Four Decades of Organic–Inorganic Fertilization Enhances Maize Yield Through Synergistic Improvements in Soil Quality

ABSTRACT

The long‐term benefits of organic amendments on soil health are recognized, yet the mechanistic pathways by which integrated fertilization sustains crop productivity in brown soils remain poorly quantified. We addressed this using a unique 46‐year field experiment with six treatments: an unfertilized control (CK), chemical fertilizer alone (NPK), low‐ and high‐rate organic fertilizer alone (M1, M2), and their combinations with NPK (NPKM1, NPKM2). To capture the integrated soil quality, a soil quality index (SQI) was established based on key physicochemical and microbial properties. We found that combined organic–inorganic fertilization (NPKM) achieved the highest maize yield and SQI. Crucially, structural equation modeling unraveled the causal mechanisms: (1) Available phosphorus (AP) directly enhanced SQI and microbial biomass carbon (MBC), which boosted yield, overriding its concurrent direct negative effect. (2) Reduced soil bulk density suppressed fungal richness, thereby improving SQI and yield. Furthermore, long‐term organic amendments drove a beneficial restructuring of the microbial community, selectively enriching beneficial genera (e.g., Bacillus ) while suppressing pathogenic fungi such as Epicoccum . Our findings establish a systemic perspective that integrated soil health rather than single‐factor optimization governs long‐term productivity, providing a mechanistic basis for sustainable brown soil management.