Urea Peroxide Embedded in Alginate–Chitosan Compound Coating Material Containing Selenate Altered Redox Potential and Selenium Speciation in Paddy Soil

Low redox potential (Eh) leads to low bioavailability and utilization efficiency of several nutrients, including selenium (Se), in paddy soil. Applying oxidants with a slow-release function to maintain soil Eh at a sufficiently high level during the key nutrient stage is beneficial to plant-nutrient accumulation. A multi-component coating material consisting of alginate, chitosan, nano-silicon dioxide, carboxymethyl cellulose, and selenate was used to embed urea peroxide to prepare a slow-release fertilizer of oxidative substances in this study. The effects of the components on oxidative substance release and mechanical properties were investigated, and their contribution to altering soil Eh and Se speciation was verified by paddy soil assay. The formula containing 20~25 mg g−1 alginate, 15 mg g−1 chitosan, 30 mg g−1 silicon dioxide nanoparticle, 10 mg g−1 carboxymethyl cellulose, 0.1 mg g−1 sodium selenate, and 150 mg g−1 urea peroxide performed the best among treatments. It had satisfactory mechanical properties and an elevated Eh with an amplitude of over 100 mV for at least 40 d compared to the soil with a single Se treatment, and increased the contents and proportions of soluble, exchangeable, and a part of iron/manganese-binding Se, especially the hexavalent one, in soil. Notably, selenate not only supplied Se but also contributed to structure reinforcement. The findings provided a slow-release carrier for soil nutrient management, but it still needs to be improved for more coordinated release of Se and oxidative substances.