Chapter One - Management-Induced Changes to Soil Organic Carbon in China: A Meta-analysis

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Abstract

Soil carbon (C) sequestration is an environmentally friendly and efficient strategy to offset emissions of greenhouse gases and mitigate climate change. However, inappropriate farming practices can deplete soil organic carbon (SOC) stock and degrade soil quality. Thus, we conducted a meta-analysis to assess and identify the effects of improved farming practices on SOC sequestration in China by compiling a data set of 83 studies. The results indicated that SOC concentration and stocks at 0–30 cm depth significantly increased by 1.00 ± 0.26 g kg−1 and 0.97 ± 0.24 Mg ha−1 when plow tillage with residue removal was converted to no-till with residue retention (NT); 1.11 ± 0.21 g kg−1 and 2.09 ± 0.46 Mg ha−1 when no fertilization was changed to chemical fertilization (CF); and 1.99 ± 0.62 g kg−1 and 3.09 ± 0.99 Mg ha−1 when CF was changed to manure application (MF) (P < 0.05), respectively. However, increases in SOC were primarily observed in the surface layer and decreased with soil depth. Therefore, the adoption of NT and MF in conjunction with CF is an effective strategy to enhance SOC stock in the surface layer. Further, in single-crop farming regions, the effects are more significant at 0–10 cm depth; and the new equilibrium can occur within 11–20 years after the adoption of NT. In double-crop farming regions, conversion to MF enhanced the SOC at 0–20 cm depth over 16 years. Additional research is warranted to credibly assess the rates of residue and manure input, soil “C saturation,” and soil type on the potential SOC sink capacity in China's croplands.

Introduction

The Fifth Intergovernmental Panel on Climate Change (IPCC) reported that the global mean surface temperature has significantly increased since the late nineteenth century: the global combined land and ocean temperature increased by 0.89 °C (0.69–1.08 °C) between 1901 and 2012 (IPCC, 2013). Climate change is attributed to anthropogenic emissions of greenhouse gases (GHGs), which include CO2, CH4, and N2O (Lal, 2004a). The use of fossil fuels and land use conversion have released 545 (460–630) Pg (Pg = petagram = 1015 g = 1 giga ton) of carbon (C) to the atmosphere, leading to an increase in atmospheric CO2 concentration from 275–281 ppmv in 1750 to 390.5 ppm in 2011 (IPCC, 2013) and 400 ppmv in 2013 (WMO, 2014). Thus, identifying strategies of reducing GHGs emissions and mitigating climate change are global issues (Paustian et al., 2000, Lal, 2004c, Lal, 2007, Lal et al., 2007). Soil C pool is the third principal global C stock containing 1220–1550 Pg to 1 m and 2376–2450 Pg to 2 m depth as soil organic carbon (SOC) and 695–748 Pg to 1 m depth as inorganic C (Lal et al., 1995, Batjes, 1996). The potential of SOC sequestration is estimated to be 0.4–1.2 Pg C yr−1 throughout the world's croplands (Lal, 2004c). Thus, enhancing SOC sequestration is important to partially offsetting anthropogenic emissions and mitigating climate change. In addition, SOC is a key soil property and an important determinant of soil quality (Reeves, 1997, Sá and Lal, 2009, Brandão et al., 2011). However, conversion of natural to agricultural ecosystems may deplete the SOC pool by as much as 60% in temperate regions and 75% or more in tropical regions, degrading soil quality and biomass productivity, exacerbating risks of food insecurity, and aggravating climate change (Lal, 2004c, Lal, 2010, Lal et al., 2007). Thus, promoting farming practices which can restore SOC stock is important to mitigating climate change, improving soil quality, and advancing food security (Lal, 2007).

The SOC pool is affected by a wide range of agricultural management practices including tillage (West and Post, 2002, Ussiri and Lal, 2009, Dalal et al., 2011, Zhang et al., 2014), residue management (Lu et al., 2009, Ding et al., 2014, Liu et al., 2014b), fertilization (Lu et al., 2009, Ding et al., 2014), manure application (MF) (Ding et al., 2014, Maillard and Angers, 2014), water management and soil drainage (Abid and Lal, 2008), etc. Thus, a wide range of C-smart practices have been adopted and popularized to replace traditional management practices. Conservation agriculture (CA) is widely practiced and typically leads to minimal soil disturbance (e.g., no-till, NT) and residue retention on the surface as mulch. In addition to enhancing the SOC pool, CA has numerous benefits of relevance to the environment and crop production (Delgado et al., 2013, Zhang et al., 2014). Thus, conversion of conventional tillage (e.g., plow tillage) to NT can result in redistribution of SOC within the soil profile (Powlson et al., 2014) and in soil-specific situations also enhance the SOC pool (West and Post, 2002, Zhang et al., 2014), particularly in surface soil (West and Post, 2002, Lu et al., 2009, Zhang et al., 2013b). Conversion to CA also enhances soil quality, increases aggregation, and improves aeration by enriching the surface SOC (Doran and Parkin, 1994, Franzluebbers et al., 2007). Agronomic yield in degraded soils can be increased by restoring the SOC pool (Lal, 2004c). Adoption of recommended management practices (RMPs) and integrated nutrient management are some of the strategies that can be used to restore SOC stock in depleted and degraded soils. However, the rate of SOC restoration is affected by numerous factors including climate (rainfall, temperature, evaporation, and seasonal distribution), soil texture and structure, farming system, and specific RMPs of soil and crop management (Lal, 2004c, Johnston et al., 2009). SOC sequestration is to enhance the SOC stock compared to the pretreatment status due to soil humus through land unit plants, plant residues, and other organic solids that originate from the atmospheric CO2 pool (Olson, 2013, Olson et al., 2014). Because of the complexity of SOC sequestration, the amounts of SOC sequestration attained under different farming practices are not clear and have numerous uncertainties. For example, the impact of NT on SOC concentration and pool follows different trends in the long- or short-terms due to experimental durations (West and Post, 2002, Ussiri and Lal, 2009, Dalal et al., 2011, Wang et al., 2011) and other site-specific factors. The increase in SOC concentration and pool in the soil profile, especially in the surface layer, primarily depend on the quantity of residues retained (RR) in the field (Lu et al., 2009, Li et al., 2010). Yet, the effectiveness of CA and residue management in enhancing SOC concentration and pool is a debatable issue (Lu et al., 2009, Ding et al., 2014, Liu et al., 2014b). MF enhances SOC over the long-term and provides a secondary benefit to fix more C than chemical fertilization application (CF) (Ding et al., 2014, Maillard and Angers, 2014). Furthermore, the effects of CF and MF are influenced by cropping intensity, climate, soil type, and even the manure type and its management (Lu et al., 2009, Ding et al., 2014, Maillard and Angers, 2014). Therefore, improving scientific understanding of how management practices affect SOC concentration and pool is important to assessing the contribution of SOC sequestration to climate change mitigation and other ecological and environmental benefits.

