Intensive Agriculture and the Soil Carbon Pool

Abstract

There is a strong link between food security and the carbon pool in terrestrial ecosystems, notably the soil organic carbon (SOC) pool. Quantity and quality of the SOC pool are essential to soil quality, agronomic production, and use efficiency of inputs. Furthermore, limiting global warming to a 2°C increase in Earth's mean temperature requires the identification of appropriate sinks for atmospheric CO2. Sequestration of C in soils is almost an obvious climate solution, which ironically has been overlooked by policy makers. Yet, it is a proverbial “low-hanging” fruit with numerous co-benefits. Most soils of the agroecosystems are depleted of their antecedent SOC pool by 25%−75%, equivalent to 10−30 Mg/ha, which is also the technical sink capacity of C sequestration. The rate of SOC sequestration in soils of the tropics and subtropics is 100−500 kg/ha/year. The potential global soil C sink capacity is 1.2−3.1 Pg C/year, 25−50 years. While mitigating climate change, an increase in SOC pool also enhances crop yield because of improvements in soil physical (i.e., aggregation, available water capacity), chemical (i.e., cation and anion exchange capacity [CEC/AEC], soil reaction, nutrient transformation), and biological properties (i.e., microbial biomass C earthworm activity). The adoption of conservation-effective measures can avoid erosion-induced emissions by minimizing decomposition of SOC transported by erosional processes. The mean residence time of SOC depends on a range of exogenous and endogenous factors. Recommended management practices (RMPs) for SOC concentration include conservation agriculture based on no-till and mulch farming, use of cover crops and green manure, application of manure and biochar, and use of perennial culture, including agroforestry. Balanced application of fertilizers is crucial. Water conservation, water harvesting and recycling, use of drip subirrigation, and growing aerobic rice are some important practices for enhancing water-use efficiency. There is a strong need for research in understanding processes of SOC sequestration, identifying and validating soil-/site-specific agronomic practices, and making policy interventions that reward farmers through payments for ecosystem services. Farming C and trading C credits are needed to create another income stream for farmers and to promote the adoption of RMPs. Cropland soils of south Asia in general and those of the Indo-Gangetic Plain in particular are severely depleted of their SOC pool. Despite high inputs of fertilizers and water, crop yields are either stagnating or declining because of the ever-declining use efficiency of inputs (i.e., fertilizers, irrigation, high-yielding varieties) and degrading soils and water resources. A low SOC concentration of cropland soils (0.1%−0.5% in the plow layer of 0−20-cm depth) is the principal cause of decline in soil quality (e.g., low aggregation and plant available water capacity). Thus, RMPs that create a positive soil C budget and enhance the SOC pool must be identified. The balanced use of fertilizers and the application of farm yard manure and other biosolids (i.e., crop residues, green manure) are essential to SOC sequestration, improving the input-use efficiency, and increasing crop yields. There is a strong and positive correlation between the SOC pool in the root zone and the grain yield of wheat, maize, soybean, and other crops. The adoption of RMPs, which enhance SOC pools and sustain high agronomic yield, necessitates payments to farmers for providing ecosystem services through the trading of C credits and so on. Farming C is an important strategy to mitigate climate change, advance food security, and improve the environment.