Combined Organic–Inorganic Fertilization and Plastic-Film Mulching Enhance Stable Carbon Fractions, Nutrient Availability, and Microbial Biomass in Semi-Arid Loess Plateau Soil

Soil degradation in semi-arid drylands demands integrated strategies that simultaneously enhance carbon storage, nutrient supply, and biological activity. Fertilization and plastic-film mulching are widely adopted on China’s Loess Plateau, yet their interactive effects on soil quality remain poorly quantified. A three-season field experiment (autumn 2022 to June 2025) was conducted on the western Loess Plateau using a full factorial design that crossed four fertilization regimes (unfertilized control, CK; manure alone, M; chemical fertilizer alone, NPK; combined manure plus chemical fertilizer, MNPK) with three mulching configurations (conventional tillage, CT; full-film mulching with soil-covered hole sowing, FMS; full-film mulching without soil covering, FMN). Ten soil quality indicators spanning stable carbon fractions, total and available nutrients, and microbial biomass were integrated into a composite Soil Quality Index (SQI). Fertilization dominated soil quality variation, explaining 43 to 89% of total variance, while mulching contributed an additional 2 to 26%, with the strongest effects on biologically active pools. The MNPK×FMS combination raised soil organic carbon from 10.11 to 14.61 g kg−1 (44.5%), with a significant interaction (F = 3.73, p = .009) generating a supra-additive gain of 1.64 g kg−1 (16.3% above additive expectation). Alkali-hydrolyzable nitrogen rose from 13.9 to 35.5 mg kg−1, and microbial biomass carbon increased by 134.2% (120 to 281 mg kg−1). The SQI ranged from 0.08 (CK×CT) to 0.91 (MNPK×FMS), whereas NPK×FMS reached only 0.48, and the difference between those two figures rests on the organic input. Fertilization appears to act mainly by supplying substrate and mulching by altering the topsoil environment, although soil temperature and water content were not measured in this experiment. The combination improved the soil properties measured here, and residual film remains a concern.