ------5 1.4.1 General objective ------5 1.4.2Specific objectives ------5 1.5 Research hypotheses ------5 CHAPTER TWO ------7 LITERATURE REVIEW ------7 2.1 Effects of Eucalyptus Plantation on Soil Physico-chemical Properties ------7 2.1.1 Soil Bulk Density ------7 2.1.2 Soil Moisture ------7 2.1.3 Soil Texture ------7 2.2 Effects of Eucalyptus Plantation on Soil Chemical Properties ------8 2.2.1 Soil pH ------8 2.2.2 Soil Exchangeable Acidity (Al3+ and H+) ------9 2.2.3 Soil Organic Carbon and Soil Organic Matter ------9 2.2.4 Soil Total Nitrogen and Soil Available Phosphorus ------11 2.2.5 Soil Exchangeable Cations (Na+, Ca2+ and K+). ------12 2.2.6 Soil Trace Elements/Micro-Nutrients (Zn2+, Fe2+, Mn2+ and Cu2+)------13 CHAPTER THREE ------16 METHODS AND MATERIALS ------16 3.1 Description of study Area ------16 3.1.1 Location ------16 3.1.2 Climate ------16 3.1.3 Hydrology ------17 3.1.4 Topography and Geology ------17 3.1.5 Ecological Resources ------18 3.1.6 Population and ethnicity ------18 3.1.6 Land use and tenure ------19 3.1.7. Site selection ------19 3.3 Sampling Procedure, Mapping and Sample Preparation ------20

3.4 Laboratory analyses ------21 3.4.1 Soil Dry Bulk Density ------21 3.4.2 Soil Texture ------21 3.4.3 Soil Moisture ------22 3.4.4 Soil Exchangeable Bases (Ca2+, Na+ and K+) and CEC ------23 3.4.5 Soil pH ------23 3.4.6 Soil Total Organic Carbon (TOC)------23 3.4.7 Soil Total Nitrogen (TN) ------24 3.4.8 Soil Available Phosphorus ------25 CHAPTER FOUR ------26 RESULTS AND DISCUSSION ------26 4.1 Effects of Eucalyptus Plantation and Native Forest on Soil physico- chemical Properties ------26 4.1.1Soil Bulk Density ------27 4.1.2 Soil pH ------28 4.1.3 Soil electrical conductivity(EC) ------30 4.1.4 Soil Organic matter (SOM) ------31 4.1.5 Soil Total Nitrogen (TN) ------32 4.1.6 Soil Exchangeable Cations (Ca2+and K+) ------33 4.1.7 Soil Available Phosphorus (P) ------33 CHAPTER FIVE ------35 5.0 SUMMARY OF FINDINGS, CONCLUSION AND RECOMMENDATIONS ------35 5.1 Summary of Key Findings ------35 5.2 Conclusions ------36 5.3 Recommendations ------37 REFERENCES ------38

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LIST OF FIGURES

Figure 1: A map showing the location of the study area...... 17 Figure 22: Population density by sub-county (person/km2)3 ...... 19 Figure 3: Variation in dry bulk density, available phosphorus, exchangeable potassium, exchangeable calcium, soil organic matter, total nitrogen, soil pH and electrical conductivity along the Eucalyptus and natural forest land use transects in Luwero district, central Uganda ...... 29 Figure 4: Variation of dry bulk density, electrical conductivity, exchangeable calcium and total nitrogen with distance from the edge of Eucalyptus and edge of natural forest in Luwero district, central Uganda ...... 30

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LIST OF TABLES

Table 2.1: pH Ranges and their Interpretation Guide. …………………………………9

Table 2: Simple linear regression parameters for soil organic matter, available phosphorus (P), exchangeable potassium (Exch. K) and soil pH...... 26

LIST OF PLATES

Plate 3.1: Eucalyptus plantation in Nandere parish Luwero Uganda...... 20

Plate 3.2: Natural Forest in Nandere parish Luwero Uganda...... 20

Plate 3.3: soil sampling under eucalyptus plantation using auger...... 21

Plate 3.4: Soil sampling under Natural forest using auger...... 21

CHAPTER ONE INTRODUCTION 1.1 Back ground of the study. Eucalyptus is a name issued from New Latin, genus name, from eu- + Greek kalyptos covered, from kalyptein to conceal; from the conical covering of the buds (Merriam Webster Dictionary).

Eucalyptus is a genus of more than 500 species, and it has become the most planted genus of trees in the world (Demel 2000). The major planting of this tree species, outside its home environment, was started in 1904 in Brazil. Eucalyptus is native to Australia, the Malaysian region and the Philippines. Today eucalyptus plantations cover at least 12 million hectares throughout the tropical zone, 90 percent of which has been established since 1955 (Turnbull

1999). This species was introduced to East Africa between late 19th and early 20th century.

Already in the early 1970s, the area covered by eucalyptus in Ethiopia, Rwanda, Uganda, Kenya and Sudan reached 95684 ha (FAO 1979). The largest plantations during the same time existed in Ethiopia and Rwanda, with 42,300 and 23,000 ha, respectively. Concerns about negative impacts of eucalypts on the environment have raised worries. These concerns include issues of high transpiration rates, soil fertility depletion, disruption of biodiversity conservation and their allelopathic effects that inhibit\undergrowth regeneration (Baragali et al., 1993; Heilman and

Norby, 1997; Calder et al., 1997; Jagger and Pender, 2000; Aweto and Moleele, 2005; Zewdie,

2008; Alemie, 2009; Cao et al., 2010; Demessie et al., 2012; Tererai, 2012; Ravina, 2012;

Tererai et al., 2014; Cortez et al., 2014). Lane et al. (2004) in China found that the expansion of

Eucalyptus spp. plantation lowers water tables and reduces water availability for irrigation on lands previously used for arable crops or occupied by indigenous trees and grass due to soil hydrophobicity (water repellency) and its deep and dense root network. Wen et al. (2009) and

Zhu et al. (2009) indicated that in China, afforestation of exotic Eucalyptus spp. plantation is

2 reported to adversely affect the soil physical and chemical properties and plant community biodiversity. In Argentina, a lot of controversy has arisen about the nutrient cycling capacity, ecological and economic implications of Eucalyptus spp. in terms of sustainable forestry (Goya et al., 2008). Within Africa, previous authors have reported that Eucalyptus spp. plantations (EP) have devastating effects on the soil physico-chemical properties, depleting soil organic matter content and negatively impacts soil hydrology (Aweto and Moleele, 2005; Kindu et al., 2006a, b;

Kenya forest services, 2009; Oballa et al., 2010; FAO, 2011; Tererai, 2012; Tererai et al., 2014).

For instance, El-Amin et al. (2001) reported that in Sudan, Eucalyptus spp. caused crop yield reduction due to nutrient depletion and production of toxic exudates (allelochemicals). A study in

Ethiopia, 1993 compared the nutrient status in plantations of Eucalyptus, cypress and cedar

(Teketay, 2003). The study showed that soils under Eucalyptusplantations had lower nutrient content compared to those in cedar and cypress forests. Alemie (2009) who conducted a study on

Eucalyptus sp. in Koga watershed in Ethiopia, attributed the drought status of previously functional nearby water stores in the watershed to the deep and dense roots network of the

Eucalyptus spp., which have high potential to suck water in addition to increased soil hydrophobicity induced by decomposition of their litter and poor undergrowth, which reduced infiltration and water table in the watershed. He further added that increasing plantations of

Eucalyptus spp. would create competition between agricultural food crops and trees for land area, major resources (water and soil nutrients) and light. Eucalyptus spp. was introduced in

Uganda 1902 by the British colonial government to provide fuel wood for the Uganda-Kenya railway (Oballa et al., 2010) but it has been reported that Eucalyptus spp. grown as a short rotation crop for high biomass production, led to rapid soil nutrient depletion (Uganda Forestry

Authority, 2009; Oballa et al., 2010). Previous studies have also found increased concentrations

