Sustainability Science © Integrated Research System for Sustainability Science and Springer 2007 10.1007/s11625-007-0034-9

Original Article Evaluation of carbon stock variation in Northern Italian soils over the last 70 years

Ciro Gardi1 and Francesca Sconosciuto1

(1) Department of Environmental Science, University of Parma, Viale delle Scienze, 33A, 43100 Parma,

Ciro Gardi Email: [email protected]

Received: 4 December 2006 Accepted: 23 May 2007 Published online: 27 July 2007

Abstract Carbon (C) sequestration in soils is gaining increasing acceptance as a means of reducing net carbon dioxide (CO2) emissions to the atmosphere. Numerous studies on the global carbon budget suggest that terrestrial ecosystems in the mid-latitudes of the Northern Hemisphere act as a large carbon sink of atmospheric CO2. However, most of the soils of North America, Australia, New Zealand, South Africa and Eastern Europe lost a great part of their organic carbon pool on conversion from natural to agricultural ecosystems during the explosion of pioneer agriculture, and in Western Europe the adoption of modern agriculture after the Second World War led to a drastic reduction in soil organic carbon content. The depletion of organic matter is often indicated as one of the main effects on soil, and the storage of organic carbon in the soil is a means of improve the quality of soils and mitigating the effects of greenhouse gas emission. The soil organic carbon in an area of Northern Italy over the last 70 years has been assessed In this study. The variation of top soil organic carbon (SOC) ranged from −60.3 to +6.7%; the average reduction of SOC, caused by agriculture intensification, was 39.3%. This process was not uniform, but related to trends in land use and agriculture change. For the area studied (1,394 km2) there was an estimated release of 5 Tg CO2-C to the atmosphere from the upper 30 cm of soil in the period 1935–1990.

Keywords Carbon stock - Land use - Permanent grasslands - Carbon sequestration

Introduction

Atmospheric levels of CO2 and other greenhouse gases have been increasing (IPCC 2001). Increasing concern over the effect these gases may have on world climate systems has resulted in increased awareness of the global carbon cycle. Studies of the global carbon budget suggest that terrestrial ecosystems in the mid-latitudes of the Northern Hemisphere act as a large carbon sink of atmospheric CO2 (Tans et al. 1990; Ciais et al. 1995). In particular, recent analyses based on atmospheric CO2 observations and models indicate that North America acts as the largest carbon sink (Fan et al. 1998), with storage of 1.7 Pg C year−1 during 1988–1992, whereas other studies show that North America carbon storage for the same period was much lower (Field and Fung 1999; Houghton et al. 1999; Tian et al. 1999; Schimel et al. 2000). Other terrestrial carbon sinks are located in Northern Asia (Bousquet et al. 1999; Wang et al. 2002) and in Europe (Cannell 2003).

