Soil carbon dynamics and crop productivity as influenced by climate change in a rainfed cereal system under contrasting tillage using EPIC
Research highlights
▶ WinEPIC predicts accurately yields for typical Italian cropping and tillage systems. ▶ WinEPIC simulates the soil C dynamic with good precision. ▶ Simulated future yields are affected by tillage and by climate change. ▶ Predicted soil organic C is affected mainly by tillage. ▶ No tillage leads to SOC sequestration, depending from the climate scenarios.
Introduction
A key question still lacking global consensus in climate change research relates to the fate of C stored in soil organic matter (SOM), and whether the amount of C in this pool will increase or decrease with global warming (Kirschbaum, 1995, Kirschbaum, 2006). Moreover, the potential of conservation agriculture for carbon sequestration and climate change mitigation in all environments is still questionable (Blanco-Canqui and Lal, 2008). In current climate conditions, soil deep ploughing and other nonconservative cultivation techniques are known to increase CO2 emissions from soils (Reicosky, 1997, Six et al., 1999), while C sequestration could be increased by the adoption of agronomic practices such as diversified cropping systems, reduction of soil erosion, application of organic fertilizer (Lal, 2004), including farm-yard manure and green-manure, improved grassland management (Franzluebbers and Doraiswamy, 2007), and conservation-tillage. There is a general agreement that increasing temperature and atmospheric [CO2] will affect both plant productivity, which provides the input to soil organic C, and the rate of soil organic matter decomposition, which determines the loss of soil organic C. As a general rule, higher CO2 levels in the atmosphere, increase crops growth and yield which in turns provides the input to soil organic C (Hendrey and Kimball, 1994, Long et al., 2005, Long et al., 2006). Free-air concentration enrichment (FACE) studies confirm that CO2 enrichment responses under field conditions consistently increase biomass and yields in the range of 5–15% with CO2 concentration elevated to 550 ppm (Ainsworth and Long, 2005). A positive interaction between CO2 concentration and temperature with plant growth under experimental conditions was ascertained (Kimball et al., 1993, Kimball et al., 2002).
Another key issue to understand changes in soil C cycling is the quality change of plant residues grown under CO2-enriched atmosphere that is returned to soil (Amthor, 1995, Torbert et al., 2000). The increase in C:N ratio with elevated CO2 is well documented in agricultural plants (Cotrufo et al., 1998, Rogers et al., 1994). However, Lamborg et al. (1984) have argued that increased soil microbial activity resulting from greater biomass C inputs under elevated CO2 could lead to increased soil organic matter decomposition (i.e. ‘the priming effect’) and, therefore, atmospheric CO2 enrichment would not result in accumulation of soil organic C. Moreover, the impact of these CO2-induced changes on residue decomposition can be species dependent (Wood et al., 1994, Prior et al., 1997). At present, more long-term CO2 research with major crops is required to accurately determine if shifts in the quantity and quality of residue will influence decomposition processes in highly managed agricultural systems (Prior et al., 2004).
Results of the studies on soil C sequestration magnitude under climate change are more controversial. Soil C increase of 1.2–1.4% yr−1 under elevated CO2 was reported (de Graaff et al., 2006, summarizing results from 117 studies). Global temperature increase is considered either not influencing (Giardina and Ryan, 2000, resuming results of 82 experiments) or influencing to different extents (Reichstein et al., 2005) or depending by the different turnover rates of the pools of SOM (Knorr et al., 2005).
There are two main critical aspects to face when studying carbon cycling in soils under CO2 enriched atmosphere and increased temperature. The first is represented by the many factors interacting in the processes of mineralization/humification, the second by the logistical problems caused by reproducing a CO2 enriched environment and increased temperature in complex field experiments, comparing tillage treatments, fertilizer rates, crop sequence, etc. Therefore, simulation models represent a valid approach to understand how agricultural systems are influenced by projected climate change. In order to overcome the problems of extensive and costly experiments, most of the known factors acting on SOC dynamic have been included, through mathematical algorithms, into several models, allowing an integration of the various factors involved (climate, pedology, cropping system, soil and crop management) and their complex interactions. So, they can be used to predict changes in SOM under the different management and climatic conditions that may occur in the future (Jones and Donnelly, 2004).
Moreover, soil C sequestration is of special interest in Mediterranean areas, where rainfed cereal cropping systems are prevalent, inputs of organic matter in soils are low and mostly rely on crop residues, while losses are high due to climatic and anthropic factors such as intensive and non-conservative farming practices. Estimates indicate that 74% of the land in Southern Europe is covered by soil containing less than 2% of organic carbon (i.e. 3.4% organic matter) in the top-soil (Zdruli et al., 2004). In Mediterranean areas, the effects of cropping systems and tillage reduction on SOC sequestration under current climate conditions have been widely studied (Alvaro-Fuentes et al., 2009, Hernanz et al., 2009), while very few studies have addressed the potential effects that climate change under different tillage practices could have in SOC dynamics (Alvaro-Fuentes and Paustian, 2010).
The first objective of our study was to calibrate the EPIC model (Izaurralde et al., 2006, Williams and Renard, 1985) in a Mediterranean cropping system, using experimental data from a long-term field experiment in Central Italy under conventional tillage (CT) and no-tillage (NT). The second objective was to simulate the impact of future climate change scenarios and CO2 enrichment on crop productivity, and soil organic matter dynamics in terms of soil C sequestration.
Section snippets
Experimental site description
The model has been tested using data from a long-term experiment on tillage and N rate fertilization carried out since 1994 in Marche region (43°32′N, 13°22′E, altitude 88 m), at the experimental farm “P. Rosati” of Marche University with a slope of 12% (Iezzi et al., 2002). Mean annual temperature is 14.4 °C, with monthly means ranging from 4.9 °C in January to 24.0 °C in August. Mean annual rainfall is 700 mm. Soil is silty-clay (Table 1), and was classified as Calcaric Gleyic Cambisols (FAO, 2006
Calibration of WinEPIC
The calibration process was performed on crop yields and soil C dynamics parameters (Fig. 1 and Table 5), on the basis of model default values, literature and measured data. The first step, i.e., an accurate modeling of crop yield and total aboveground biomass, is a prerequisite to quantify C additions as residues and their subsequent transformations in the soil (Izaurralde et al., 2006).
Since plant-C input is a main driver of soil C dynamics, it was important to confirm that the EPIC crop
Conclusions
In our simulation, there are sources of uncertainty that ought to be considered. First of all we used fixed [CO2] for each time period for the simulation, while the [CO2] is expected to increase with time. Another cause of uncertainty is that we used the same crop variety and cropping system for all simulations, thus assuming no adaptation to expected climate change.
Nevertheless, our simulated yields under projected climate change are comparable to those simulated by other authors and measured
Acknowledgements
The research is part of the Italian research project “SOILSINK”, Climate change and agro-forestry systems: impacts on soil carbon sink and microbial diversity, funded by the Integrated Special Fund for Research (FISR). We greatly acknowledge the coordination of the field work of Dr. Giuseppe Iezzi, and the consultance of Giuseppe Corti, Alberto Agnelli, and Stefania Cocco (Dipartimento di Scienze Ambientali e delle Produzioni Vegetali, Università Politecnica delle Marche) which described the
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