3 of micro-nutrients, especially Fe and Mn in soils under Eucalyptus spp. compared to those under plantations such as tea of similar age (Oballa and Langat, 2002; Uganda Forestry Authority,

2009). Furthermore, Aweto and Moleele (2005) reported that Eucalyptus species were not efficient in cycling macro-nutrients such as nitrogen and phosphorus, as litter from the species have high carbon to nitrogen (C: N) ratio (Castro-Díez et al., 2011) and hence low rates of mineralization (Bernhard-Reversat and Schwartz, 1997), which results in low rates of nutrient release. The species also have allelopathic effect on other plants as a result of the allelochemicals that are released during the decomposition process, which inhibit the growth of other plant species underneath them (El-Amin et al., 2001; Mfahaya et al., 2009; Oballa et al., 2010;

Zhang et al., 2010; FAO, 2011; Ruwanza et al., 2014). These allelopathic chemicals or allelochemicals can also persist in the soil, affecting both neighboring plants as well as those planted in succession (Ferguson et al., 2013). Owing to the perceived negative ecological impacts of Eucalyptus spp. on soil and water resources, interest in the plant has waned and the population of Eucalyptus spp. in state forests, farmlands, stream banks and catchment areas in

Uganda have reduced drastically (Oballa et al., 2010; FAO, 2011). Consequently, farmers have been advised not to establish plantations of Eucalyptus spp. within 15 m of the riverine vegetation and 6 m from any neighboring farm (Mfahaya et al., 2009). The effects of Eucalyptus spp. plantations on soil properties and depletion of the soil essential nutrients still remain unclear

(Bernhard-Reversat, 1999; El-Amin et al., 2001) and conventional scientific reports are scanty

(Oballa et al., 2010). Therefore, additional documentation on the specific effects of Eucalyptus spp. plantations in relation to soil physical (soil moisture and soil bulk density) and chemical properties (pH; organic carbon; available phosphorus; total nitrogen; exchangeable bases is required in Luwero district.

1.2 Problem statement Improper land use and soil management, which often leave the soil with less or no vegetation, are often cited as the major causes of soil quality deterioration. Fast-growing tree plantations may also lead to soil quality decline when they are poorly planned and not properly managed. According to Jagger and Pender (2000), the impact of tree plantations upon soil resources has been very much debated and there is no complete consolidated view, partly due to the fact that the impact is much dependent on variable site and forest conditions. This is true for Eucalyptus as well, according to a FAO expert report (Davidson 1985). Eucalyptus is a dominant tree species planted in Bamunanika sub county Luwero district of central Uganda. Although these species are planted in various spatial patterns in the district to meet the demand for fuel wood, construction poles, timber, electricity poles, water conservation, charcoal production, erosion control, and some farmers alternatively embark on trees for income generation when the markets for their agricultural produce fail or are low. The impact of Eucalyptus on soil physico-chemical properties in Bamunanika is yet to be quantified, hence the need for this study.

1.3 Justification for the study A number of reports have been made in other countries concerning the negative impact of Eucalyptus plantation on edaphic characteristics of soils. for example, Bernhard-Reversat (1999) reported that ecological implications of exotic trees like Eucalyptus spp., are often questioned but their effects on the soil ecology has not been adequately studied. In view of the above reports, this study is aimed at investigating the effects ofEucalyptus plantation on selected soil physical and chemical properties in the bamunanika sub county with the view to helping land management planners to make efficient land use decisions in the study area and to better inform farmers’ decisions to undertake good agronomic and sustainable land management practices in the region to promote ecological sustainability and crop growth. To safeguard and maintain the sustainability of the soil and water in Bamunanika sub county, it is imperative that effects of Eucalyptus spp. plantation on the soil physicochemical properties be evaluated and ascertained.

Land management planners can use this information in their decisions on land use in the study

area and to understand the particular choices made by farmers concerning Eucalyptus spp.

plantation in Luwero district. The results from this study will help National Environmental

Management Authority (NEMA), National Forestry Authority (NFA), community leaders,

extension officers, farmers, opinion leaders, NGOs and stakeholders to understand, redesign and

improve as well as take steps to minimize negative impacts of Eucalyptus spp. plantations. The

outcome of this research would be beneficial to local, district, County and National governments

as it will generate a baseline data for further studies and research, and policy dialogues. The

findings will also facilitate monitoring and evaluation of Eucalyptus spp. plantations regarding

their effects on the soil properties in Luwero district and Uganda at large.

1.4. Objectives of the study 1.4.1 General objective The general objective of the study was to evaluate the effects of Eucalyptus spp. plantation on selected soil physical and chemical properties pertinent to crop growth and yields.

1.4.2 Specific objectives The study addresses the following specific objectives:

i. To evaluate the variation in soil physical properties (soil texture and soil bulk density)

between Eucalyptus spp. plantation and native forest. ii. To quantify the change in soil physico-chemical properties (pH; organic carbon;

available phosphorus; total nitrogen; exchangeable bases) with distance from the edge

Eucalyptus spp. plantation and native forest.

1.5 Research hypotheses i. No significant differences exist between Eucalyptus spp. plantations and native forest in their effects on soil physico-chemical properties in Luwero district.

ii. No significant difference in the intercept for the linear relationship between a soil physico-

chemical property with the distance from the edge of land use type (Eucalyptus plantation

Vs natural forest). iii. No significant differences in the gradient for the linear relationship between a soil physico-

chemical property with distance from the edge of the land use types (Eucalyptus plantation

vs natural forest).

CHAPTER TWO LITERATURE REVIEW 2.1 Effects of Eucalyptus Plantation on Soil Physico-chemical Properties 2.1.1 Soil Bulk Density FAO (2011) found increased soil bulk density from 0.58 mg m-3 to 0.70 mg m-3 for Eucalyptus spp. plantations in Australia. Ravina (2012) found higher soil bulk density at 1.24 g cm-3 under Eucalyptus spp. plantation compared to native forest of 0.66 g cm-3 within 0 - 20 cm depth of soils in Brazil. Aweto and Moleele (2005) also found that Eucalyptus spp. plantations increased soil bulk density more than the native forest of Acacia spp. in Botswana. Increased soil bulk density is associated with reduced soil infiltration, increased surface run off, with consequent reduction in soil moisture.

2.1.2 Soil Moisture Cao et al. (2010) reported low soil moisture contents, ranging from 20.2 to 30.5 % in the top soil (0-10 cm depth) of four areas under Eucalyptus spp. plantations in China. A study in India on the hydrological impacts of Eucalyptus spp. plantation indicated that in some regions, Eucalyptus spp. plantation exhibited greater water use than recorded rainfall over the same time period implying short to medium term soil moisture reductions (Calder et al., 1997). According to Calder et al. (1997), a phenomenon known as “soil water mining” was occurring, whereby, the trees’ extensive root systems were able to tap into deeper soil layers that other species were unable to reach.They concluded that, the implication of this is adisturbance in the water table and potential draining of underground aquifers, reducing groundwater recharge. Alemie (2009) further reported soil moisture reduction under Eucalyptus spp. plantations in Koga watershed in Ethiopia with the reductions being highest at distances closer to the Eucalyptus spp.Tererai et al. (2014)also reported that invaded sites of Eucalyptuscamaldulensis showed significantly reduced soil moisture levels than their counterpart of un-invaded sites in winter and spring seasons of riparian soils of the Berg River catchment in Western Cape Province, South Africa. 2.1.3 Soil Texture Aweto and Moleele (2005) found that between 0 and 20 cm soil depth, the mean proportion of clay was significantly higher under Eucalyptus spp. plantation soils compared to an Acacia forest plantation in Botswana.They further argued that higher clay content of soil means higher

8 cation exchange capacity of the Eucalyptus spp. plantation. Balamurugan et al. (2000) also reported increase in clay and decrease in sand content under Eucalyptus spp. plantation.