Carbon (C) sequestration in soils is gaining increasing acceptance as a means of reducing net carbon dioxide (CO2) emissions to the atmosphere. The global quantity of carbon (C) present to a depth of 1 m in soil organic material (SOM) is about twice the 750 Pg present in the atmosphere as CO2, i.e. approximately 1,576 Pg (Eswaran et al. 1993) or 1,394 Pg (Post et al. 1982). The size of the SOM pool indicates that even small changes in the global stock of SOM could cause a significant change in atmospheric CO2 and, consequently, the global C cycle (Schimel et al. 1994). Thus one of the largest terms of the global C balance is the exchange of C between soils and the atmosphere. The balance between the quantity and quality of inputs on the one hand, and the decomposition of SOM and the weathering of carbonate (naturally occurring or from applied lime) on the other hand, will determine whether the soils are net sources or sinks for CO2. Among agricultural soils, other factors, for example tillage operations, manure and fertilizer application and crop rotation, and land use change (Ford-Robertson et al. 1999; Schrot et al. 2002; Fearnside 1997; Kätterer and Andrén 1999) can affect the soil organic matter balance. Reduction in tillage and no tillage generally lead to accumulation of organic carbon in the soil (Lal et al. 1994; Campbell et al. 1995; Paustian et al. 1997), although some researchers have shown that the higher SOC concentration in the upper centimeters of the soil is compensated by less SOC at lower depths (Angers et al. 1997; Needelman et al. 1999; Wander and Yang 2000). It has been estimated that 50 years of conventional agriculture can reduce soil organic carbon by 50%, whereas addition of manure can double soil carbon in 40 years (Tilman 1998). Data on the world’s agricultural soils are contrasting, showing a trend of carbon accumulation if referring to the recent past or the near future whereas comparison with the 20th or 19th century indicates a general reduction of soil organic carbon content. Recent projections of agricultural soil carbon accumulation estimate a rate of approximately 17–39 Mt carbon year−1 for the United States (Lal et al. 1998) and a rate of 40 Mt carbon year−1 for Canada (Dumanski et al. 1998; Lindwall 1999). A net carbon accumulation of 340 Mt carbon year−1 has also been estimated for the former Soviet Union in the event of application of no-tillage techniques to all suitable soils (Kolchugina et al. 1995). However, if we consider the variation over the last century (20th), it is evident that most of the soils of North America, Australia, New Zealand, South Africa, and Eastern Europe have lost a great part of their organic carbon pool on conversion from natural to agricultural ecosystems during the explosion of pioneer agriculture (Huggins et al. 1998; Lal 2001). In Western Europe the transition toward modern agriculture after the Second World War, with the adoption of short crop rotation or monoculture, deep tillage operations, and the progressive abandonment of manure application, led to drastic reductions in soil organic carbon content. The depletion of organic matter is indicated by the European Soil Bureau to be one of the main effects on soil, and the Sixth Environment Action Programme of the European Community proposes storage of organic carbon in the soil as a means of improving the quality of soils and mitigating the effects of greenhouse gas emission. The soil organic carbon in an area of Northern Italy over the last 70 years has been assessed In this study.

Materials and methods Study area

The study area is located in the Po Valley (Northern Italy), the main Italian alluvial plain, and includes 27 municipalities in the Provinces of Modena and (Fig. 1). The area investigated was 1,394 km2; soil types, according to the Fao-Unesco classification, were cambisols, calcisols, and vertisols. The climate is subhumid, with rainfall ranging from 570 to 1,163 mm year−1 and the mean annual temperature is 12.7°C.

Fig. 1 Location of the study area

This area was of interest because of different land-use dynamics within the territory and the availability of historical data on soil properties such as organic carbon content. The entire area is characterized by intensive agriculture with a prevalence of cereals and industrial crops in the eastern part and livestock husbandry in the west.

Data sets

Over 3,800 top soil organic carbon (SOC) measurements, in two different data sets, were used for this investigation. The first data set (781 samples) was produced in the period 1935–1950 by the Italian Sugarbeet Association (ISA). The analytical method used to obtain these data was the “Istcherekow” method, based on wet oxidation of organic carbon by potassium permanganate and oxalic acid (Draghetti 1957).

The second data set, consisting of 3,038 samples, was produced in the period 1980–1990 by the Soil Survey Office of the Emilia-Romagna Region, and soil organic carbon was determined by the Walkley and Black (1934) method, based on wet oxidation of organic carbon by chromium dichromate and sulfuric acid. For comparison of the existing data correlation between the two methods was investigated. On a set of 30 soil samples the organic carbon was determined using both the Istcherekow and Walkley–Black methods; regression analysis was performed on the data obtained and the regression equation was used to convert the ISA data (Fig. 2).

Fig. 2 Results from regression analysis of SOC data determined by use of the Istcherekow and Walkley–Black methods on 30 soil samples

Detailed data on land use, crop type, and animal husbandry were also obtained for the two periods; the data were derived from the agriculture inventories conducted periodically by the Italian government.

All data have been georeferenced using ArcView, and a historical comparison of agricultural land use, livestock husbandry, and soil organic carbon storage for 1935–1990 was compiled using the spatial analysis tools of a geographic information system (GIS). Statistical analysis of data was by one-way ANOVA; before ANOVA the homogeneity of the variances was tested by use of Levene statistics.

Results and discussion Changes in land use and agriculture structure

During the period studied a significant change in the land-use pattern of the entire investigated area was observed. Table 1 shows the variation of land-use types, crops, and livestock, grouped into main classes and divided among municipalities. The entire area was subject to intensification of land use. The main changes between the two periods are reduction in the number of farms and contraction of permanent grasslands and alfalfa area.

