Microbial responses to stoichiometric imbalances regulate soil organic carbon loss in degrading alpine ecosystems

Alpine grassland degradation is a major threat to global carbon cycles, yet the microbial mechanisms driving soil organic carbon (SOC) loss remain poorly understood. Ecological stoichiometry theory provides a framework for understanding how resource imbalances constrain microbial activity and metabolism. Here, we investigated how grassland degradation altered the stoichiometric imbalances between soil microbes and their resources and how microbes coped with such imbalances, as well as the implications of their responses for SOC stock. We established a degradation gradient (non-, light, moderate, and heavy) in both an alpine meadow and an alpine steppe on the Qinghai-Tibet Plateau, China, with analyzing vegetation nutrient storage, soil physicochemical properties, microbial biomass, dissolved organic nutrients, extracellular enzyme activities, and nutrient mineralization rates. Our results showed that C:N stoichiometric imbalance exhibited a hump-shaped response to grassland degradation with a maximum around moderate degradation, while C:P and N:P stoichiometric imbalances significantly decreased with increasing grassland degradation levels in both ecosystems. However, microbial responses were ecosystem-specific: meadow microbes showed strong C:N:P homeostasis, while steppe microbes showed weaker C:N and C:P homeostasis, indicating higher stoichiometric plasticity. Mechanistically, microbes coped with these shifting imbalances by adjusting extracellular enzyme stoichiometry, net N mineralization, and soil microbial respiration. For instance, C:P and N:P stoichiometric imbalances were strongly linked to the relative production of P-acquiring enzymes across both ecosystems, with slightly stronger correlations in meadows. These response mechanisms were significantly correlated with SOC stock, suggesting that microbial metabolic adjustments are a key pathway regulating the 14.8–71.5% decline in SOC stock decline observed during degradation. Our findings provide a mechanistic link between grassland degradation, microbial stoichiometric response, and carbon cycling, highlighting that ecosystem-specific microbial strategies are critical determinants of SOC vulnerability in these sensitive high-altitude ecosystems.