Moreover, Alem et al. (2010) found in Ethiopia that the percentage of clay particles in soil collected from Eucalyptus grandis plantation was significantly higher than that of the adjacent native sub-montane Rain Forest.Khanmirzaei et al. (2011) further reported in Iran that clay and coarse silt contents were significantly greater under Eucalyptus spp. plantations, whereas sand fraction was less.

2.2 Effects of Eucalyptus Plantation on Soil Chemical Properties 2.2.1 Soil pH Rhoades and Binkley (1996) found a reduction in soil pH from 5.9 to 5.0 after eight years of

Eucalyptus saligna establishment. Cao et al. (2010) also found reduced soil pH between 0 to10 cm soil depths of four areas, which ranged from 4.2 to 4.5, under Eucalyptus spp. plantations in

China. Similarly, Alemie (2009) found reduced soil pH and strongly acidic values ranging between 3.5 to 4.0 under Eucalyptus spp. plantations in Koga watershed in Ethiopia.

Aweto and Moleele (2005) argued that Eucalyptus sp. immobilizes soil exchangeable bases, especially calcium leading to low soil pH. Bailey et al. (2005) adds that acidification usually leads to depletion of the soil base cations (e.g., K+, Mg2+, Ca2+). This depletion arises from replacement of the basic cations by Al3+ and H+ ions at the exchange site. According to Castro-

Díez et al. (2011), low soil pH limits the growth and activities of decomposer soil microorganisms as soil biological activities are reduced in acidic soils. Berendse (1998) indicated that at soil pH of below 5.5, soil trace nutrients like Manganese (Mn) and Aluminium

(Al) availability increase to levels that become toxic for most plant growth. Further, soil

9 nutrients such as phosphorus and nitrogen tend to form insoluble compounds with Al and Fe in acidic soils, become adsorbed and therefore, made inaccessible for plant uptake.

Table 2.1: pH Ranges and their Interpretation Guide. Source: Horneck et al. (2011)

pH Interpretation ˂5.1 Strongly Acidic 5.2-6.0 Moderately Acidic 6.1-6.5 Slightly Acidic 6.6-7.3 Neutral 7.4-8.4 Moderately Alkaline ˃8.5 Strongly Alkaline

2.2.2 Soil Exchangeable Acidity (Al3+ and H+) Soils that have reduced pH and are strongly acidic have been found to contain higher exchangeable acidity. Oyedele et al. (2009) indicated that hydrolysis of Al3+ ions contribute to development of soil acidity. Frimpong et al. (2014) indicated that when the soil pH drops below 5.5, aluminum and manganese toxicities may occur. In this regard, increasing soil exchangeable acidity (H+ + Al3+) concentration would lead to lower the soil pH. Soils with higher exchangeable acidity have been reported to cause immobilization of soil essential nutrients such as P, N, Ca, Mg, and K under Eucalyptus spp. plantations (Aweto and Moleele, 2005).

2.2.3 Soil Organic Carbon and Soil Organic Matter Benefits of improved organic matter in the soil include improved water and nutrient holding capabilities; better soil structure which enhances root growth and increases aeration; a more hospitable environment for soil organisms; and a reserve of plant nutrients. Lal (2005) pointed out that the soil carbon content depends on complex interactions among climate, soils, tree species and management, and the chemical composition of the litter, as determined by the dominant tree species. Substantial changes in soil organic matter (SOM) have been reported after substitution of the natural vegetation by short-rotation Eucalyptus spp. plantations in Brazil

(Lima et al., 2008). Sharmsher et al. (2002) found that 83 % in the surface soil and 94 % in the sub-soil of soil samples in Pakistan were low (< 1 %) in organic matter under Eucalyptus spp. plantation soils. Reports by Jan et al. (1996) also indicated significant organic matter changes in Eucalyptus spp. plantation soils relative to Shorearobusta natural forest soil in Uttar Pradesh, Pakistan. Similar findings have been reported by Bernhard-Reversat (1998) who found that the organic carbon concentration in the 0–10 cm depth of native Acaciaseyal woodland in Keur Maktar, Senegal was twice that in a corresponding layer of soil under Eucalyptusspp.plantation.

Baber et al. (2006) conducted a study on the effect of Eucalyptuscamaldulensis on soil properties and soil fertility in Khan District in Pakistan. They found that the organic matter content in the surface soil at 0-15 cm depth ranged from 0.38 to 1.10 %. By comparing the results with the established criteria for soil fertility, the soil samples were low in organic matter.

Organic matter contents are considered low if it is less than 4 %, medium if it is between 4 % and 8 %, and high if it is above 8 % (University of Connecticut, Cooperative Extension System,

2003). According to Cortez et al. (2014), the results for soil total organic carbon (TOC) indicated that organic C decreased in the first year of the conversion of native forest to

Eucalyptus spp. plantations in Brazil.Chen et al. (2013) indicated that during deforestation of native vegetation and establishment of Eucalyptus spp. plantation, soil macro-aggregates are disrupted and exposed to microbial breakdown, with the consequence of organic matter being lost from soil.

Zhang et al. (2012) add that lower organic matter inputs are provided to the soil since young

Eucalyptus spp. stands may provide a low amount of inputs of plant residues to the soil.

However, Leite et al. (2010) found in Brazil that contents of soil organic matter were considerably higher in Eucalyptus spp. soils than in pasture areas, which they attributed to the greater amount of residues produced by the Eucalyptus spp. plantation (leaves, branches, bark

11 and especially roots) that remained in the soil. Hou (2006) also argued that Eucalyptus spp. have a high growth rate and C fixation potential; therefore, part of the assimilated C is transported to the soil through litter fall and rhizodeposits, and hence the potential of increasing soil organic C content with time (Lima et al., 2006). Therefore, it is likely that the changes in soil chemical properties, particularly in SOM, differ after several Eucalyptus spp. plantation rotations, varying with the soil type and dominant climate conditions (Leite et al., 2010).

2.2.4 Soil Total Nitrogen and Soil Available Phosphorus FAO (2011) made a comparison of 1 to 8 years old Eucalyptus spp. plantations and natural mixed broad leaved forest in the central Himalayas. The study found soil total N and available P decreased as a result of reforestation with Eucalyptus spp. plantation. A study by Leite et al. (2010) pointed out that large nutrient amounts are exported when Eucalyptus spp. plantation are harvested, causing a reduction in the soil nutrients content such as total N and available P.

Jagger and Pender (2000) pointed out that when considering which tree species should be chosen for afforestation or agro-forestry, the depletion of soil nutrients is one of the most commonly cited criticisms associated with the Eucalyptus spp. In contrast to other trees commonly used, such as Leucaena and Acacia, Eucalyptus spp. do not fix nitrogen from the atmosphere as the leguminous species do. They, therefore, concluded that fast growing non-leguminous

Eucalyptus spp. are not recommended for intercropping with annual crops. Tererai et al. (2014) found that soil total N decreased with an increase in Eucalyptuscamaldulensis cover compared to the site not covered by the Eucalyptus spp. in South Africa. This was attributed to the high biomass and higher nutrient consumption rates of the Eucalyptus spp. Alemie (2009) also found that concentration of soil TN decreased under plantations of Eucalyptus spp. in Ethiopia.

Nitrogen concentration in soils is reflected in part by the rate of decomposition of the plant material (Demessie et al., 2012). Previous authors have reported that Eucalyptus spp. produce

12 litter with low nutrient concentrations, which decomposes slowly to release low concentrations of nutrients (Aweto and Moleele, 2005; Baber et al., 2006; Cao et al., 2010; Demessie et al.

2012). Aweto and Moleele (2005) indicated that soil available phosphorus was relatively low in soil under Eucalyptus spp. plantation (˂ 5µg g−1) at 0–20 cm depth of the soil, which was attributed to the long-term immobilization in the plantation standing biomass. In Australia,

Polglase et al. (1992) found that the concentration of soil available phosphorus in the topsoil (0–

5 cm depth) under Eucalyptus spp. plantation declined from an initial concentration of 34 to 2.3

µg g−1 after 16 years. Alemie (2009) further reported that soil available phosphorus concentration in Eucalyptus spp. plantation within 2 cm depth of soil was in the very low range (< 5 mg kg-1) in Ethiopia.