Table 1 Variations of land use, agriculture structure, and livestock in the 27 municipalities of the studied area between 1929 and 1990 Forage Permanent Farm mean size crops P grasslands (% Cows ha−1 (ha) (% h a.a.) Municipality a.a.) r.v. r.v. r.v. r.v. r 1929 1990 1929 1990 1929 1990 1929 1990 1929 1990 (%) (%) (%) (%) (% Bagnolo in 5.5 6.9 25.0 11.7 8.5 −28.0 40.5 1.7 −95.8 1.07 1.46 37.1 0.84 6.86 7 Piano Cadelbosco di 6.3 7.5 18.4 13.1 11.7 −11.0 37.7 35.6 −5.5 1.21 1.41 16.1 0.34 12.83 3 Sopra Campagnola 6.5 8.3 28.5 1.2 0.7 −41.5 43.5 22.5 −48.3 1.33 2.12 59.3 0.74 3.39 3 Emilia 4.3 8.1 87.3 59.0 56.6 −4.1 29.6 19.8 −33.1 1.44 2.09 45.7 0.90 8.52 8 Campogalliano 3.7 6.2 66.0 6.3 0.8 −86.7 36.0 24.5 −32.1 1.34 1.07 −20.1 1.16 6.12 4 Carpi 3.8 6.5 71.5 2.1 0.4 −80.8 39.1 26.8 −31.5 1.11 0.70 −36.6 0.76 1.93 1 Castelnuovo di 4.8 9.1 90.1 27.9 33.1 18.5 29.5 36.7 24.6 1.27 1.85 45.7 0.75 3.18 3 Sotto Cavriago 1.1 5.2 390.8 27.2 0.8 −96.9 26.3 77.9 196.4 1.27 4.16 227.6 1.04 9.66 8 Correggio 5.9 5.8 −2.1 1.5 1.1 −23.3 47.1 30.7 −34.9 1.24 1.46 17.6 0.87 6.85 6 6.7 11.3 69.0 0.6 0.0 −100.0 48.7 17.7 −63.7 1.29 1.20 −6.8 0.77 1.96 1 6.7 10.9 62.0 15.6 5.0 −67.9 33.2 36.1 8.7 1.41 2.06 46.0 0.80 3.34 3 4.7 6.0 28.0 2.4 2.1 −12.9 41.8 23.2 −44.4 1.17 1.97 69.0 0.54 0.63 1 3.9 7.0 77.8 3.1 0.1 −98.1 41.3 48.4 17.1 1.52 1.77 16.8 0.84 6.61 6 5.3 7.6 43.4 0.2 0.1 −37.0 42.4 40.6 −4.3 1.49 3.17 112.4 1.29 7.18 4 Modena 3.6 10.6 190.5 13.4 3.4 −74.2 31.9 22.7 −28.9 1.40 1.59 13.7 0.97 5.76 4 Montecchio 5.2 5.8 11.7 76.0 68.4 −10.0 22.1 10.7 −51.5 1.47 3.20 118.0 1.17 1.08 − Emilia 5.8 12.6 116.6 0.6 0.4 −32.5 49.5 33.2 −32.9 1.18 3.32 180.5 0.74 5.11 5 Novi di 2.8 5.7 104.3 2.9 0.0 −100.0 41.1 29.2 −29.0 1.15 1.49 29.4 1.05 2.84 1 Modena 5.4 7.7 42.6 18.3 5.9 −67.6 53.4 44.8 −16.1 1.07 1.43 33.3 0.05 0.50 9 Reggio Emilia 5.1 8.8 72.2 30.2 26.6 −11.8 34.6 34.3 −1.1 1.30 1.13 −13.2 2.37 4.87 1 7.8 12.2 55.7 0.6 0.0 −100.0 45.4 45.1 −0.7 1.61 3.01 87.0 0.97 4.73 3 6.3 7.0 11.7 1.3 0.8 −38.8 42.9 28.1 −34.5 1.17 1.85 58.2 0.66 2.95 3 Rolo 4.6 6.5 41.8 0.2 0.0 −100.0 50.5 21.9 −56.6 1.33 1.45 8.8 1.03 7.48 6 5.3 6.4 20.5 10.9 4.4 −59.9 41.1 33.7 −17.9 1.28 2.06 60.2 1.08 6.57 5 Sant’Ilario 6.9 6.4 −6.1 40.3 40.2 −0.2 27.5 7.7 −72.1 1.27 1.83 43.7 0.98 6.66 5 d’Enza San Martino in 5.0 12.1 140.7 5.6 1.4 −75.3 44.2 32.3 −27.0 2.23 1.47 −34.3 0.91 3.40 2 Rio r.v., relative variation; a.a., agricultural area In relative terms, the most important variation in land use affected the permanent grasslands, which represented 13.8% of the investigated area in 1929 and 10.1% in 1990—a relative reduction of 27%. The same behavior, even more pronounced, characterized most of the agricultural areas of Northern Italy, where traditional irrigated permanent grasslands (“prati stabili”, “marcite”) were replaced by corn for silage, Italian ryegrass, and alfalfa meadows. In the area investigated the poliennial meadows, for example alfalfa, were also subjected to a significant reduction, from 39.5% in 1929 to 30.1% in 1990 (−23%). In this case the variation was determined by the progressive introduction of corn silage in the dairy farms (except for “Parmesan cheese” farms).

