Aridity Index Explains the Accumulation of Soil Organic Carbon Sources in Grassland Ecosystem

Plant-derived carbon (PDC) and microbial-derived carbon (MDC) are the crucial sources of soil organic carbon (SOC), and their contributions strongly affect the stability and turnover of soil organic matter. However, the mechanism underlying the contribution of PDC and MDC to SOC across different grassland types is poorly understood. In this study, we selected four grassland types in Ningxia, China, at a regional scale, including meadow steppe, typical steppe, desert steppe, and steppe desert. We analysed the characteristics of PDC and MDC in topsoil (0–20 cm) and subsoil (20–40 cm) across different grassland types and their contributions to SOC. Our results showed PDC and MDC contents in meadow steppe were significantly larger than those in the other grassland types (p < 0.05), while the contribution of PDC and MDC to SOC in desert steppe was greatest (p < 0.05), and bigger in subsoil than in topsoil. Vanillyl phenols-derived carbon (19%) and fungal necromass carbon (69%) were the main contributors to PDC and MDC, respectively. In addition, random forest model results showed that climate, vegetation, and soil explained 55% and 85% of the variation in PDC and MDC, respectively (p < 0.001). A structural equation model revealed that aridity index was the primary factor influencing SOC sources in grassland ecosystems. This study examined the contributions of PDC and MDC to SOC across different grassland types, as well as the underlying mechanisms of SOC sequestration, providing insights into the carbon cycle processes within grassland ecosystems.