2.2.5 Soil Exchangeable Cations (Na+, Ca2+ and K+). Aweto and Moleele (2005) indicated that decline in pH in plantation soil would lead to a lowering of soil base saturation, as these exchangeable bases (Ca, Na, and K) are immobilized, resulting in soil exchangeable bases depletion over time.Leite et al. (2010) found that soil exchangeable K, Ca, and Mg declined in the Eucalyptus spp. plantation soils compared to the native forest soils in Brazil. Tererai et al. (2014) in their study in South Africa found that soil exchangeable cations such as Ca and Mg were generally higher in un-invaded sites and declined along the invasion gradient of Eucalyptus camaldulensis(lightly, moderately and heavily invaded) in all seasons. Aweto and Moleele (2005), in their study in Gaborone, South Eastern Botswana, found lower exchangeable nutrient cation content, especially calcium, magnesium and potassium of both 0‒ 10 and 10–20 cm depths of soil underEucalyptus spp. plantation. Bernhard-Reversat (1988) observed that the calcium, magnesium and potassium concentrations were considerably lower in the 0–20 cm depth of soil under 8-year Eucalyptus spp. plantation in Senegal than in soil under native semi-arid Sudano-sahelian savanna. Jaiyebo (1998) indicated a reduced concentration of soil exchangeable bases under plantations of Eucalyptus spp. in northern Nigeria.

Alem et al. (2010) observed higher soil exchangeable Na concentration in Eucalyptus grandis compared to that in native forest in Ethiopia. Adetunji (1996) indicated that soils with exchangeable Na of 1 c mol kg-1 should be regarded as potentially sodic. When Na is present in the soil in significant quantities, particularly in proportion to the other cations present, it can have an adverse on crops and physical conditions of the soil (Adetunji, 1996; Bashour and

Sayegh, 2007).

2.2.6 Soil Trace Elements/Micro-Nutrients (Zn2+, Fe2+, Mn2+ and Cu2+) Soil micro-nutrients availability often correlate well with soil pH and organic matter levels (Horneck et al., 2011). According to Horneck et al. (2011), availability of most micro-nutrients decreased as pH increased. They further reported that micro-nutrient deficiencies rarely occur when the soil pH is below 6.5 and that micro-nutrient deficiencies generally occur only when soil pH is 8.0 or above. Baber et al. (2006) found in Khan District in Pakistan that soils under Eucalyptus camaldulensis plantation contained reduced levels of soil micro-nutrients such as Zn2+, Fe2+, Mn2+ and Cu2+ but this was because the soil pH range was in the basic range of 8 to 8.5.Soil available Fe2+, and especially Mn2+, followed the same pattern. Khanmirzaei et al. (2011) carried out a study on four Eucalyptus spp. plantation plots in Iran. The study found significant increases in soil available micro-nutrients especially Fe and Mn contents at 0 – 20cm soil depth. In the study, Fe increased from 1.85 to 3.60 mg kg-1 and Mn from 2.10 to 11.28 mg kg-1.Generally, Montagnini (2000) indicated that soil major nutrient availability decreases and micronutrients increase in plantation of fast-growing tree species with high nutrient uptake, high immobilization of nutrients in the plant biomass and with decreased mineralization. Lemma et al. (2007) reported that as an insulating layer, litter protects the soil from extreme changes in moisture and temperature, intercepts thorough-fall, and improves infiltration.According to Demessie et al. (2012), the quantity and quality of litter production and the decomposition process play an important role in soil fertility management in terms of nutrient cycling, carbon budgeting and formation of soil organic matter under plantations, where part or all of the biomass accumulated during the production period is hauled out of the site after harvest. Under such circumstances, it will be likely that continuous cropping with short rotation crops will

14 result in nutrients depletion and deterioration of physical and biochemical activity of the soil (Parrotta 1999; Fisher and Binkley, 2000; Wang et al., 2007).Eucalyptusspp. are associated with large quantities of litter with high C: N ratios, high lignin and phenolic content (Castro-Díez et al., 2011). This results in lower net N mineralization, as soil available N gets bound in microbial processing of the low N-litter (Briones and Ineson, 1996; Bernhard-Reversat and Schwartz, 1997). For instance,Dalla Tea and Marcó (1996) reported fourteen-year-old Eucalyptus spp. plantations in Argentina to have 25 Mg ha–1 of litter with a low concentration of nitrogen (8 kg mg–1) and phosphorus (0.25 kg mg–1).

The recalcitrant litter and consequently low decomposition rates of Eucalyptus spp. litter

(Tererai et al., 2014) also explains the reduced levels of soil total N, P and K concentrations usually found under Eucalyptus spp. plantation soils. Many invasive alien plants tend to speed up litter decomposition and increase nutrient pools for plant uptake (Ehrenfeld, 2003), in particular N content (Lake and Leishman, 2004; Follstad Shah et al., 2009). However,

Eucalyptus spp. plantation have been reported to produce nutrient-poor litter, with slow decomposition rate, thus resulting in little or no effect on soil-nutrient pools (Castro-Díez et al.,

2011), especially those mostly in monocultures (Forrester et al., 2006).According to Wang et al.

(2007) and Pandey et al. (2007), when natural forests are replaced by monoculture plantations, the forest species diversity decreases, the total litter fall gets reduced and eventually the pattern of nutrient release through decomposition changes.Scarcity of humus formation and a high annual disappearance of organic matter in the forest floors under Eucalyptus spp. plantations have been reported (Goya et al., 2008). Most of the studies that have reported elevated C and N in soils are for N-fixing species growing on nutrient-poor soils (Yelenik et al., 2004; Gaertner et al., 2011). For cases of Eucalyptus spp. plantation, elevated soil nutrients have mostly and usually been reported in vegetation assemblages when they are mixed with leguminous species

15 like that of Acacia species (Bauhus et al., 2000; Laclau et al.,2005; Forrester et al., 2006;

Gaertner et al., 2011).

CHAPTER THREE METHODS AND MATERIALS 3.1 Description of study Area 3.1.1 Location

Luwero is a district in the Central Region of Uganda. Luwero is the site of the district headquarters. Luwero District is bordered by Nakasongola District to the north, Kayunga District to the east, Mukono District to the southeast, Wakiso District to the south, and Nakaseke District to the west. The district headquarters at Luwero are approximately 75 kilometres (47 mi), by road, north of Kampala, Uganda's capital and largest city. The coordinates of the district are 00

50N, 32 30E (Latitude:0.8333; Longitude:32.500). Luwero District was the site of a fierce insurgency by the rebel group National Resistance Army and a brutal counter-insurgency by the government of Milton Obote, known as the Luwero War or the "Bush War", that left many thousands of civilians dead during the early to mid1980s. The area affected by the war has come to be known as the Luwero Triangle. In 2005, Nakaseke County was split from Luwero District to form Nakaseke District. Luwero District is administered by the Luwero District

Administration, with headquarters at Luwero. There are several town councils within the district, each with its own urban town council: Bombo, Luwero, Bamunanika, Kalagala, Kalule, Ndejje,

Ziroobwe.

3.1.2 Climate Climate in Luwero District can be described as modified equatorial climate. Mean diurnal maximum temperatures range between 180 and 350 while the corresponding minimum diurnal range is 80 and 250. The rainfall is well distributed throughout the year, with the average annual rainfall being 1,300 mm. The peak rain period is March – May and October – November. The reliability of rainfall generally declines northwards. Dry seasons occur from December – February and June – July.

Figure 1: A map showing the location of the study area. 3.1.3 Hydrology Wetlands and rivers cover approximately 7% of the District. Major rivers in the district include Kafu, Sezibwa, Lugogo, Lwajali, Mayanja, and Danze. Ground water resources supply water from boreholes, protected and unprotected springs, swallow wells. 3.1.4 Topography and Geology Hilly uplands dominate the south regions of the district ancient granitic rock out-rocks rise up in the north and wide interlocking valleys break up the low hills in the central region and are seasonally flooded, bringing added diversity to the region.

