Sequestering carbon in soils of agro-ecosystems☆
Research highlights
► The study addresses the potential and challenges of carbon sequestration in agricultural soils. ► Technical potential of C sequestration is 1.2-3.1Gt/yr. ► Increasing C pool in the root zone by 1t/yr can increase production in developing countries by 24-32 million t/yr for food grains and 6-10 million t/yr for roots and tubers. ► The strategy is to adopt conservation tillage, cover cropping, manuring, agroforestry, biochar and other amendments. ► Rather than subsidies, adoption of these technologies can be promoted by payments for ecosystem services.
Introduction
Anthropogenic activities have led to an increase in atmospheric concentration of CO2 from 280 ppm in the pre-industrial era to almost 400 ppm at present (CDIAC, 2009, WMO, 2008), and is increasing at the rate of about 2.2 ppm/yr (IPCC, 2007a). Despite the vigorous debate on global warming (Kerr, 2009, Solomon et al., 2009), mean global temperature has increased by 0.8 °C since 1880, and may increase by an additional 3–7 °C by 2100 under business as usual scenario (IPCC, 2007a, Allen et al., 2009). Some effects of the global warming since 1880 include the reduction in the terrestrial and arctic snow cover (Post et al., 2009), rise in sea level (IPCC, 2007a, Rhamstort, 2007), decline in crop yield (Schlesinger and Roberts, 2009), reduction in ecosystem services (Walker et al., 2009), increase in frequency of extreme events (e.g., the hurricane Katrina in 2007) especially drought (e.g., monsoon failure in India in 2009), change in biodiversity (IPCC, 2007b) because of pole-ward shift of principal biomes, and increase in global hunger and food insecurity (Cline, 2007).
The recent increase in the world’s food-insecure population to 1.02 billions (FAO, 2008, FAO, 2009a, FAO, 2009b) is attributed most immediately to the increase in the price of food grain staples in 2007–2008 (Piesse and Thirtle, 2009). However, underlying this were a number of factors, including the decrease in production caused by drought in Australia and South Asia (SA), diversion of grains to biofuels, and soil degradation (Lal, 2009). Of the 1020 million food-insecure people (FAO, 2009a, FAO, 2009b), almost 90% (642 million in the Asia/Pacific region and 265 million in Sub-Saharan Africa or SSA) are concentrated in regions where soils are severely depleted of their soil organic carbon (SOC) and nutrient reserves, and are strongly degraded. The projected increase in temperature and decrease in effective precipitation because of climate change may further exacerbate the food insecurity in these ecologically-sensitive regions. Decline in rice yield in China and wheat yield in SA are of special concern even with 2 °C increase in temperature (IPCC, 2007a).
There is a strong link between food insecurity, soil degradation, and climate change (Fig. 1), because of a strong positive feedback among the underpinning processes. Yet, the twin crisis of climate change and food insecurity, which may define the future (Homer-Dixon, 2009), can be addressed through restoration of the SOC pool and the attendant improvement in soil quality. Among several solutions being debated to mitigate climate change (Jacobson, 2009), an important option is sequestration of carbon (C) in agroecosystems, especially in agricultural soils. Soils depleted of SOC not only yield less but also have low use efficiency of added inputs, and are able to sequester less atmospheric CO2. Furthermore, the process of sequestering atmospheric CO2 will itself enhance SOC pool and off-set anthropogenic emissions while benefitting both agricultural productivity and mitigating temperature rise. Thus, the objective of this article is to describe potential and challenges of C sequestration in soils of the world’s agroecosystems. The strategy is to increase soil resilience, enhance adaptation to extreme events, and mitigate climate change by off-setting emissions through soil C sequestration.
Section snippets
Soil carbon pool and its management
Terrestrial C pools (soil and biota) are important components of the global carbon cycle (Le Queré, 2008). The SOC pool, estimated at 2500 billion tons to 2-m depth (Batjes, 1999), has been considerably depleted by the conversion of natural to agricultural ecosystems and by several soil degradation processes such as erosion, salinization, and nutrient depletion/imbalance. In regions where the biomass productivity of the soils is low because of inherent constraints (i.e., drought stress in arid
Soil quality and food security
For crop production, the threshold/critical level of SOC concentration in the root zone (top 20 cm) for most cropland soils is 1.1% for the tropics (Aune and Lal, 1998) and ∼2% for the temperate regions. Severely depleted agricultural soils managed by extractive farming practices, such as those in SA and SSA, have SOC concentrations < 0.5% and sometimes as low as 0.1%. Therefore, enhancing the SOC concentration in these soils is essential to improving soil quality through beneficial impacts on the
Incentivization through payment for ecosystems services
Adoption of RMPs has been a slow process, especially in SSA and SA, in regions where improvements in agronomic production are needed the most. Lack of or low rate of adoption of RMPs is partly due to the limited resources available to small landholders who can neither afford the required inputs nor are these inputs available in the remote regions because of poor roads, market and credit facilities, and weak infra-structure. In this regards, creation of C market (Capoor and Ambrosi, 2008) is a
Economics of soc sequestration versus geological sequestration
One of the challenges to creating an effective C market is the research leading to the development of marginal abatement cost curves from agricultural ecosystems. Such abatement cost curves have been produced for UK (Moran et al., 2008, MacLeod et al., 2010), and US (McCarl and Schneider, 2001). However, similar data are needed for emerging economies with large agricultural and forestry land areas (e.g., China, India) and developing countries of SSA and elsewhere. Some of the policy and
Limitations of soil carbon sequestration
Despite the vast potential and numerous co-benefits of SOC sequestration, there are several challenges which need to be addressed. There is insufficient knowledge about the impact of microbial processes on the pathways of C within the global carbon cycle. There is also a strong need for understanding additional requirements of nutrients (N, P, S) and water for C sequestration in soils. There is also a question about the permanence of C sequestered especially because of the projected climate
Conclusions
World soils can be a major source or sink of atmospheric CO2 depending upon the land use and management. Soils are an important sink of CO2 and CH4 through conversion to a restorative land use and adoption of RMPs which create positive C and elemental (N, P, S, K) budgets. Recarbonizing the pedosphere with a C sink capacity of > 2 billion tons C/yr for 25–50 years can have a strong impact on the global carbon cycle. Increasing the C pool of the pedosphere by 10% over the 21st century (+250 billion
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While the Government Office for Science commissioned this review, the views are those of the author(s), are independent of Government, and do not constitute Government policy. To illustrate see the PDF of the paper http://dx.doi.org/10.1016/j.landusepol.2009.08.017.