Soil-profile distribution of organic C and N after 6 years of tillage and grazing management

Summary Stocks of soil organic carbon (SOC) and total soil nitrogen (TSN) are key determinants for evaluating agricultural management practices to address climate change, environmental quality and soil productivity issues. We determined SOC, TSN and particulate organic C and N depth distributions and cumulative stocks in response to 6 years of tillage (conventional and no‐tillage) and cover‐crop management (without and with cattle grazing) on an Acrisol in the southeastern USA. Total and particulate organic C concentrations were greater under no‐tillage than under conventional tillage at a depth of 0–20 cm only. No differences in concentrations at various depths to 150 cm or of cumulative stocks were observed in total and particulate organic C and N in response to cover crop management, suggesting that animal grazing had no negative effect on SOC and TSN. Compared with perennial grass as a control, the stock of SOC was significantly reduced under cropping at a depth of 0–40 cm, was trending toward reduction at a depth of 0–90 cm and was not different at a depth of 0–150 cm. A declining significance of management with depth indicated an important consideration in attempts to determine the whole‐profile response of SOC and TSN to management: rigorous sampling approaches are needed to overcome the problems of small concentrations and large variation with increasing soil depth. The results of this study clearly indicate that (i) greater SOC and TSN concentrations can be expected in the surface layer under no‐tillage than under conventional tillage in Acrisols of the warm, moist climatic region of the southeastern USA and (ii) compensation for stratified SOC and TSN with no‐tillage does not occur deeper in the soil profile, such that the same relative numeric difference that occurred at the surface remains unchanged with greater cumulative depth.