Film Mulching with Straw Return and Controlled-Release and Conventional Urea Blend Alters Aggregate-Associated Nitrogen Distribution While Sustaining Grain Yield in Dryland Maize

Plastic film mulching (FM) improves crop productivity in dryland agriculture but risks soil quality degradation under continuous chemical fertilization. Under FM with optimized nitrogen (N) input from a controlled-release and conventional urea blend (CCU), straw return (SR) management may either contribute to grain yield or alter aggregate-associated N pools. A two-year film-mulched experiment was conducted with three treatments: no fertilization (CK), CCU alone (CU), and CCU plus SR (CS). Compared with CU, CS did not further increase grain yield despite greater total N uptake during critical growth stages. However, CS increased the proportion of large macroaggregate (LM) by an average of 10.0 percentage points and the mean weight diameter by 35.7%, and also increased N concentration in most aggregate fractions. A significant increase in N stock was detected in the LM fraction in both years. In contrast, N stock in the microaggregate (MI) was significantly lower under CS in 2020, whereas N stocks in small macroaggregate (SM), silt and clay (SC), and bulk soil were not significantly affected in either year. SM N stock was positively correlated with grain yield in the pooled dataset, but this association was attenuated after controlling for year and should therefore be considered exploratory. Overall, CS maintained grain yield comparable to CU while altering aggregate-associated N distribution, suggesting a potential decoupling between short-term yield and aggregate-associated N pools; however, the origin of the additional aggregate-associated N could not be resolved without isotopic tracing.