Soil enzyme activity and stoichiometry indicates that litter quality regulates soil microbial nutrient demand in a Tibetan alpine meadow

The effects of litter quality on soil microbial communities and enzyme activities have been widely documented; however, the specific relationship between soil enzyme activity, stoichiometry and their interactions with litter and soil properties across varying litter qualities remain unclear. Freshly fallen leaves of six species were collected and divided into low- and high-quality litter based on decomposition rates. We assessed the activities of carbon (C)-, nitrogen (N)- and phosphorus (P)-acquiring enzymes—β-1,-4-glucosidase (BG), β-1,4-N-acetylglucosaminidase (NAG), leucine aminopeptidase (LAP), and acid phosphatase (AP)—along with biotic and abiotic factors affecting enzyme activities (dissolved organic matter and microbial biomass in litter and soil) at five time points over 673 d. Enzyme vector analysis showed that vector lengths (microbial C limitation) were the largest across all treatments after 309 d, and all vector angles were > 45°, suggesting that soil microbes were more limited by P than by N during decomposition process. Redundancy analysis (RDA) and structural equation modeling (SEM) demonstrated that soil enzyme activity and stoichiometry were driven by different variables, depending on litter quality. In the control, soil dissolved organic carbon (SDOC) and phosphorus (SDOP) were the primary predictors of soil enzyme activity, while under low-quality litter addition, litter dissolved organic carbon (LDOC) and soil dissolved organic nitrogen (SDON) were the most influential factors, and under high-quality litter addition, litter microbial biomass carbon (LMBC), SDOC, and SDON were key drivers. Furthermore, SDOC was significantly and negatively correlated with vector length, explaining the greatest variation in soil enzyme stoichiometry across all treatments. Vector length and angle were better explained by LDOC and litter microbial biomass phosphorus (LMBP) under low-quality litter addition, in contrast, by litter microbial biomass nitrogen (LMBN) and litter dissolved organic nitrogen (LDON) under high-quality litter addition. Our results highlight that litter quality modulates soil microbial metabolism by influencing dissolved organic matter and microbial biomass in both litter and soil layers. This study reveals the mechanism mediating soil microbial metabolism during litter decomposition, which is crucial for understanding C and nutrient cycling in alpine grassland ecosystems.