Soil organic carbon and nitrogen sequestration following grazing exclusion on the Loess Plateau, China

Enhancement of soil organic carbon (SOC) and nitrogen (N) storage in degraded ecosystems is of great significance for improving soil quality and mitigating climate change. Grazing exclusion is an effective management practice to restore degraded ecosystems in drylands. However, the spatio-temporal dynamics and regulating factors of SOC and N sequestration in degraded ecosystems following grazing exclusion has not been clearly explored. Using synthesized data from 38 peer-reviewed publications, we investigated the rates of SOC (RSOC) and soil total N (RN) stock changes and their controlling factors following grazing exclusion in the Loess Plateau of China. Grazing exclusion significantly increased SOC and total N stock in most sites. The average RSOC was 0.57, 0.64 and 0.7 Mg ha-1yr- 1 and the average RN was 0.033, 0.031 and 0.036 Mg ha-1yr- 1 at 0-20, 20-40 and 40-60 cm soil depths, respectively. RSOC showed a Poisson function relation with the duration of grazing exclusion. RSOC and RN did not significantly change with mean annual precipitation, but they were negatively correlated with mean annual temperature at 0-20 cm. RSOC at each soil depth was positively correlated with altitude. The strong coupling relationship between RSOC and RN resulted in no significant change in the ratio of SOC to total N between grazing exclusion and grazing sites over time. The results show that altitude, age of GE and initial SOC and N contents rather than precipitation controls these processes in the Loess Plateau. Grazing exclusion promotes SOC and N sequestrations more than 30 years in the degraded lands of the Loess Plateau. The inherent increase of soil N would be beneficial to soil carbon sequestration during the recovery process.