Active carbon fractions mediate microbial responses to long-term organic–inorganic fertilization and full-film mulching in a Loess Plateau wheat system

Aims Soil organic carbon (SOC) on the Loess Plateau is widely depleted (< 1.2%), yet how fertilization and full-film mulching jointly regulate carbon quality and microbial functioning over the long term remains unresolved. We partitioned their effects on SOC fractions in dryland winter wheat ( Triticum aestivum L.) and identified which fractions govern microbial abundance and enzyme activity. Methods A six-year (2019–2025) 4×3 factorial experiment crossed four fertilization regimes (CK, M, NPK, MNPK) with three planting–mulching methods (CT, FM + SC, FM − SC). Fulvic acid carbon (FAC), humic acid carbon (HAC), light fraction organic carbon (LFOC), bulk SOC, microbial biomass, culturable community size, enzyme activities and grain yield were analysed by two-way ANOVA, regression, redundancy analysis and structural equation modeling (SEM). Results MNPK × FM + SC produced the greatest gains in LFOC (+ 73.5%) and HAC (+ 61.7%), while M × FM + SC maximized FAC (+ 58.1%) relative to CK × CT. Active carbon fractions, particularly LFOC, predicted culturable microbial abundances (R² = 0.593–0.872) and carbon-cycling enzyme activities (R² = 0.661–0.818), whereas total SOC was unrelated to any biological parameter (R² < 0.11). Phosphatase was decoupled from all carbon pools, indicating enzyme-specific regulation. SEM identified microbial activity as the dominant pathway driving organic carbon accumulation (β = 0.96, R² = 0.93). Grain yield increased by 33.6–38.7% under fertilization combined with full-film mulching. Conclusions Carbon quality rather than quantity governs soil biological function in this dryland system. Combined organic–inorganic fertilization with full-film soil-covered hole sowing offers a practical strategy for rebuilding active carbon and sustaining yield on the Loess Plateau.