Soil and water conservation measures reduce erosion but result in carbon and nitrogen accumulation of red soil in Southern China

Soil erosion is continually overlooked in the hilly red soil region of southern China, and there is a substantial lack of knowledge about the effects of soil and water conservation measures (SWCMs) on erosion prevention and soil carbon and nitrogen accumulation for the eroded sloping arable land in this region. Herein, based on a long-term experiment in the red soil hilly region in China, we investigated the effects of 17 years of SWCMs on soil organic carbon (SOC), total nitrogen (TN), microbial biomass carbon and nitrogen (MBC and MBN, respectively) as well as soil erodibility (K factor) at soil depths of 0–100 cm in the top, middle and bottom parts of the slope. The SWCMs included straw mulching (SM), downslope ridge cultivation (DC), downslope ridge cultivation + contour hedgerow planting (DCH), and contour ridge cultivation tillage (CC). A bare land treatment without any crops was used as a control (CK). We found that SWCMs increased SOC, TN, MBC, and MBN by 8–57%, 1–14%, 25–68%, and 17–35%, respectively, with greater increases in these variables in the CC and SM treatments and smaller increase in the DC treatment. Additionally, the effects of the SWCMs were greater at the top and middle of the slope than at the bottom of the slope, and the effects varied with soil depth. Moreover, the SWCMs decreased soil erodibility, with a lower K factor in the SWCMs treatments (0.018–0.049 t ha h ha−1 MJ−1 mm−1) than in the CK (0.042–0.068 t ha h ha−1 MJ−1 mm−1) and a greater decrease in the CC treatment. The effects of the SWCMs on soil erodibility mainly occurred through the regulation of SOC according to structural equation modeling. Overall, this study suggested that straw mulching and contour ridge cultivation tillage were the most effective strategies for restoring soil carbon and nitrogen and preventing soil erosion.