With few exceptions, most of the geological formation in the District consists of the basement complex systems as the oldest, overlain in places by a succession of sedimentary strata which will have under gone a variable degree of metamorphosis. These major geologicalformations are characterized by the presence of young intrusive rocks, mostly acidic and less commonly basic.

The youngest formation of Pleistocene is represented by the sands, quartz and clays of alluvial or lacustrine origin.

3.1.5 Ecological Resources The major ecosystems in Luwero District includesForest – Savannah woodland and plantations, swamp forests, Wetlands and rivers, which cover approximately 7% of the District area. Wetlands species are highly specialized and if their inhabitants are disturbed the impact is great, leading to decline and even extinction of some species. Grassland Bush land dominates the northern part of the district. Modified biodiversity ecosystems are rich in wildlife species and include agro-systems, forest plantations, urban and systems. The high ecosystem diversity in the district implies high species diversity. These include birds, invertebrates, primates, plant species, butterflies, and reptiles, fish. 3.1.6 Population and ethnicity

The 1991 national census estimated the population of the district at 255,400. The national census conducted in 2002 estimated the population at 341,300. In 2012, the population was estimated at 440,200. The August 2014 national population census enumerated the population at 458,158.

The districts have always been a multi-ethnic area with many people of different origins and ethnic backgrounds. Among them include Baganda, the original inhabitants of this district. Other ethnic groups include Banyarwanda, Banyankole from Western Uganda, Luo speakers and

Nubians of Sudanese origin.

Figure 22: Population density by sub-county (person/km2)3

3.1.6 Land use and tenure

The total area of Luwero District is approximately 5572.2 km2 of which 5112.2 km2 is dry land and the rest is rivers and swamps. Farming, settlement, public and commercial establishments are the main land uses in the district. Agriculture is the mainstay of the district economy. It has been estimated that 85 percent of the district population are engaged in agriculture. 3.1.7. Site selection

A study area with both natural forest and Eucalyptus plantation was selected. A 20m transect emanating from each of the two land use types (Eucalyptus vs. natural forest) was set up. At each transect, a soil sample was taken from 0, 5, 10, 15 and 20m from the edge of the land use type. Four undisturbed core samples were taken using pF rings and core sampler from each of the sampling points (within 40 cm of each other on either side of the transect) and uniquely labelled prior to transportation to the lab for bulk density determination via oven-drying. Four cores were taken at each sampling point, parallel to the edge of the land use type and composited. The samples were uniquely labelled and taken to the lab for analysis. The samples were air-dried, crushed and sieved to pass through 2-mm mesh and then analyzed for the chemical properties (pH, total N, soil organic matter, available P and exchangeable bases).

Plate 3.1: Eucalyptus plantation in Nandere parish Luwero Uganda

Plate 3.2: Natural Forest in Nandere parish Luwero Uganda.

3.3 Sampling Procedure, Mapping and Sample Preparation Soil samples were collected randomly at the distance of 5m between sampling points from each eucalyptus plantation using the screw auger at a 20m transect within and near the eucalyptus land use type. Five composite samples were taken at each of the EP and NF. A digital GPS- locations was used to map coordinates of the various sampling points. These collected soil samples were placed in polythene bags and transported to the lab for analysis. Soil samples were air-dried at 65 ºC for 48 hours in shallow wooden trays in a well-ventilated place protected from rain and contamination. All soil samples were ground and made to pass through a 2-mm sieve

after all the gravel, live and dead organic residues have been removed. The samples were retained and handed in for the selected soil physico-chemical analysis in the laboratory.

Plate 3.3: soil sampling under eucalyptus Plate 3.4: Soil sampling under natural forest plantationin Nandere parish Luwero Uganda. in Nandere parish Luwero Uganda.

3.4 Laboratory analyses 3.4.1 Soil Dry Bulk Density The soil bulk density was determined 24 hours after rainfall using the Core method prescribed for undisturbed soils (Kolay, 2000). 1 to 2 cm of soil surface was removed and a core sampler driven into the soil. The soil from around the core sampler was evacuated and the soil beneath was cut off. Both ends of the core sampler were trimmed and flushed with a straight edge knife. The core sample was oven dried to a constant weight using the WTC Binder Oven at 105 oC for 24 hours. The soil bulk density was calculated using the formula in equation 2 below: Bulk Density (g cm-3) = Wt. of soil core (oven-dry basis) (g) ...... Equation 1 Vol. of soil core (cm3)

3.4.2 Soil Texture Soil texture analysis was done using the Bouyoucos hydrometer method (Hinga et al., 1980;

Klute, 1986). Oven-dry soil sample weighing 50 g was put in a plastic shaking bottle and 50 ml

Calgon (dispersion agent) added. A blank sample (without soil sample) was also prepared

(represented reading for blank sample, Rb).The mixture was placed on a mechanical shaker and shaken for 2 hours. The suspension was transferred into the sedimentation measuring cylinder, and top up to the 1000 ml mark. The mixture was stirred well using a metal plunger to bring the particles into suspension. By the use of a soil hydrometer, a reading was taken after 40 seconds

(represented the first reading, R1). After two hours, the hydrometer was used to take the second reading (R2). The second reading gave the clay content. The calculations were done as follow

(equations 2, 3, and 4):

% Clay = R2-Rb x 100…………………………………………………………………. Equation 2 50

The first reading gives the sand + Clay

That is % (sand + Clay) = R1/50 *100…………………………………………...... Equation3

Therefore, % Silt = this reading minus % clay

% silt = 100 - % (Sand + Clay) …………………………………………………………Equation 4

After getting the percentage sand, silt and clay, the soil textural triangle was used to classify the soil.

3.4.3 Soil Moisture Soil moisture was determined by the Gravimetric method as provided by Kolay (2000). A metal TM Can with a lid was weighed accurately with a Scout Pro 2000 g Balance (W1). About 100 g of soil was placed in the Can and weigh accurately along with the lid (W2). The Can covered with the lid and the soil was oven-dried for 48 hours. The Can was then removed from the oven covered tightly with the lid and placed in a desiccator to cool for about 30 minutes. After cooling, the Can was weighed accurately with the oven-dry soil in it and the weight recorded

(W3). Percent moisture content on oven-dry basis was then calculated using the formula in equation 5 below:

Percentage Soil Moisture = Wt. of moist soil – Wt. of oven dried soil * 100…………. Equation 5 Wt. of oven dried soil

3.4.4 Soil Exchangeable Bases (Ca2+, Na+ and K+) and CEC An air-dried 2 mm sieved soil sample weighing 5 g was put into an extraction bottle and 50 ml of 1.0 M NH4OAc solution at pH 7.0 was added. The bottle with its content was placed in a shaking machine and shaken for 1 minute. At the end of the shaking, the suspension was centrifuged at 3000 rpm for 5 minutes. The supernatant solution was filtered through No. 40 Whatman filter paper. 25ml aliquots of the extract were used for the determination of Ca, Na, and K (Rhoades, 1982). Calcium, potassium and sodium by Sherwood 410 Flame Photometer. The Cation Exchange Capacity (CEC) was calculated by summing all the exchangeable cations together as shown in equation 6: CEC =Exch. K+ + Exch. Ca2+ + Exch. Na+ ……………………………Equation 6

Where, CEC = Cation Exchange Capacity

Exch. = Exchangeable

3.4.5 Soil pH The pH of the soils was determined using ST 10 OHAUS pH meter. All pH measurements were performed using a 1:2.5 (w/v) soil: water ratio by weighing 20 g of 2 mm air-dried soil into a 100 ml plastic beaker and adding 50 ml distilled water. The soil was mixed with the distilled water and stirred intermittently using a stirrer for 30 minutes before pH was determined (Anderson and Ingram, 1993).

