Soil and crop management to promote formation of stable soil organic matter

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Two-thirds of the terrestrial organic carbon (C) in soil is contained as soil organic matter (SOM), a pool of C more than twice the size of that in the atmosphere or in the living plant biomass. SOM is critical for supporting plant, animal and microbial life because of its role in enhancing the physical and chemical properties that regulate soil quality. This includes retention of essential plant nutrients and increasing water-use efficiency, promoting aggregation thereby reducing erosion, and its direct contribution to soil fertility. Intensive soil cultivation for agriculture combined with the growing of annual crops for food production have depleted SOM levels by 30-50% compared to that in their native counterparts. Soil management (tillage practices) and cropping systems influence the quality and quantity of plant residue inputs, and the rates of plant residue decay and SOM turnover. Although agricultural soils, which account for ~10% of global organic C, have been a significant contributor to atmospheric CO2 emissions, they have the potential to be a major global reservoir for atmospheric carbon dioxide C where it can be sequestered as stable humus that is hundreds to thousands of years old. Adoption of appropriate cultural and management practices, including complex crop rotation and conservation tillage (reduced or no-till) systems, and returning crop residues and organic wastes are being actively promoted to protect and enhance SOM levels. Recently quantitative and qualitative measurements of the components of SOM in agricultural soils have become a major research focus due to their potential for mitigating climate change.