Reduced Precipitation Frequency Decreases the Stability of the Soil Organic Carbon Pool by Altering Microbial Communities in Degraded Grasslands

Decreasing precipitation frequency (DPF) has the potential to alter soil microbial community structure, enzyme activity, and the stoichiometry of microbial biomass in grassland ecosystems. Grasslands have undergone degradation, often driven by anthropogenic activities such as overgrazing, which further intensifies their sensitivity to environmental changes such as altered precipitation. Changes in soil microbial communities can in turn impact the soil organic carbon pool (SOCP) and its stability, particularly in degraded grasslands shaped by agricultural practices. Here, we evaluated how DPF affects different types of soil carbon pools, soil microbial community structure, the stoichiometry of microbial biomass, and the potential activity of exoenzymes related to microbial nutrient acquisition in three steppe grasslands representing a degradation gradient (from light to moderate to severe degradation). We also developed a systematic model linking microbial stoichiometry, community structure, enzyme activity, and the SOCP and its stability. Our results showed that DPF significantly reduced the soil total carbon pool (STCP), SOCP, and dissolved organic carbon pool (DOCP) in all degraded grasslands, while it increased the DOCP/SOCP ratio in the grasslands with light to moderate degradation, indicating lower stability of the SOCP. Decreased precipitation frequency reduced microbial biomass in grasslands with light to moderate degradation but had the opposite effect on grasslands with severe degradation. Additionally, the promoting effects of DPF on the fungi/bacteria ratio and β-1,4-xylosidase activity diminished with increasing grassland degradation. The fungi/bacteria ratio, microbial biomass carbon/nitrogen ratio, and β-1,4-xylosidase activity were identified as the main predictors for the SOCP and its stability. In lightly and moderately degraded grasslands, decreased soil water content (SWC) and increased soil moisture variation induced by lower precipitation frequency promoted β-1,4-xylosidase activity by decreasing the microbial biomass carbon/nitrogen ratio. The lower stability of the SOCP in degraded grasslands under altered precipitation frequency highlights the challenges posed by climate change regarding soil carbon sequestration in these fragile ecosystems. Our results also stress the importance of targeted water management for soil carbon sequestration in agriculture and livestock management, which could be achieved by altering soil microbial activity and stoichiometry, For example, fertilization increases nutrient availability, enhances microbial growth, and shifts C/N/P ratios, promoting carbon allocation to biomass over respiration and thus enhancing soil carbon retention.