3.4.6 Soil Total Organic Carbon (TOC) Soil total organic carbon was determined by the concentrated sulphuric acid and aqueous potassium dichromate mixture. Oven-dry soil sample weighing 0.3g was scooped into a block digester tube. Added 5ml of potassium dichromate solution and 7.5ml conc. H₂ SO₄ . The tube

24 was placed in a pre-heated block at 145-155℃ for exactly 30 minutes and there after complete oxidation and external heating (Nelson and Sommers, 1975), the digester tube was removed and added about 200 mL DI water, then added 10 mL concentrated H3PO4 using a dispenser, and allowed the mixture to cool. Added 3-4 drops diphenylamine indicator and solution was titrated against ferrous ammonium sulphate solution until the color changes from violet blue to green. Also prepared three blanks, containing all reagents but no soil, and treated them in exactly the same as the soil suspensions. Calculations

Total organic carbon (%) =0.003X0.2(Vb-Vs) x 100 x 1.334 /sample

weight……………equation 7

Where:

V b = Volume of (NH4) 2SO4.FeSO4.6H2O solution required to titrate the blank (mL).

Vs = Volume of (NH4) 2SO4.FeSO4.6H2O solution required to titrate the sample (mL).

1.334 = correlation factor.

Organic matter (%) = 1.724 x total organic carbon (%)……………equation 8

3.4.7 Soil Total Nitrogen (TN) Soil TN was determined by the Kjeldahl distillation method as provided by Bashour and Sayegh (2007). Air-dried soil sample weighing 0.3g was weighed into 500 ml Kjeldahl flask. Five millilitres distilled water was added to wet the soil thoroughly. A scoop of digestion accelerator mixture (sulphuric-salicylic acid mixture) was added to each tube. Five millilitres of conc.

H2SO4 was added and the mixture was digested by heating on rack in a fume cupboard gently at first until vigorous effervescence subsided. Heating was continued for 1 hour after digest was clear with no charred organic matter remaining. The digest was cooled and 20 ml distilled water added and the soil was allowed to settle. The supernatant solution was decanted into 100 ml volumetric flask. The process was repeated washing the sand particles and quantitatively transferring supernatant to the flask and made to the 100 ml mark. Five millilitres of 40 % NaOH and 100 ml of distilled water were added. The mixture was distilled, collecting the

distillate into 5 ml of the boric acid (H3BO3) solution -indicator mixture. The distillate was

titrated with 0.01 M H2SO4 from green to pinkish end point and the titre value recorded. The soil TN was calculated using the formula in equation 9:

Kjeldahl N (%) = (ST - BT) X 0.005 X14 x 50x100 /0.3g x 5x1000 ………………equation 9

ST = ml of standard acid(HCl) with sample

titration

SB = ml of standard acid (HCl) with blank

titration

3.4.8 Soil Available Phosphorus Soil available phosphorus was determined by the Mehlich double acid method (Maghanga etal., 2012). An oven–dried soil sample weighing 2.0 g was put into a 100 ml shaking bottle and 20

ml of the extracting solution (a mixture of 0.1 M HCl and 0.025 M H2SO4) added. The mixture was shaken for 5 minutes and filtered through a Whatman No. 40 filter paper. A 5 ml of the extract was transferred to a 25 ml flask and diluted to the mark. 5 millilitres of L-Ascorbic Acid was added to each sample and waited for the blue colour formation. After 5 minutes the concentration -of P in the filtrate was determined at 830 nm wavelength using JENWAY MODEL 6405 Ultra Violet /VIS Spectrophotometer.

CHAPTER FOUR RESULTS AND DISCUSSION 4.1 Effects of Eucalyptus Plantation and Native Forest on Soil physico- chemical Properties There was no significant difference at 95% confidence level between the EP and NF for soil pH, organic matter, available phosphorus, exchangeable K (Table 2) unlike for soil bulk density, TN,

EC and exchangeable calcium with respect to their intercepts.

Table 2: Simple linear regression parameters for soil organic matter, available phosphorus (P), exchangeable potassium (Exch. K) and soil pH along the transect from Eucalyptus and from natural forest land in Luwero district, central Uganda

Parameter Land use Intercept Statistics Slope Statistics t (6) t pr t (6) t pr Soil organic Eucalyptus 1.94 ± 0.286a 6.78 <0.001 -0.015 ± 0.0234a -0.65 0.540 matter (%) Natural forest 2.31 ± 0.286a 8.05 <0.001 0.021 ± 0.0234a 0.90 0.404 F pr 0.083 R2 (%) 46.8 Available P Eucalyptus 3.28 ± 1.050a 3.10 0.021 -0.015 ± 0.0234a -0.65 0.540 (ppm) Natural forest 2.21 ± 1.050a 2.09 0.081 0.021 ± 0.0331a 1.10 0.315 F pr 0.139 R2 (%) 36.1 Exch. K Eucalyptus 0.41 ± 0.221a 1.85 0.114 -0.016 ± 0.0180a -0.91 0.400 -1 (cmolc kg ) Natural forest 0.22 ± 0.221a 0.99 0.360 0.013 ± 0.0180a 0.71 0.500 F pr 0.665 R2 (%) - Soil pH Eucalyptus 5.36 ± 0.284a 18.86 <0.001 0.037 ± 0.0232a 1.58 0.164 Natural forest 6.05 ± 0.284a 21.27 <0.001 -0.016 ± 0.0232a -0.70 0.512 F pr 0.403 R2 (%) 4.7 Regression parameter values for the same variable followed by the same lower case letter are not significantly different at 95% confidence level

The null hypothesis that no significant differences exist between EP and NF in their intercepts for the linear relationship of soil physico-chemical properties with distance from the edge was therefore rejected for pH, organic matter, available phosphorus, exchangeable K, soil bulk

27 density, TN, EC and exchangeable calcium. There was no significant difference in the gradient for the linear relationship at 95% confidence level between the EP and NF for soil pH, organic matter, available phosphorus, exchangeable K unlike for soil bulk density, TN, EC and exchangeable calcium.Therefore, the null hypothesis that no significant differences in the gradient for the linear relationship exist between EP and NF in their effects on soil physico- chemical properties was accepted for pH, Organic matter, available phosphorus, exchangeable

K, soil bulk density, TN, EC and exchangeable calcium.

4.1.1 Soil Bulk Density The highest values of soil bulk density recorded in the EP and NF soils were 1.6 and 0.99 g cm-3 respectively (Fig 3). The lowest values of soil bulk density for the EP and NF soils recorded were 1.1 and 0.6 g cm-3 respectively. And the median values recorded were 1.3 and 0.7 g cm-3 for EP and NF soils respectively. The bulk density for EP soils had much longer whisker than that for NF soils therefore bulk density varied more widely between the eucalyptus and natural forest land use types. The results revealed that bulk density of EP soils decreased as the distance away from edge of eucalyptus trees increased, contrary to the natural forest where bulk density increased with increased distance away from edge of natural forest(Fig. 4). Soil bulk density is used as a measure of soil compaction and health (Field Ecology Guide, 1998). Higher soil bulk density means less amount of water held in the soil at field capacity and at lower soil bulk densities, soils are less compacted and are able to retain water (Kakaire et al., 2015). Ravina (2012) found a higher soil bulk density of 1.24 g cm-3 under Eucalyptus spp. plantation compared to 0.66 g cm-3 in native forest in a Brazilian soil (0-15 cm). Kolay (2000) indicated that bulk density of productive natural soils generally ranges from 1.1 to 1.5 g cm-3. Since the soil bulk densities found in the EP were very higher than for NF soils and were not within critical range, the findings from the study confirmed those of Aweto and Moleele (2005), who concluded that Eucalyptus spp. plantations increased soil bulk density more than the native forest in Botswana.

