Food security has been threatened by the increase in industrialization, urbanization, and desertification. The fertilizer can be applied to grow, while the fertilizer utilization rate is only 40% in China. The annual loss of nitrogen has reached up to 120 Tg, due to the ammonia evaporation volume and nitrogen oxide emissions. The unabsorbed nutrients are lost during leaching and runoff pathways, leading to soil acidification, the eutrophication of the water bodies, and greenhouse gas environmental issues. Therefore, it is an urgent need to improve the utilization rate of the fertilizer, in order to alleviate the agricultural surface pollution due to the high production in the food industry. In this study, a one-step coating was proposed to prepare the castor oil-based polyurethane-coated urea (PCU), in order to solve the complex procedure and the unfriendly environment of the conventional petroleum-based polyurethane-coated urea. The castor oil (CO) and Polymethylene polyphenyl isocyanate (PM-200) were used as the raw materials, while the paraffin was introduced as a modifier. A systematic investigation was then implemented to determine the water absorption, porosity, swelling capacity, and water contact angle of the liquid paraffin-modified polyurethane membranes before and after 7 days of immersion. The membrane with the 0.5% liquid paraffin also exhibited the lowest porosity (1.01%) and swelling capacity (1.67). Notably, the water contact angle of the 0.5% liquid paraffin-modified membrane reached 102.8°, indicating a 4.35° increase, compared with the unmodified polyurethane membrane. The optimal slow-release ratios were explored using a UV spectrophotometer, Kjeldahl nitrogen tester, and scanning electron microscope. The slow-release performance, nitrogen loss, and microstructure were determined to be the influencing factors on the castor oil content, paraffin wax addition, coating temperature, and rotational speed. The soil column drenching and field tests were carried out to validate the application. Paraffin tests showed that the slow-release period of PCU with 5% coating reached 66 days, which was 34.7% longer than that of the unmodified system, under the optimal conditions of 60% castor oil, 0.5% paraffin addition, 60 ℃ coating temperature, and 20 r/min rotational speed; Paraffin was significantly improved the hydrophobicity, where the pores of the film layer were filled to reduce the surface energy. As a result, the cumulative ammonia evaporation volume of PCU with 5%-9% coating decreased from 1 200 mg/kg (normal urea) to 170-340 mg/kg within 30 days, indicating a reduction of 71.67%-85.83%. Additionally, the 30-day leaching loss rate of the total cumulative nitrogen decreased by 77.94%-91.01%, and the losses of ammonium nitrogen, nitrate nitrogen, and amide nitrogen dropped to 119.6, 151.4, and 50.2 mg/L, respectively, indicating a decrease of 82%-93%. A field experiment was conducted in Xinxiang City, Henan Province, China. Furthermore, the maize plant height, stem thickness, chlorophyll content (SPAD value) and dry matter accumulation of the homemade film-wrapped urea-treated group were 73.13 cm, 1.883 cm, 61.20, and 322 g, respectively, under the condition of 20% yield reduction, compared with the ordinary urea-treated group, which were 11.07%, 2.3%, 9.76%, and 10.30% higher, respectively. The yield reached 11 914.35 kg/hm², which was 22.1% higher than the normal urea treatment group. The optimal raw material ratio and paraffin hydrophobic modification can be expected to overcome the technical limitations of the conventional multi-step modification of the vegetable oil-based polyols, which was 22.1% higher than that of the control. The finding can provide a promising potential approach for high-efficiency slow-release fertilizers, in order to prevent and control agricultural surface pollution.

Chemical fertilizers have been widely used for crop production in recent years. However, the current utilization rate of chemical fertilizers is generally low. Particularly, the excessive application has also led to the enrichment of the soil nutrients in the short term, far exceeding the nutrient requirements of plant growth. This nutrient imbalance can cause a serious waste of the fertilizer resources, and a series of chain environmental issues, such as soil compaction, microbial imbalance, ammonia volatilization, and nitrogen leaching. Multiple threats have been posed to the ecological environment. In this study, a composite coated urea was prepared to match the nutrient demand of the crops using slow release, in order to improve the soil water use efficiency with the help of the water retention function. The superabsorbent polymer (SAP) in the laboratory was used as the outer coating. The inner coating was taken as the polyurethane that was synthesized by polymethylene polyphenyl polyisocyanate (PAPI) and castor oil (CO). The composite coated urea (PCUB) was prepared with multiple functions, such as water retention and slow release. A systematic investigation was made to explore the effect of the amount of SAP on the slow release and loss control performance of the fertilizer. The number of days was 65 days for 80% slow release of polyurethane-coated urea without SAP (PCU). The PCUB with 5% SAP (PCUB5) effectively prolonged the slow-release period of fertilizer to 72 days. The hydrogel layer was formed into a three-dimensional network structure. The nitrogen release was then delayed after water absorption. The SAP significantly increased the water absorption of the fertilizer. Specifically, the water absorption of the PCUB5 reached 16.53 g/g. The experimental results showed that the ammonia volatilization of the PCUB5 treatment group was 17.65% lower than that of the PCU ones within 30 days, and 76.67% lower than that of the pure urea ones. At the same time, the soil with the PCUB shared a significant increase in the water holding and retention rate. Therefore, the SAP enhanced the soil water interception and then extended the effective water supply cycle of the soil. The soil column leaching experiment shows that the 30-day total nitrogen cumulative loss concentration of the PCUB5 treatment group was 79.03% lower than that of the pure urea treatment group. The field experiment showed that the PCUB5 treatment group, with a 20% reduction in dosage, significantly improved the growth performance of the summer maize, compared with the pure urea treatment. Among them, the plant height, stem diameter, chlorophyll content, dry matter content, and yield increased by 4.37%, 5.59%, 4.9%, 6.71%, and 24.7% (P<0.05), respectively. The nitrogen release cycle of the PCUB was more suitable for the nutritional needs of the summer maize. The synergistic effect of the hydrogel barrier and hydrogen bond can greatly contribute to the nitrogen release and crop fertilizer requirement cycle. The water-fertilizer regulation can provide a promising solution to sustainable agriculture in the arid areas. The nutrient release cycle of the PCUB can fully meet the nutritional needs in the growth cycle of the summer maize. The yield of summer maize can be significantly enhanced under the condition of reducing the 20% urea. A kind of fertilizer product can also be obtained with excellent prospects.