Preparation of hydrophobically modified sodium alginate-chitosan hydrogel and its characteristics in urea controlled-release fertilizers

Urea is widely used in agriculture because of its high nitrogen content and low cost, but its utilization efficiency is often limited by volatilization, leaching, and nitrification-denitrification losses. In this study, sodium alginate (SA) was hydrophobically modified by free-radical grafting with methyl methacrylate (MMA) to obtain SA-PMMA, which was subsequently ionically cross-linked with chitosan (CS) to construct hydrogel carriers for controlled-release urea fertilizers. The resulting materials were characterized by FTIR, XRD, TG-DTG, XPS, and SEM-EDS, confirming successful grafting and indicating interactions between urea and the SA-PMMA/CS matrix. The effects of grafting ratio on water absorption, urea loading, nitrogen release, degradation, and fertilizer performance were systematically investigated. Among the five formulations, sample Z3 exhibited the best overall release behavior, with a cumulative nitrogen release of 77.33% over 56 days, meeting the national standard for slow-release fertilizers. Release-kinetic analysis showed that nitrogen release was best described by the first-order model and was governed by Fickian diffusion. In spinach pot experiments conducted at the same nitrogen input, the hydrogel fertilizers, especially Z3, promoted root development, leaf expansion, chlorophyll a ccumulation, dry matter production, and plant nitrogen uptake compared with conventional urea. These results demonstrate that hydrophobically modified SA/CS hydrogels are promising carriers for improving urea retention, regulating nitrogen release, and enhancing agricultural nitrogen-use efficiency.