4.1.2 Soil pH The pH of the soils ranged from 5.0 to 6.1 and 5.4 to 6.3 for EP and NF respectively (Fig 3). Though a bit lower, the pH of the soils under the EP was similarto that found in the NF soil. The low pH of the EP is consistent with the low exchangeable bases such as Ca and K found in the EP soils. Mensah (2015) reported that the bases released by litter decay can check acidification. Aweto and Moleele (2005) argued that Eucalyptus spp. immobilize soil exchangeable bases especially calcium and magnesium leading to low soil pH. Inferring from Horneck et al. (2011) that most nutrient elements occur in available forms at a soil pH range of 5.5 to 6.5, the NF soil was likely to be more fertile with a greater availability of plant nutrients compared to the EP soil. These findings also agree with those of Aweto and Moleele (2005), who found that soil pH was significantly lower in soil under Eucalyptus in both the 0–10 and 10–20 cm soil depths in Botswana, compared to native Acacia forest. Similarly, Sanginga and Swift (1992) reported that soil pH was lower under the plantation of Eucalyptus grandis than in the native savanna woodland in Zimbabwe. Jaiyebo (1998) concluded that the soil acidity under Eucalyptus spp. was likely to increase with time.

Avai Bulk lable den pho sity sph (g orus cm- mg kg-1) 3)

Exc Exc han han gea gea ble ble pota calci ssiu um m (cm (cm olck olck g-1) g-1)

Tota Soil l orga nitro nic gen matt (%) er (%)

Soil Elec pH trical con ducti vity (µS cm- 1)

Figure 3: Variation in dry bulk density, available phosphorus, exchangeable potassium, exchangeable calcium, soil organic matter, total nitrogen, soil pH and electrical conductivity along the Eucalyptus and natural forest land use transects in Luwero district, central Uganda

4.1.3 Soil electrical conductivity(EC) The highest EC recorded was 70 and 200µS cm-1 for EP soils and NF respectively while minimum values recorded for EC in EP and NF soils were 55 and 85 µS cm-1 respectively (Fig. 3).This means the soils were non-saline in nature, despite significant increase with distance from the edge of NF land use type, unlike with that from EP land use type (Fig 4). Contrary to the NF the EC values increased with increased distance away from edge of NF land use type.

1.8 Eucalyptus 250

Natural forest

)

1 -

) 1.5 3

- 200 y = 79.8 + 5.36x 1.2 y = 1.7 - 0.0306x 150 0.9 y = 0.63 + 0.0132x 100 0.6 y = 60.6 - 0.18x

Dry bulk density (g cm (g density bulk Dry 50 0.3 R2 = 90.8% R2 = 83.4%

F pr < 0.001 cm (µS conductivity Electrical F pr = 0.003 0 0 0 5 10 15 20 0 5 10 15 20 Distance from edge of land use (m) Distance from edge of land use (m)

2.5 0.36

)

1 -

kg y = 1.973 + 0.0035x c 2.0 0.30

) y = 0.219 + 0.0014x 0.24 1.5 0.18 1.0

0.12 y = 0.1025 - 0.00047x Total nitrogen (% nitrogen Total y = 0.585 - 0.0097x 0.5 R2 = 81.6% 0.06 R2 = 69.4%

F pr = 0.004 F pr = 0.017 Exchangeable calcium (cmol calcium Exchangeable 0.0 0.00 0 5 10 15 20 0 5 10 15 20 Distance from edge of land use (m) Distance from edge of land use (m)

Figure 4: Variation of dry bulk density, electrical conductivity, exchangeable calcium and total nitrogen with distance from the edge of Eucalyptus and edge of natural forest in Luwero district, central Uganda Due to the fact that along edge of natural forest was other land uses such as banana plantations which were managed by application of manures and compost into the soils therefore causing increased organic matter content and hence makes soil to be rich in nutrients,that may have contributed to greater electrical conductivity.

4.1.4 Soil Organic matter (SOM) The highest percentages of SOM recorded in the EP and NF were 1.85 and 2.75 % respectively

(Fig 3) and medium percentages recorded were 1.75 and 2.35% for EP and NF respectively together with lowest values recorded for EP and NF also were 1.51 and 2.25% respectively. The linear regression analysis showed that OM in soils of the EP was significantly lower (t (6) = -

0.65, P >. 5) than in the soils of NF. The organic matter contents were lower in EP soils than those of natural forest. The relatively lower SOM in EP soils is possibly due to the differences in the rates of plant litter decomposition under the EP and NF vegetation (Demessie et al., 2012).

Sharmsher et al. (2002) found that 83 % of the soil samples under Eucalyptus spp. plantation areas were found low (< 1%) in organic matter. Jan et al. (1996) also made observation of reduced organic matter concentrations in Eucalyptus spp. soils relative to Shorearobusta native forest soil in Uttar Pradesh, Pakistan. Similar results have also been found by Bernhard-Reversat

(1998) in Keur Maktar, Senegal, where the amount of organic carbon in the 0–10 cm depth of native Acaciaseyal woodland was twice the concentration in corresponding layer of Eucalyptus spp. plantation. Again, in the Sudan Savanna zone of Cameroon, total carbon in the topsoil of

Eucalyptus spp. plantation fallow declined while that of Acasia polyacantha plantation fallow increased significantly over a period of seven years (Brady and Weil, 1996). Furthermore, the organic carbon levels under 5 years and 10 years’ plantations of Eucalyptus spp. plantation, near

Pointe Noire, Congo, were below the level in a savanna control plot, suggesting that there was no substantial build-up of organic carbon in soil under the Eucalyptus spp. plantation during the first 10 years following plantation establishment (Brady and Weil, 1996).

4.1.5 Soil Total Nitrogen (TN) The highest concentrations of soil TN recorded were 0.13 and 0.27 % for EP and NF respectively (Fig 3). The lowest concentrations of TN recorded were 0.054 and 0.20% for EP and NF respectively. The TN found in the EP soils was very low compared to that in the NF. The results(graph) revealed that TN decreased as distance from the edge of EP increased. Contrary to the natural forest where TN increased as distance from edge of NF trees increased. Alemie (2009) found that concentration of soil TN decreased under plantations of Eucalyptus spp. in Ethiopia. Nitrogen concentration in soils is reflected in part by the rate of decomposition of the plant material (Cao et al., 2010). Previous authors have reported that Eucalyptus spp. produce litter with low nutrient concentrations, which decomposes slowly to release low concentrations of nutrients (Aweto and Moleele, 2005; Baber et al., 2006; Cao et al., 2010; Demessie et al., 2012), and coupled with the prevailing low soil pH, N mineralization from the EP litter would have been considerably slow. Additionally, Tererai et al. (2014) attributed the low total available N under Eucalyptus spp. plantations in South Africa to the high soil nutrient uptake rates of the plant. Thus, as observed in this study, the lower TN concentration found in the EP soils compared to those of the NF was not unexpected. Monoculture Eucalyptus spp. plantations have been found to alter the forest species diversity due to its allelopathic effects (Pandey et al., 2007; Wang et al., 2007). The sparse undergrowth observed under the EP was indicative of reduced biodiversity. As regards the EP, the few plant species found growing were usually located on the fringes of the EP some 2 or 3 m away from the plantation. On the contrary, the NF was associated with undergrowth consisting of many different plant species including Cynodon dactylon, Podocarpus sp., Acacia mearnsii, Croton macrostachyus, and many other legume species. This observation indicates greater biodiversity and hence a greater variety of litter fall, which would possibly result in balanced ecosystem functioning due to more soil organic matter accumulation and probably a faster rate of decomposition to form humus. These observations confirm report by FAO (1993) that the biodiversity of a natural forest is often greater than that of Eucalyptus spp. plantations as the species composition of natural ecosystems are very diverse, whilst that of Eucalyptus spp. plantations is limited. Forrester et al. (2006) also demonstrated that mixing Acacia mearnsii with Eucalyptus globulus increased the quantity and rates of N and P cycled through aboveground litter fall when compared with

Eucalyptus globulus monocultures. Other researchers have also found higher quantities of N and P cycled through litter fall in mixtures of N-fixing trees and non-N-fixing than non-leguminous monoculture Eucalyptus spp. plantations alone (Binkley and Ryan, 1998; Parrotta, 1999).

