Controlled-Release Fertilizer Improving Paddy Yield and Nitrogen Use Efficiency by Reducing Soil Residual Nitrogen and Leaching Losses in the Yellow River Irrigation Area

The unreasonable application of nitrogen (N) fertilizer leads to high nutrient losses and severe potential of agricultural non-point source contamination, which threatens water quality in the upper Yellow River Basin. Therefore, the aim of this study is to explore the effects of N application rates and various control measures on rice yield and N leaching in paddy fields in the Yellow River irrigation area. Four treatments were employed in this study, CK (no N fertilizer application, 0 kg N∙ha^-1), CRU (controlled-release urea application, 180 kg N∙ha^-1), OPT (optimal N fertilizer application, 210 kg N∙ha^-1), and FP (N fertilizer application based on farmer experience, 240 kg N∙ha^-1), to examine paddy yield, N use efficiency (NUE), N concentrations in leaching water at various soil depths, and N contents along the 0-100 cm depth of the soil profile. The results indicated that the amount of TN leached was 25.14-48.04 kg∙ha^-1 after different N applications, and the TN leaching coefficients of FP, OPT, and CRU were 10.88%, 11.27%, and 7.07%. Compared to FP and OPT, the CRU significantly reduced the concentrations of TN, ammonium N (NH₄⁺-N), and nitrate N (NO₃^--N) in the surface and soil water, with average TN leaching decreasing by 31.55% and 27.35% in the years 2022 and 2023, respectively. NO₃^--N was identified as the primary form of N leached from the paddy fields. Compared to FP and OPT treatments, the CRU treatment increased the average paddy yield by 19.99-20.66% and improved the average NUE by 19.04-16.38%. This study revealed that the application of high amounts of N positively affected soil N leaching, and controlled-release urea demonstrates superior efficacy compared to conventional fertilization. The application of controlled-release urea at a rate of 180 kg N∙ha^-1 not only ensures a good paddy yield but also reduce N losses, which should be recommended to local farmers.