Driving factors of soil organic carbon sequestration under straw returning across China's uplands.

Straw returning is suggested as a sustainable agricultural practice to promote soil organic carbon (SOC) sequestration, whose magnitude can be influenced by climatic, edaphic and agronomic factors simultaneously. However, the driving factors regulating straw returning-induced SOC increase in China's uplands remain uncertain. This study conducted a meta-analysis by collecting data from 238 trials at 85 field sites. The results showed that straw returning significantly increased SOC content by an average of 16.1% ± 1.5% with an average sequestration rate of 0.26 ± 0.02 g kg yr. The improvement effects were significantly better in the northern China (NE-NW-N) than in the eastern and central (E-C). SOC increases were more pronounced in C-rich and alkaline soils, in cold and dry climates, and under larger amounts of straw-C and moderate nitrogen fertilizer inputs. Longer experimental period resulted in higher SOC increase rates but lower SOC sequestration rates. Furthermore, partial correlation analysis and structural equation modelling revealed that total straw-C input was the key driving factor of SOC increase rate whereas straw returning duration was the dominant limiting factor of SOC sequestration rate across China. Climate conditions were potential limiting factors of SOC increase rate in NE-NW-N and SOC sequestration rate in E-C. It was suggested that straw returning with large application amounts should be more strongly recommended in uplands in NE-NW-N especially in the straw applications at the beginning, from the perspective of SOC sequestration.