Co-Application of Urea and Slow-Release Nitrogen Fertilizer Mitigates Nitrogen Loss and Enhances Silage Maize Yield on Sloping Red Soil Cropland

Aims: This study aimed to evaluate how the combined application of slow-release nitrogen fertilizer and urea influences nitrogen losses via runoff and the yield performance of silage maize in red soil sloping cropland, in order to support region-specific strategies for soil and water conservation and efficient nitrogen management in maize-based systems. Methods: A one-year field runoff plot experiment was conducted under natural rainfall conditions. Three fertilization treatments with equal nitrogen input were established: conventional urea application (CK), 25% controlled-release nitrogen fertilizer combined with 75% urea (P1), and 50% controlled-release nitrogen fertilizer combined with 50% urea (P2). Results: (1) Co-application of slow-release fertilizer and urea significantly improved soil and water conservation compared to CK; total runoff was reduced by 9.17% and 8.29% under P1 and P2, respectively; and soil loss was reduced by 16.45% and 12.25%. (2) The co-application of slow-release fertilizer effectively reduced nitrogen losses from sloping farmland. Compared to CK, both P1 and P2 reduced losses of total nitrogen (TN), nitrate nitrogen (NO3−-N), and ammonium nitrogen (NH4+-N) during the maize growth period, with the reductions under P2 being most pronounced. (3) The co-application of slow-release fertilizer and urea enhanced maize nitrogen uptake and increased maize yield. Conclusions: Based on the results of this single-year field experiment, the combined application of controlled-release nitrogen fertilizer and urea showed potential advantages in reducing runoff-related nitrogen losses while improving crop productivity in red-soil sloping cropland. Among the treatments, P2 (50% controlled-release nitrogen substitution) exhibited relatively better overall performance. However, the broader applicability of these findings requires further verification through multi-year field experiments under different climatic conditions.