Carbon Sequestration in Agricultural Soils.

Agricultural soils, those under croplands and grazing lands, comprise 5 Bha of managed land use. The soil organic carbon (SOC) pool to 1-m depth is estimated at 152 Gt (~10%) in croplands, 425 Gt (26.9%) in grazing lands, and 577 Gt or 36.5% of the global SOC pool in agricultural soils. Combined with plantation forestry, these soils have a C flux of 3 Gt C/y for cropland, 26 Gt C/y for grazing lands, and 5 Gt C/y for plantations. Soils of these managed ecosystems account for 57% of the global annual flux. Therefore, farmers have a custody of more environment than any other group. Since agricultural soils contain lower SOC pool than their capacity in relation to climate and other ecological factors, they have a large C sink created by the historic loss through land use conversion, soil degradation and extractive farming practices. The soil C sink capacity can be filled by conversion to a restorative land use and adoption of recommended management practices. The latter include those which can create positive C and nutrient (N, P, S) budgets such as no-till farming with residue mulch, cover cropping, integrated nutrient management including manuring, soil amendments such as biochar, agroforestry systems, etc. The rate of SOC sequestration ranges from 100 kg C/ha/y in warm and arid climates to 1500 kg/ha/y in cool and humid regions, with a technical potential of 1-2 Gt C/y for about 50 years.