A cellulose carbamate–grafted sodium alginate semi-interpenetrating network fertilizer with enhanced water retention and controlled nutrient release for sandy soil improvement

To address agricultural water shortage and low fertilizer utilization efficiency, this study developed a biodegradable carbamate cellulose‑sodium alginate/urea-formaldehyde resin hydrogel (CCS-NaAlg/UF) with dual water retention and nutrient slow-release functions. The material was prepared via chemical crosslinking with citric acid combined with physical crosslinking by urea-formaldehyde resin. Its structure was characterized using FT-IR, XRD, XPS, and SEM. Water absorption kinetics, cyclic stability, and soil water retention were evaluated, while nitrogen slow-release characteristics were assessed through leaching experiments. Degradation performance and practical application effects were investigated via soil burial tests and millet pot experiments. Results showed that the hydrogel reached swelling equilibrium in deionized water within 40 min with a water absorption ratio of 35.38 g/g. At 3 wt% addition to sandy soil, the maximum water retention rate increased to 94.8%, with the effective retention period extended to three times that of blank soil. Nitrogen slow-release performance was excellent, with cumulative release rates of 48.9% in water and 35.6% in soil within 30 days, with both release profiles conforming to the Korsmeyer-Peppas model (R > 0.93). Under drought stress, millet growth was significantly superior to the untreated group at both 7 and 14 days. The material achieved 43.07% soil burial degradation after 49 days with no microplastic residues. This work provides a green, efficient strategy for integrated water-fertilizer management in arid and semi-arid regions.