Cereal productions in China increased by about 32% between 2003 and 2011, which was more than double the world average rate of increase (Zhang et al., 2013a). This drastic increase was attributed to the adoption of RMPs and especially to higher rates of agricultural inputs (Zhang et al., 2013a). However, high inputs also have some severe negative impacts. For instance, overuse of chemical fertilizer increases reactive nitrogen (N) emissions, which may cause acid rain and eutrophication, jeopardize air and water quality, and endanger human health (Liu et al., 2013a, Zhang et al., 2013a). Moreover, the C footprint of China's crop production has been estimated at 0.78 ± 0.08 Mg CE ha−1 yr−1 for land use between 1993 and 2007 (Cheng et al., 2011). Additionally, soil degradation by inappropriate land use and management exacerbates the depletion of SOC, which thus far is estimated to be 8–14 Pg C in China, of which 50–66% can be restored through land use conversion and restoration of degraded soils and ecosystems (Lal, 2004b). Thus, there is an urgent demand to adopt RMPs for sustainable development of China while improving resilience of soils and agroecosystems. Understanding how farming practices affect SOC sequestration is essential to identifying site-specific C-smart practices for diverse agroecoregions of China.

Meta-analysis is an effective tool to integrate and compare multiple individual studies and identify a general conclusion and patterns at regional and global scales (Gurevitch et al., 2001, Luo et al., 2006). This approach has been widely adopted to determine changes in SOC concentration and pool upon conversion of farming practices. Meta-analysis is a practical method to assess the effects of changing farming practices on SOC and the probable influence of other factors. Therefore, the objectives of this chapter are to (1) assess changes in SOC concentration and stock through adoption of RMPs; (2) determine changes in the depth distribution of SOC concentration and stock; (3) evaluate how soil sampling depth, experimental duration, cropping intensity, and microclimate affect SOC; and (4) identify the relationship between changes in SOC concentration and pool upon adoption of RMPs in China.

Section snippets

Data Sources

Published data were collated from 83 peer-reviewed papers from across China, which reported changes in SOC dynamics due to conversions of farming practices. These practice changes included conversion of plow tillage with residue removal (PT0) to no-till with residue removal (NT0) (only the tillage practice change was considered); residue removal (R0) to residue retained (RR); PT0 to NT; no fertilization (F0) to CF or MF; and CF to MF. The literature reviewed was from the Web of Science

Mean Difference of SOC Concentration and Stock after Conversions

Generally, the SOC concentration significantly increased upon adoption of RMPs (P < 0.05) (Table 2). The MD of SOC concentration was the highest for conversion of PT0 to NT among the three paired tillage and changes in residue management practice, and for conversion of F0 to MF among changes in the fertilization management practices at 0–30 cm depth, respectively. The SOC concentration also increased more at 0–30 cm than at 0–60 cm depth. This trend indicated that NT and MF enhanced the SOC

Effects of Changes in Practices on SOC Concentration and Stock

The results presented herein show a more significantly positive effect in enhancing SOC concentration and stock under NT than NT0 or RR. Conversion of PT0 to NT significantly increased both SOC concentration and stocks in China (P < 0.05) at 0–30 and 0–60 cm soil depths (Table 2). The relative increase was more significant for 0–30 cm depth upon conversion of PT0 to NT than that from PT0 to NT0 and R0 to RR. Lam et al. (2013) also observed that adoption of conservation tillage is effective only to

Conclusions

The meta-analysis in this chapter was used to assess changes in SOC concentration and stock by conversion to RMPs based on the experimental data set collected from throughout China. The results indicate that RMPs can enhance the SOC concentration and stock. Moreover, among RMPs examined, NT and MF were more effective in enhancing SOC than other practices. However, the increase of SOC concentration and stocks were limited to the top soil (0–10 cm depth after conversion of PT0 to NT, and 0–20 cm

Acknowledgments

This research was funded by Special Fund for Agro-scientific Research in the Public Interest in China (201103001, 201503136) and Program for New Century Excellent Talents in University of Ministry of Education of China (NCET-13-0567).

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