Effects of grassland restoration from cropland on soil carbon sequestration and the controlling factors across China

Based on a search of literature databases, such as CNKI, Google Scholar, and Web of Science, for the 1950－2016 period, we screened 123 articles (126 study sites) related to China’s “Grain for Green” project and soil organic carbon (SOC). In total, we extracted 735 observations from these papers at both farmland and restored grassland sites, such as SOC, soil available nutrient, restoration time, and climate. The objective was to investigate the impact of the “Grain for Green” project on SOC, as well as identify the dominant factors influencing this impact. The results indicated that, First, after the “Grain for Green” project, SOC exhibited a first decreased but then increased response along with restoration time (with a turning point at around 6 years), and the project totally enhanced SOC by 19.8%. Second, the increase of SOC weakened with increasing soil depth, and this enhancement was not significant when the depth exceeded 1 m. Third, soil available nitrogen was the main factor affecting the recovery of SOC, while soil available phosphorus and plant functional groups had a minimal impact. Fourth, along an increasing water gradient, the recovery of SOC changed from negative to positive, and the transition threshold was 25.15 (aridity index). Overall, our work indicates that the “Grain for Green” project shows a positive effect on SOC, but this enhancement is jointly determined by climate and recovery time, which could provide a scientific basis for decision-making regarding land use change and soil carbon management in China.