4.1.6 Soil Exchangeable Cations (Ca2+and K+) Fig 3 shows the concentrations of exchangeable cations. Among the exchangeable cations, the concentration of exchangeable calcium was highest with values 1.2 and 2.2 c mol kg-1 for soils under EP and NF respectively. And the highest concentrations of exchangeable potassium recorded were 0.3 and 0.42 c mol kg-1 for EP and NF soils respectively. The exchangeable calcium as its revealed by results(graphs) that concentration of calcium decreased gradually as distance from edge of both EP and NF land use types increased. This can be because of presence of other land uses such as crop growing mainly perennials (bananas, coffee, sugar canes etc). However, these were significantly different from each other. But no significant differences were found among the concentrations of Ca and K in the EP and NF soils. This observation could be explained as the EP soil was strongly acidic compared to the NF soil. Bailey et al. (2005) reported that acidification usually leads to depletion of the soil base cations (e.g., K+, Mg2+, Ca2+) due to replacement of the basic cations by Al3+ and H+ ions at the exchange site. Aweto and Moleele (2005) also indicated that low soil pH leads to low soil base saturation. Contrary to observations made in this study, Aweto and Moleele (2005) found lower exchangeable nutrient cation (calcium, magnesium and potassium) of both 0–10 and 10–20 cm depths in soil under Eucalyptus spp. plantation, explaining that the Eucalyptus sp. immobilizes soil nutrients in their standing biomass faster than they recycle such nutrients back to the topsoil, which resulted in soil exchangeable bases depletion over time. Bernhard-Reversat (1988) and Jaiyebo (1998) indicated a reduced concentration of exchangeable bases under plantations of Eucalyptus spp. in Senegal and northern Nigeria, respectively.

4.1.7 Soil Available Phosphorus (P) The highest values of soil available P recorded were 4.2 and 5.5 mg kg-1 for EP and NF, respectively (fig 3) and mean concentration of available P recorded was 3.8 and 3.9 mg kg-1 for EP and NF respectively. The concentration of available P in the EP soil was significantly lower

(t (6) = -0.65, P ˂. 5) compared to that in the NF soil. The available P in the EP soil falls below the medium sufficiency range of at least 16 mg kg-1. The relatively lower available P registered for the EP soil may be due to the low pH value which might have resulted in higher amount of soil available P fixed/adsorbed. Aweto and Moleele (2005) reported that Eucalyptus spp. have a higher capacity of immobilizing phosphorus, and make them inaccessible for plant use.

Berendse (1998) adds that at soil pH of below 5.5, soil trace nutrients like Aluminium (Al) availability increases to levels that are unsuitable for most plant growth and soil soluble phosphorus (P) tends to form insoluble compounds with Al and Fe in acidic soils, which are inaccessible to native plants. These findings are in agreement with those of Polglase et al.

(1992), who found that the concentration of inorganic phosphorus in the 0–5 cm depth of soil under Eucalyptus spp. plantation declined from an initial concentration of 34 to 2.3 µg g−1 when the plantation attained the age of 16 years in Australia. Aweto and Moleele (2005) also found that available phosphorus was low (below 5µg g−1) in soil under Eucalyptus spp. plantation in both the 0–10 and 10–20 cm depths of soil in Botswana. Furthermore, Alemie (2009) reported that available phosphorus concentration in Eucalyptus spp. plantation within 2 cm depth of soil was in the very low range (< 5 mg kg-1) in Ethiopia.

CHAPTER FIVE 5.0 SUMMARY OF FINDINGS, CONCLUSION AND RECOMMENDATIONS 5.1 Summary of Key Findings Based on soil samples collected from the field and laboratory analyses the following findings were made:

The soil bulk density under the Eucalyptus spp. plantation (1.6 g cm-3) was significantly higher compared to that of the native forest (1.06 g cm-3). The soil bulk density under the

Eucalyptusspp. Plantation. The bulk density for EP soils had much longer whisker than that for

NF soils therefore bulk density varied more widely between the eucalyptus and natural forest land use types. The results (graphs) revealed that bulk density of EP soils decreased as the distance away from edge of eucalyptus trees increased, contrary to the natural forest where bulk density increased with increased distance away from edge of natural forest.

The pH level of the Eucalyptus spp. plantation (5.2) was significantly lower compared with the control native forest (6.5). The total N found for the Eucalyptus spp. (0.13 %) was significantly lower compared to that of the control native forest (0.27 %). The SOM recorded as 1.60 and

2.73 % for soils under Eucalyptus spp. plantation and native forest respectively. The level of

SOM in soils of the Eucalyptus spp. plantation was found to be significantly lower (P < 0.05) than that of the soils of the control native forest. Lower exchangeable cations (Ca and K) concentration was recorded under the Eucalyptus spp. plantation compared with the control native forest. However, there was no significant differences in the gradient and intercept for linear relationship between two land use types with distance from edge of the EP and NF trees.

The level of exchangeable calcium (1.2 and 2.2 c mol kg-1) and potassium (0.3 and 0.42 c mol kg-1)were found between Eucalyptus spp. plantation and native forest respectively.

Accumulation of soil available P under Eucalyptus spp. was significantly lower (4.4) compared to that in the NF soil (6.0 mg kg-1). The EC values were higher in NF soils than that of EP soils.

5.2 Conclusions The results obtained in this study indicated that Eucalyptus spp. plantation significantly increased the soil bulk density and lowered the soil pH, leading to a decline in soil total nitrogen and soil total organic carbon concentrations. Soils under the Eucalyptus spp. were also found to be acidic which would have caused decline in soil available phosphorus concentration, due to increased phosphorus fixation rendering it unavailable for plant use. Decomposition of the Eucalyptus spp. litter also caused significant decline in concentrations of plant total N and total P, due to higher rates of immobilization of these nutrients in the Eucalyptus spp. litter.

In conclusion, the study rejected the null hypothesis thatno significant differences exist between

Eucalyptus spp. plantations and native forest in their effects on soil physical properties (soil bulk density). The null hypothesis that no significant difference exist between EP and NF in their effects on soil chemical properties was also rejected for soil pH, soil available phosphorus, total nitrogen, total organic carbon, soil exchangeable bases (Ca, K). Also accepted for The null hypothesis that no significant differences in the gradient for linear relationship between two land use types with distance from edge of the EP and NF for soil pH, Organic matter, available phosphorus, exchangeable K. and rejected the null hypothesis that no significant difference in intercepts for linear relationship between two land use types with distance from edge of the EP and NF for soil bulk density, soil pH, Organic matter, available phosphorus, TN, EC , exchangeable calcium and exchangeable K. The intercepts for these parameters were significantly different for both EP and NF. The study demonstrated that sole cultivation of

Eucalyptus had the tendency to affect the soil physico-chemical properties and so it is advisable to interplant Eucalyptus spp. with other leguminous species, in addition to good agronomic

practices such as nitrogenous and phosphate fertilizer application to replenish the loss of

essential soil nutrients such as N and P associated with Eucalyptus planting and also to ensure

sustainability of Eucalyptus cultivation on the soil resources.

5.3 Recommendations i. The soil should be possibly limed to increase soil pH and base saturation at the end of each rotation before replanting. This will make soil exchangeable cations (such as Ca, Mg, and K), soil nitrogen and phosphorus which are mainly deficient in Eucalyptus plantations available for plant growth. ii. Planting of Eucalyptus speciesshould be supplemented with fertilizer application using

appropriate phosphate fertilizers forex ample NPK+TE, DAP. This will ensure that soil nutrients

such as nitrogen and phosphorus are not exhausted plus trace elements such as boron. iii. Leguminous species should be planted at the site where Eucalyptus spp. plantation was

harvested previously to help quicken and restore the lost fertility. This will ensure successful

plantation of the next crop on the site. iv. Wise decisions should be made about eucalyptus species to plant, where to plant, how to plant

and how to manage it, depending on the purpose of the planting. v. Policy on Eucalyptus planting that it should be planted 15 m away from water sources and 6 m

away from the neighbor’s farm should be enforced.

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