Improving properties of soy protein–based hydrogel composites by incorporating bamboo biochar towards slow release and water retention of fertilizers and enhanced plant growth

The development of environmentally friendly slow-release fertilizers with effective water retention is an urgent need in modern agriculture. Although biochar can improve soil fertility as a nutrient carrier, it suffers from poor slow-release performance and water retention. Conversely, soy protein hydrogels, characterized by their hydrophilic nature with three-dimensional cross-linked networks, can retain large amounts of water and facilitate the slow release of fertilizers and water due to their high specific surface area. Hence, a novel slow-release composite material with high water retention was prepared by introducing bamboo biochar into a soy protein–based hydrogel (SPB) network through graft copolymerization. The findings indicated that the bamboo biochar promoted the SPB cross-linked network density, which improved the swelling rate of SPB materials and soil water-holding capacity. Moreover, SPB-2–4% and SPB-3–4% exhibited superior slow-release capabilities for nitrogen fertilizer. Cucumber seedlings treated with SPB materials containing bamboo biochar demonstrated enhanced growth and chlorophyll content than those treated with biochar-free SPB materials. Compared with the control, the cucumber plants treated with SPB-2–4% displayed a significant increase in fresh weight, root length, and leaf area by 139.32%, 99.20%, and 149.45%, respectively, which can be attributed to the positive synergistic effect of soy protein and bamboo biochar. Furthermore, the nutrients and porous structures of bamboo biochar favor the proliferation of microorganisms, enriching the soil microbial community. Therefore, the bamboo biochar-soybean protein hydrogel composites have great application prospects for sustainable agriculture and provide a new direction for the development of slow-release and water-retention fertilizers.