Enhanced-Efficiency Nitrogen Fertilizers Coated with Sulfur, Inhibitor, and Epoxy Resin: Preparation and Effects on Maize Growth and Nitrogen Utilization

Priorities in controlled-release fertilizer research include reducing the amount of epoxy resin needed in the coating, adjusting the solubility of urea and inhibitors, reducing the environmental impact of nitrogen fertilization, and ensuring that fertilizer is released at the same rate as crop requirements. In this study, sulfur-coated urea was combined with inhibitors to create composite-coated stabilized urea (CCSU) for improved fertilization efficiency, specifically four different methods of the coating were used: CCSU1 (HQ and DCD are sprayed on the surface of sulfur-coated urea, followed by an epoxy resin coating.), CCSU2 (HQ and DCD combined with epoxy resin are sprayed directly onto the sulfur-coated urea surface.), CCSU3 (spraying HQ on the sulfur-coated urea surface first, then spraying a mixture of DCD and epoxy resin), and CCSU4 (DCD is sprayed onto the sulfur-coated urea surface, followed by a mixture of HQ and epoxy resin.). The microstructure and controlled-release performance of CCSUs were assessed by means of scanning electron microscopy, hydrostatic release, and ammonia volatilization tests. The pot experiment was used to study the fertilization efficiency of CCSUs. The SEM results showed that the CCSUs have smooth, complete surfaces and a uniform thickness of film with tiny nitrogen release channels. The result of hydrostatic release tests showed that CCSUs delayed the time to 80% nitrogen release by 9–23 days in comparison with the epoxy resin coated urea (the treatment R). CCSU3 showed the best inhibition of ammonia volatilization, with a 16.24% reduction in ammonia volatilization at 28 days compared to R. Pot experiment studies with maize showed that CCSUs can continuously maintain N supply and improve corn yield and nitrogen use efficiency (NUE). In particular, the CCSU3 exhibited better agronomic effectiveness, with 23.81% higher yield and 23.79% higher NUE than R. It is recommended to mix DCD with epoxy resin and spray HQ alone onto the surface of sulfur-coated urea to form the CCSU, which could ensure the controlled release of nutrients while reducing the amount of epoxy resin and improve the release and conversion of nitrogen as well as the synchronization effect with the nitrogen demand of corn, thereby further enhancing the NUE.