Bermudagrass management in the Southern Piedmont USA. VII. Soil-profile organic carbon and total nitrogen

Estimates of potential soil organic C (SOC) and total N (TN) sequestration at depths below the traditional plow layer (i.e., 0-0.3 m) in agricultural systems are needed to improve our understanding of management influences on nutrient cycling and potential greenhouse gas mitigation. We evaluated the factorial combination of nutrient source (inorganic, inorganic + cover crop, and broiler litter) and forage utilization (unharvested, hayed monthly, and low and high grazing pressure) on profile distribution of and changes in SOC and TN during the first 5 yr of 'Coastal' bermudagrass [Cynodon dactylon (L.) Pers.] management. Nutrient source did not affect SOC and TN in the soil profile. Contents of SOC and TN under haying were lower than under other management systems throughout the soil profile. Averaged across nutrient sources, SOC sequestration to a depth of 0.9 m was 3.6 Mg ha(-1) (P = 0.06) under low grazing pressure and 2.4 Mg ha(-1) (P = 0.19) under high grazing pressure. Sequestration of TN was 0.49 Mg ha(-1) (P = 0.03) under low grazing pressure and 0.56 Mg ha(-1) (P = 0.02) under high grazing pressure. The minimum change in SOC and TN needed to detect significant (P = 0.1) sequestration increased an average of 0.6 and 0.10 Mg ha(-1), respectively, for each additional 0.3-m layer of soil. This study demonstrated that plow-layer accumulation of SOC and TN occurred, but that increased variability with depth and small loss of SOC and TN with an additional 0.3 m below the plow layer erased the significance of surface effects.