Biodegradable dual-crosslinked chitosan/oxidized starch for slow-release urea with water retention

Low fertilizer utilization efficiency, water scarcity, and the non-degradability of synthetic polymer matrices necessitate sustainable alternatives in agriculture. This study proposes a fully biomass-based, dual-crosslinked chitosan/oxidized starch (CS/OS) hydrogel for controllable degradation and efficient water-fertilizer management. CS/OS hydrogels were constructed via a Schiff base reaction and borate coordination, followed by urea loading. A multi-scale approach integrating FT-IR, XPS, and SEM was employed to elucidate the degradation mechanism. The optimized CS/OS (1:1) hydrogel exhibited a water absorption capacity of 2278% and superior soil water retention. Mechanistically, the hydrogel degraded by 69.91–77.32% over 35 days, a process initiated by oxidized starch breakdown that triggered 3D network collapse prior to chitosan decomposition. This controllable degradation dictated nitrogen release kinetics, with CS/OS/Urea achieving 52.09–77.95% release in soil over 35 days, governed by a transition from swelling-driven diffusion to matrix erosion-dominated transport. XPS evolution confirmed synchronous polymer decomposition and NH4 + liberation. Benefiting from this synchronous water-nutrient supply, CS/OS/Urea 1:1 increased pakchoi fresh and dry biomass by 38.07% and 20.52%, respectively, versus pure urea. This study presents CS/OS/Urea as a multifunctional slow-release urea formulation that enhances nitrogen use efficiency and crop productivity while mitigating environmental contamination. The findings underscore the practical viability of biomass-based hydrogels in sustainable agricultural systems and provide mechanistic insights for designing future hydrogel-based fertilizers.