Soil abiotic properties, not microbial composition or functional gene abundance, determine enzyme activities in alpine grassland soils

Enzyme activities are jointly regulated by microbial traits and soil physicochemical properties, but their relative importance remains unclear in alpine grasslands, where cold climate and nutrient limited. To address this, we combined metagenomic analyses of microbial functional genes with assays of potential C- and N-cycling enzyme activities across five alpine grassland types on the Tibetan Plateau. We tested whether enzyme activities are driven more by biotic factors (microbial community composition, functional gene abundance) or abiotic filters (soil properties). Seven enzymes were measured, including cellulase, β-glucosidase, and α- N -acetylglucosaminidase, and their associations with microbial taxa, CAZy/KO gene abundances, and soil properties were evaluated. The results showed weak or non-significant linear correlations between enzyme activities and the corresponding gene abundances (all P > 0.05), even after accounting for multiple environmental factors. Piecewise structural equation modeling revealed that total phosphorus (TP) strongly promoted cellulase gene (K01179) abundance (β = 0.592, P < 0.01), while soil organic matter (SOM) significantly enhanced cellulase activity (β = 0.587, P < 0.05). In contrast, microbial taxonomic composition contributed little once abiotic factors (e.g. pH, TP, SOM, C/N) were considered. Collectively, TP and SOM act as dominant abiotic filters that decoupled functional gene abundance from enzyme activities. These findings highlight in alpine grasslands, soil properties, rather than community or gene abundance, primarily regulate enzyme activities. This provides new mechanistic insight into soil enzyme dynamics under nutrient limitation and extreme environments, emphasizing that environmental filters override microbial traits in shaping ecosystem function. • Functional gene and enzyme activities decoupled in alpine grassland on Tibetan Plateau. • TP and SOM strongly predicted enzyme activities in alpine grasslands. • Microbial taxonomic composition provided limited predictive power compared to soil abiotic properties. • Functional redundancy and environmental filtering shape microbial enzyme patterns in alpine ecosystems.