Precipitation Gradients Regulate Soil Carbon Stabilization Mediated by Microbes in an Arid Region of China

Desert ecosystems are important components of the terrestrial carbon cycle, yet the mechanisms by which altered precipitation regulates soil organic carbon (SOC) stabilization remain poorly understood. We evaluated long-term responses to precipitation manipulation in a desert shrub ecosystem in northwestern China. The experiment was established in 2011 with seven treatments ranging from −50% to +50% of ambient precipitation, and the comprehensive plant, soil, and microbial dataset analyzed in the present study was collected in 2025 after 15 consecutive growing seasons of manipulation. We examined plant growth traits, soil moisture, microbial biomass, extracellular enzyme activities, bacterial and fungal communities, and particulate and mineral-associated organic carbon fractions. Plant growth traits, microbial biomass, extracellular enzyme activities, and SOC fractions generally exhibited nonlinear responses across the precipitation gradient, whereas soil moisture increased with increasing precipitation. Moderate precipitation addition (+30%) enhanced microbial biomass and enzyme activity and significantly increased SOC, particulate organic carbon, and mineral-associated organic carbon by 92.5%, 90.5%, and 112.1%, respectively, relative to the control. However, these responses declined under the +50% treatment, indicating that further increases in water availability did not continuously promote carbon accumulation. Bacterial communities were more responsive to altered precipitation than fungal communities, while microbial α-diversity showed weaker relationships with carbon stability. Partial least squares structural equation modeling indicated that precipitation effects on carbon stability were associated primarily with changes in microbial biomass and extracellular enzyme activity; microbial biomass had the largest total effect, and both models explained 87% of the variation in carbon stability. These results suggest that SOC stabilization in this dryland shrub ecosystem responds nonlinearly to altered precipitation and is more closely associated with microbial functional processes than with microbial α-diversity.