Enhancing soil carbon and nitrogen through grassland conversion from degraded croplands in China: Assessing magnitudes and identifying key drivers of phosphorus reduction

Agricultural intensification has resulted in severe degradation of croplands in China. While the conversion of degraded croplands to grasslands has been proposed as a potential solution, the effects on soil organic carbon (SOC) and nutrients remain uncertain due to the complex interactions between plant growth and nutrient cycling. To identify key drivers and evaluate general patterns, a meta-analysis of 204 studies was conducted to evaluate changes in SOC, soil nitrogen (N), phosphorus (P), and soil enzyme activities in the 0-40 cm soil layer following grassland conversion across China. Our results showed that grassland conversion increased SOC (16 %), total N (TN, 12 %), available N (AN, 7 %), microbial biomass C (MBC, 48 %), microbial biomass N (MBN, 39 %), and the activities of urease (38 %) and phosphatase (64 %). However, the conversion decreased soil pH (-1 %) and available P content (AP, -26 %). The reduction in AP could be attributed to the restored grasslands absorbing more AP for growth. The duration of conversion was a major factor influencing changes in SOC, TN, and urease activities. Furthermore, mean annual precipitation had a significant impact on soil pH, TP, AP, MBC, and phosphatase activities. Our findings suggest that converting degraded croplands to grasslands can improve soil C and N content in China; however, these findings should be interpreted in the context of the entire recovery process. Further research and mitigation measures, such as the addition of soil P sources, may be required to address this decline in AP.

Automated summary

Agricultural intensification has led to severe degradation of croplands in China. Converting degraded croplands to grasslands can increase soil organic carbon and nitrogen content, but may decrease soil pH and available phosphorus. The duration of conversion and mean annual precipitation are major factors influencing soil changes.