Microbial and anthropogenic controls on global soil nitrogen mineralization efficiency

Soil nitrogen (N) turnover regulates plant nutrient availability and potential N loss, yet how microbial communities, anthropogenic activities, and climate-change factors are associated with its global patterns remains poorly synthesized. Here, we quantified soil N turnover using the N mineralization efficiency index (NMEI), defined as gross N mineralization normalized by soil N content, together with recalcitrant and labile NMEIs. Using 2444 observations from 373 ¹⁵N-based studies, we combined global pattern analysis with meta-analysis to evaluate climatic, edaphic, microbial, land-use, fertilization, and global-change associations with NMEI. We found that NMEI was higher in croplands and alkaline soils than in natural ecosystems and acidic or neutral soils. Recalcitrant NMEI exceeded labile NMEI in croplands and alkaline soils, suggesting greater contribution of less labile organic N pools to mineral N production under these conditions. Bacterial abundance was the strongest positive predictor of NMEI, whereas fungal abundance, fungal-to-bacterial ratio, and microbial biomass were negatively associated with NMEI. Higher temperatures and soil pH, combined with lower soil organic carbon, were associated with higher NMEI, partly through shifts in microbial community towards bacterial dominance. Meta-analysis showed that land-use change and fertilization increased NMEI by 11.7% and 21.4%, respectively, whereas direct climate-change factors showed no significant overall effect. Conversion of natural forests to croplands or managed plantations increased NMEI by 17.4% and 19.8%, respectively. Balanced N-phosphorus-potassium, organic-only, and combined organic-balanced fertilization increased NMEI by 62.7%, 49.2%, and 47.7%, respectively, whereas inorganic N-only fertilization had no significant effect. These findings suggest that bacterial dominance and anthropogenic changes in land use and fertilization are key predictors of global soil N turnover. Nitrogen management should therefore interpret and optimize mineral N supply by jointly considering microbial regulation, land-use effects, and balanced or organic nutrient inputs.