Straw Return Combined With Nitrogen Fertilizer Application Improve Nitrogen Utilization Efficiency by Regulating Soil Physical and Chemical Properties in Spring Maize

ABSTRACT Straw return (SR) and controlled‐release nitrogen fertilizers application can improve soil structure, enhance soil water retention, and promote crop growth. However, the mechanisms through which SR and controlled‐release nitrogen fertilizers application jointly modulate soil physical and chemical properties remain unclear. This study, conducted during 2023–2024, investigated the effects of five nitrogen application rates: 0, 100, 150, 200, and 250 kg/ha (N0, N100, N150, N200, and N250), under SR and traditional planting (TP, without SR) conditions on soil properties and nitrogen use efficiency (NUE). The results showed that SR and controlled‐release nitrogen fertilizers significantly influenced soil nitrate nitrogen and ammonium nitrogen, bulk density, aggregate composition, mean weight diameter, geometric mean diameter (GMD), and total nitrogen accumulation in all maize organs. Compared to N0, the N200 treatments combined with SR significantly increased soil microbial biomass carbon (MBC) by 17.3%–35.5% during 2023–2024, and significantly increased the proportion of macroaggregates, and NUE, with the GMD increasing by approximately 17% (17.66% in 2023 and 16.07% in 2024). Correlation analysis revealed that NUE are influenced by two key conditions: optimized soil physical structure and appropriate nitrogen. Random forest analysis further revealed that straw returning significantly increased the relative importance of MBC, shifting the yield dependency from direct inorganic nitrogen supply toward a mechanism mediated by soil biological activity. Partial least squares path modeling clarified that straw returning primarily improved maize yield by indirectly enhancing soil physical properties ( ꞵ = 0.710, p < 0.001), while controlled‐release nitrogen fertilizer directly supplied nitrogen source ( ꞵ = 0.920, p < 0.001). And these synergistically improved NUE, thereby increasing yield. However, no significant differences were observed between the N200 and N250 treatments. Therefore, the application of SR combined with 200 kg/ha of nitrogen fertilizer (a 20% reduction compared to the 250 kg/ha nitrogen treatment) can maintain soil fertility while reducing the potential risk of nitrogen leaching to the environment. These results provide support for integrating SR into fertilization practices in spring maize systems, highlighting its potential for soil fertility improvement.