During the period considered in this study, Italian agriculture experienced a deep structural change that affected both the social structure of the rural areas and the technical aspects of agricultural production—the index of agricultural productivity doubled and the gross output per worker in agriculture increased by a factor of 30 (Federico and Malanima 2002). In the same period the number of farms in the area changed from 29,875 to 12,553. The number of animals, mainly cattle and pigs, was substantially stable, but it is important to note that between the two periods the size of livestock husbandry farms increased dramatically. Also, cropping intensities steadily increased over the period studied, as a result of the promotion of industrial crops and simpler crop rotation. The traditional farming system, based on integrated cultivation and livestock husbandry, was progressively abandoned, leading to strong specialization by farms in livestock and vegetable crops.

Municipalities showed notably different land-use changes. The western part of the area (Cadelbosco, Campegine, Montecchio, Reggio Emilia, S. Ilario) experienced only minor changes, whereas the eastern and the central part of the area were subjected to the agroecosystem intensification described above.

Changes in soil organic carbon

The average SOC content, with standard deviations, for the 27 municipalities and for the two periods are reported in Table 2. The results from ANOVA are indicative of significant differences between the two periods (P < 0.001) and significant differences between municipalities (P < 0.001). However, the SOC reduction was not uniform over the entire area. In some areas, where livestock husbandry has increased, there was stability or a slight decrease in SOC values, whereas other areas, characterized by a marked transition toward industrial agriculture, experienced the largest SOC reduction (Fig. 3). In the Modena and Carpi municipalities, where the reduction of permanent grasslands and meadow area was greater, the decrease of SOC was 59 and 60% respectively. S. Ilario and Cavriago were the only municipalities with stable SOC in the 1935–1990 period (+6.7 and +0.5% SOC, respectively); S. Ilario was characterized by a large and stable area of permanent grasslands (40% of the agricultural area) whereas Cavriago had a marked reduction in permanent grasslands but a strong increase in alfalfa meadows. Furthermore, Cavriago had the largest increase in the number of cows per hectare. The relationship between livestock husbandry and SOC seems to be determined by both manuring practices and the soil stabilizing effect of poliennial or perennial forage crops.

