Aridity-induced nonlinear shifts in soil organic carbon across Chinese grasslands

Soil organic carbon (SOC) is an important carbon pool in terrestrial ecosystems that plays a key role in global carbon cycling and climate regulation. However, the patterns of SOC variation along aridity gradients remain poorly characterized, particularly under varying aridity conditions. To address this gap, we surveyed 192 sites across nine provinces in China, spanning environmental gradients from humid to extremely arid regions. Our findings revealed a distinct threshold at an aridity level of 0.77, beyond which SOC accumulation and stability became more closely associated with microbial diversity and soil enzyme activity, while the influence of vegetation and nutrient inputs declined significantly. Notably, β-glucosidase activity showed strong correlations with SOC dynamics under severe aridity. These results indicate that intensified aridity conditions shift the biotic and abiotic correlates of SOC, emphasizing the increasing relevance of microbial and enzymatic characteristics under arid environments. This study highlights the nonlinear response of SOC to aridity and identifies a critical threshold at an aridity level of 0.77, underscoring the potential value of microbial and enzymatic indicators in monitoring soil carbon changes. These findings provide important insights into the vulnerability of grassland soil carbon stocks and offer a scientific basis for developing adaptive management strategies to conserve carbon sinks under future climate scenarios.