Soil organic carbon sequestration calculated from depth distribution

Sequestration of organic C in agricultural soils is necessary to improve soil health to meet the challenges of climate change, diminishing biodiversity, water quality deterioration, and rising demands for food and fiber production. New assessment approaches are needed to quantify how conservation agriculture might contribute to soil health improvement and soil organic C (SOC) sequestration. In the southeastern United States, conservation management has been repeatedly shown to stratify SOC concentration with depth. Starting immediately below a zone of tillage influence (i.e., 30-cm depth), SOC concentration is rarely affected by management due to low C inputs and high decomposition potential, whereas concentration increases toward the surface in a nonlinear manner, presumably with greater inputs of residue and root inputs and changing temperature and moisture conditions. Using these observations as an ecological foundation, SOC sequestration was calculated as the summation of SOC stock greater than the baseline condition at 30-cm depth. Data from literature sources were mathematically fitted with this new approach as a validation. Two on-farm surveys with different agricultural management practices (5-40 yr) in the southeastern United States yielded preliminary estimates of SOC sequestration. The interquartile range of calculated SOC sequestration was 4.2-9.4 Mg C ha(-1) for conventional-tillage cropland (n = 45 fields), 13.6-29.7 Mg C ha(-1) for no-tillage cropland (n = 97 fields), and 15.9-26.1 Mg C ha(-1) for perennial pasture (n = 29 fields). This new approach will be valuable to estimate SOC sequestration from a greater diversity of conservation agricultural systems practiced on farms within a region and independent of soil type.