Plant diversity and precipitation gradients drive litter return and SOC in alpine vegetation.

Climate and plant diversity are the major drivers of litter return and soil organic carbon (SOC) content. However, in cold and arid regions, how plant diversity and climate are affected by other factors and their contribution rates to the regulation of litter return and SOC content remains largely unknown. This study investigated the plant diversity and SOC content in the cold and arid vegetation areas of Qilian Mountains and Qaidam Basin. Results showed that when the number of plant species was between 10 ~ 15, it promoted the litter return. The relationship between the number of plant species and the litter decomposition was mainly related to the material composition of the plant itself, and there was no obvious linear relationship. Litter return (litter-to-biomass ratio = total biomass (TB) / litter biomass (LB)) was positively correlated with SOC. Under different precipitation gradients, the correlation in shallow soil depth (0 ~ 20 cm) was higher than in deep soil depth (20 ~ 40 cm). Plant diversity (SWI: Shannon-wiener Index, SRI: Species Richness Index) further regulated SOC content by affecting litter decomposition. Plant diversity (SWI: 1.75–2.25; SRI: 10–15) maximized SOC content. Climate dominated SOC regulation below 200 mm precipitation, while diversity prevailed at 200–600 mm. Based on the above, in arid regions with annual precipitation < 200 mm, climate factors (water, temperature) are the primary drivers regulating SOC. In regions with 200–600 mm precipitation, plant diversity becomes the key factor controlling SOC. When annual precipitation > 600 mm, the sensitivity of plant diversity to climate change decreases, thereby weakening its regulatory effect on SOC and litter decomposition.These results provide a scientific basis for studying species diversity and climate effects on litter decomposition and SOC content and is importance for understanding soil carbon pool changes and its influencing factors in cold and arid grassland ecosystems rgions.