Response of Soil Organic Carbon Stocks and Soil Microbial Biomass Carbon to Natural Grassland Conversion: A Global Meta-Analysis

Natural grasslands worldwide are increasingly being converted into other land-use types, such as cropland and forest, thereby impacting soil carbon cycles and stocks. Soil organic carbon (SOC) is essential for regulating soil properties and microbial communities, while microbial biomass carbon (MBC) is the most active fraction of the SOC pool, both of which play pivotal roles in the global carbon cycle. Here, we performed a meta-analysis on 623 and 85 individual observations from 85 peer-reviewed articles to quantitatively evaluate the effect of grassland conversion on SOCS and MBC. Overall, conversions significantly reduced SOCS and MBC by 10.11 % and 30.63 %, respectively. Notably, the impact varied by conversion type: converting grassland to forest, cropland, and plantation reduced SOCS by 7.69 %, 16.47 %, and 20.55 %, respectively. Meanwhile, converting grassland to cropland and abandoned land decreased MBC by 47.80 % and 38.74 %, respectively. Environmental factors such as mean annual temperature (MAT), mean annual precipitation (MAP), soil total nitrogen (TN), and soil carbon-to‑nitrogen ratio (C/N) influenced these changes. SOCS and MBC were positively correlated with MAT, soil C/N and TN. Specifically, when the C/N or TN of the converted soil exceeded 1.21 or 1.11 times that of the original grassland, SOCS would exhibit a trend of carbon sequestration. Our findings provide valuable insights for global soil carbon sequestration and land use management policies.