Soil organic carbon sequestration and agricultural greenhouse gas emissions in the southeastern USA

Agriculture in the southeastern USA can be highly productive (i.e., high photosynthetic fixation of atmospheric CO2) due to warm-moist climatic conditions. However, its impacts on greenhouse gas emissions and mitigation potential have not been thoroughly characterized. This paper is a review and synthesis of literature pertaining to soil organic C (SOC) sequestration and greenhouse gas emissions from agricultural activities in the southeastern USA. Conservation tillage is an effective strategy to regain some of the SOC lost following decades, and in some areas centuries, of intensive soil tillage and erosion. With conventional tillage (CT) as a baseline, SOC sequestration with no tillage (NT) was 0.42 ± 0.46 Mg ha−1 year−1 (10 ± 5 years). Combining cover cropping with NT enhanced SOC sequestration (0.53 ± 0.45 Mg ha−1 year−1) compared with NT and no cover cropping (0.28 ± 0.44 Mg ha−1 year−1). By increasing cropping system complexity, SOC could be increased by 0.22 Mg ha−1 year−1, irrespective of tillage management. Taking into account an average C cost of producing and transporting N fertilizer, SOC sequestration could be optimized at 0.24 Mg ha−1 year−1 with application of 107 kg N ha−1 year−1 on N-responsive crops, irrespective of tillage management. In longer-term studies (5-21 years), poultry litter application led to SOC sequestration of 0.72 ± 0.67 Mg ha−1 year−1 (17 ± 15% of C applied). Land that was previously cropped and converted to forages sequestered SOC at a rate of 1.03 ± 0.90 Mg ha−1 year−1 (15 ± 17 years). Limited data suggest animal grazing increases SOC sequestration on upland pastures. By expanding research on SOC sequestration into more diverse pasture and manure application systems and gathering much needed data on methane and nitrous oxide fluxes under almost any agricultural operation in the region, a more complete analysis of greenhouse gas emissions and potential mitigation from agricultural management systems would be possible. This information will be necessary for developing appropriate technological and political solutions to increase agricultural sustainability and combat environmental degradation in the southeastern USA.