Oligosaccharide modified alginate/biochar composites hydrogel to fabricate slow-release fertilizer

The development of slow-release fertilizers (SRFs) combining controlled nutrient release and water retention is a promising strategy to solve water-fertilizer asynchrony in agricultural applications. Herein, a green SRF system was fabricated by incorporating cellulose oligosaccharides (COS) into an alginate-biochar composite hydrogel. The multi-hydroxyl structure of COS binds water molecules and interacts with the alginate matrix, forming a multiple hydrogen-bond network. This network effectively regulates water distribution, thereby synergistically enhancing both the water retention and slow-release performance of the material. Results show that the intermediate-to-free water ratio obviously increases from 0.16 to 0.40 with the addition of 5% COS, while the material also exhibits the best long-term water retention capacity. The soil water-holding capacity increases by 85.41%. In terms of release behavior, the initial release rate is reduced from >30% to ≤22%, providing the flattest 30-day release profile. Pot experiments further verify that the 5% COS-modified hydrogel composite significantly promotes the growth and chlorophyll accumulation of pak choi and tatsoi by regulating water and urea release in a sustained manner.