Table 2 Mean soil organic carbon (SOC), standard deviations (s.d.), and number of samples in the 27 municipalities of the investigated area in 1935 and 1990 1935 1990 SOC SOC Variation Municipality (g kg−1) Number of (g kg−1) Number of (%) samples samples Mean s.d. Mean s.d. 25.48 7.18 23 14.79 4.84 74 −41.94 Cadelbosco di 24.06 5.88 36 14.30 3.97 47 −40.54 Sopra Campagnola 24.72 4.77 22 12.50 3.34 117 −49.43 Emilia Campegine 24.30 4.60 12 13.65 3.61 52 −43.82 Campogalliano 29.11 6.34 13 16.83 5.19 69 −42.19 Carpi 28.37 5.71 47 11.27 2.84 518 −60.28 Castelnuovo di 21.72 4.73 26 15.71 5.44 53 −27.68 Sotto Cavriago 20.56 6.14 7 20.67 9.24 11 0.54 Correggio 22.77 4.04 54 13.01 3.65 228 −42.89 Fabbrico 24.89 2.24 19 14.37 3.24 44 −42.26 Gattatico 20.38 3.75 29 18.01 5.26 43 −11.62 Gualtieri 23.17 3.77 25 13.41 3.78 75 −42.11 Guastalla 24.52 3.58 31 13.26 3.31 57 −45.92 Luzzara 25.20 3.39 22 12.29 3.06 167 −51.21 Modena 28.28 6.53 56 11.60 3.37 426 −58.97 22.90 6.21 14 19.26 6.18 20 −15.92 Novellara 26.64 4.19 43 11.94 3.48 163 −55.20 Novi di Modena 24.79 6.23 22 14.59 3.83 68 −41.13 Poviglio 23.19 4.43 21 15.39 4.30 87 −33.61 Reggio Emilia 24.66 6.17 137 14.88 5.96 346 −39.65 Reggiolo 27.55 4.15 23 13.21 4.47 40 −52.05 Rio Saliceto 23.10 2.17 22 14.62 4.46 25 −36.71 Rolo 29.03 8.16 8 13.52 3.17 70 −53.41 Rubiera 21.83 5.15 19 13.02 4.91 38 −40.35 Sant’Ilario 20.21 2.49 12 21.56 4.88 17 6.72 d’Enza 21.48 2.84 16 12.32 3.77 49 −42.63 Soliera 27.16 4.50 22 11.64 3.19 134 −57.13

Fig. 3 Relative variations in soil organic carbon (SOC) content during the 1935–1990 period. Data are averages for the municipalities

In the Modena and Reggio Emilia area, changes in land use and agriculture management reduced SOC by 39.3% on average. This reduction level is in agreement with data reported for the conversion of grasslands or perennial crops into arable land (Tilman 1998; Freibauer et al. 2004). The conversion of permanent grasslands, however, occurred in only 3.8% of the area investigated; it is, consequently, necessary to consider further mechanisms acting during the last 70 years that caused the observed SOC reduction:

(a) Reduction of poliennial forage crops. Poliennial crops, for example alfalfa, reduce the frequency of soil tillage and promote an increase in SOC (Al-Kaisi et al. 2004).

(b) Reduction of manuring practices. The positive effect of manure application on soil organic carbon has been well recognized since the first data from the Rothamsted experiment. Manure applications increase not only SOC concentration but also biological activity and aggregate stability (Bronick and Lal 2004). Long-term manure applications also increase SOC through an indirect effect resulting from the increased OM return in crop residues because of increased crop production (Whalen and Chang 2002).

(c) Deeper tillage operations. The scientific literature contains substantial evidence that intensive tillage reduces SOC (Al-Kaisi et al. 2004; Shukla and Lal 2004; Alvarez et al. 1995). It has been shown that CO2 losses from soil are related to the volume of soil disturbed by tillage operations (Reicosky 2001); deeper soil tillage promotes SOC reduction. The results achieved from this research are consistent with data obtained in other European countries and in North America.

Based on the available data, we estimate that total soil carbon storage in the investigated area, considering the upper 30 cm of soil, was about 13 Tg for the year 1935. From 1935 to 1990 the combination of land-use dynamics and changes in agronomic management resulted in a substantial decrease in soil carbon storage, estimated at 5 Tg C.

Conclusions

Our study indicates that land-use and agricultural management changes have had a dominant effect on carbon storage in agricultural areas of Northern Italy. During the period 1935–1990, because of agriculture intensification, land ecosystems of the studied area acted as a source of carbon to the atmosphere. This evidence confirms that, in the last century, not only did the conversion of virgin soils to agriculture act as a carbon source, but also the intensification of agriculture in soils cultivated for thousands of years. The results we obtained are comparable to other estimates and confirm the general trend of SOC reduction in Europe. The adoption of sustainable agriculture can contribute to reversing this trend and transform agricultural soils from a source to a sink of carbon; in particular the promotion of manuring practices and reduction of the depth and frequency of tillage operations can contribute to achieving this result. Use of permanent grasslands instead of alfalfa for forage production results in comparable forage yields, but enables, within 50–80 years, storage of 30–50 t ha−1 of C in the upper 50 cm of soil (Gardi et al. 2004).

Acknowledgments This paper is based on data supplied by the Italian Sugarbeet Association, the Soil Survey Office of the Emilia-Romagna Region, and the Italian Institute of Statistics (ISTAT). Special thanks go to Professor Gilmo Vianello, University of , who made useful suggestions.

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