A novel acrylamide and chitosan dual-network hydrogel urea slow-release fertilizer based on the carbamate/citric acid co-modified starch with enhanced water retention and nutrient slow-release property

The development of eco-friendly slow-release fertilizers (SRFs) based on the renewable industrial crops such as starch is crucial for sustainable agriculture. Herein, a carbamate/citric acid (CA) co-modified starch derivatives (CCAS) was synthesized via continuous esterification of natural starch (NS) using microwave-assisted hydrothermal coupled alcohol-thermal techniques, which was subsequently used as dopant for constructing the acrylamide (AM) and chitosan (CS) dual-network hydrogel (CCAS-CS-AM) through one-step radical copolymerization strategy, thereby formulating the corresponding urea SRF (CCAS-CS-AM-urea). The CCAS-CS-AM demonstrated excellent water absorption capacity of 33.32 g·g⁻¹ and remarkable soil water retention capability, maintaining 42% of initial moisture after 30 d of soil burial. The CCAS-CS-AM-urea exhibited superior nutrient slow-release behavior, with cumulative N release rates of 76.8% in water (24 h) and 80.4% in soil (30 d), substantially outperforming other controls. Nutrient slow-release kinetics analysis revealed that N release was governed by a synergistic mechanism of Fickian and non-Fickian diffusion, mainly dependent on the slow-release medium. Maize seedlings pot experiments verified that CCAS-CS-AM-urea has significantly plant growth promotion function compared to the pure urea, which was reflected in the enhancement of the plant height, biomass accumulation, and chlorophyll content. This work presents a feasible strategy for high-value conversion of starch into eco-friendly hydrogel SRF. Carbamate/citric acid (CA) co-modified starch (CCAS) was synthesized via microwave-assisted hydrothermal coupled alcohol-thermal continuous esterification of cassava starch (NS), followed by one-step radical copolymerization with acrylamide (AM) and chitosan (CS) to construct a dual-network hydrogel (CCAS-CS-AM), which was then loaded with urea to afford urea SRF (CCAS-CS-AM-urea). • A carbamate/citric acid co-modified starch (CCAS) was synthesized via two-step esterification using cassava starch (NS). • CCAS-CS-AM was prepared by one-step copolymerization for urea SRF (CCAS-CS-AM-urea) fabrication. • CCAS-CS-AM shows exceptional water absorption capacity and markedly enhanced soil water retention. • CCAS-CS-AM-urea SRF exhibits a cumulative N release rate of 76.8% (24 h) in water and 80.4% (30 d) in soil.