Fabrication of Eco-Friendly Biomass-Based Hydrogel Beads as an Effective Matrix for Slow-Release Fertilizer

Rapid dissolution of conventional fertilizers causes low nutrient-use efficiency and serious leaching losses, contributing to agricultural non-point source pollution. In this study, biomass-based slow-release fertilizer beads were prepared by ionic crosslinking of potato starch (ST), chitosan (CS), and corn-straw biochar (BC), using potassium nitrate (KNO3) as the model nutrient. The effects of ST/CS ratio and BC incorporation on bead structure, swelling, nutrient loading, release kinetics, and soil-column leaching were systematically investigated. Biochar incorporation formed a more compact and interconnected porous network and reduced the equilibrium swelling ratios of ST90/CS10, ST80/CS20, and ST70/CS30 from 188%, 176%, and 164% to 168%, 136%, and 104%, respectively. Although BC slightly decreased KNO3 loading capacity, it markedly slowed nutrient release; ST80/CS20/BC20 released 31.09%, 50.09%, and 81.82% of loaded KNO3 at 24, 72, and 504 h, respectively, which were 28.40%, 25.27%, and 11.30% lower than those of ST80/CS20. Kinetic fitting indicated that BC reduced the apparent release rate and promoted diffusion-controlled release behavior. Soil-column experiments further showed that the beads reduced NO3−-N and K+ leaching compared with free KNO3, with ST80/CS20/BC20 showing the best balance between nutrient loading and release control. These results suggest that starch–chitosan–biochar beads are a promising biodegradable matrix for slow-release fertilizer applications.