Sustained nitrate release from hydrogel encapsulated layered double hydroxides-biochar composite: Experiments and theoretical elucidation

Nitrate (NO₃⁻) remains a dominant nitrogen species and an indispensable nutrient for improvement of crop productivity. Conventional fertilizers suffer from deficient nitrate use efficiency, prompting the development of novel materials for effective retention and prolonged nitrate release. Layered double hydroxide (LDH) is a sustainable material that has long been used as a nutrient carrier. However, the inefficient anion exchange, the complexity of synthesis step and the unsatisfactory slow-release performance still limit its application. Herein, we developed a nitrate-intercalated Mg/Fe LDH-biochar composite encapsulated in calcium alginate hydrogel (LDH@BC/CA) via a one-step method, designed for maximal nitrate loading and sustained release. LDH@BC/CA composite at loading ratios of 25 % LDH and 20 % CA exhibited remarkable slow-release performance in aqueous conditions, achieving only 4.5 % cumulative release rate over a 10-day experiment. Compared with the control group, the addition of 0.5 % LDH@BC/CA resulted in minimal additional nitrate loss from the soil column over 30 days (from 3.10 mg to 3.19 mg) and a 1.26-fold increase in soil nitrate concentration. Shifts in the microbial community structure in soil amended with LDH@BC/CA provided evidence for the stimulated nitrogen cycling. Density functional theory (DFT) and molecular dynamic (MD) simulations were performed to rationalize the experimental observation of preferential Mg depletion over Fe after 30-days incubation in soil. DFT indicated notable electron depletion at both Mg and Fe sites, with Fe showing greater depletion (−1.77 e versus −1.68 e for Mg). The higher vacancy formation energy of Fe (10.24 eV versus 9.94 eV) further confirmed its stronger electrostatic retention and structural stability. MD simulations also revealed the high mobility of nitrate and partial Mg release, attributed to the weakened Mg–O coordination during the reconstruction of LDH framework. This study demonstrated the potential of LDH@BC/CA as a novel slow-release fertilizer for sustaining nutrient availability while mitigating nitrate pollution in agricultural systems. • A novel nitrate-intercalated LDH@BC/CA hydrogel was prepared for slow-release of nitrate. • LDH@BC/CA was superior in sustained release of nitrate over individual systems. • LDH@BC/CA improved soil nitrate concentration without significant nitrate leaching. • The nitrate release and reconstruction mechanism were elucidated by theoretical calculations.