Sustainable Zinc Biofortification of Mung Bean Using Cellulose-Based Slow-Release Fertilizer Hydrogel under Normal and Drought Stress Conditions

Abstract

Zinc (Zn) deficiency in edible legumes is a persistent nutritional problem that is exacerbated in systems with low micronutrient availability and recurrent drought, where conventional fertilizers often show poor synchronization between nutrient release and crop demand. This study aimed to evaluate cellulose-based slow-release hydrogel fertilizer formulations to improve Zn biofortification of mung bean (Vigna radiata L.) under normal irrigation (100% field capacity) and drought stress (40% field capacity), and to quantify treatment and irrigation effects on Zn accumulation and Zn use-efficiency indices. A controlled-environment pot experiment was conducted using a factorial arrangement of seven nutrient treatments (control, SRHG5, SRHG10, SRFHG5, SRFHG10, NPK-Fe, Zn, and Zn, Fe) across two irrigation regimes with three replicates (42 units). Cellulose-derived hydrogels were synthesized via cross-linking, loaded with Zn from ZnSO₄·7H₂O (5.6% elemental Zn) at an equivalent rate of 25 lb/A (~28 kg/ha; 0.08 g ZnSO₄ per pot), characterized for swelling and nutrient-release behavior, and applied near the root zone. At physiological maturity, seed and stover Zn concentrations were determined after acid digestion using AAS/ICP-OES, and data were analyzed by two-way ANOVA. Seed Zn ranged from 29.40-46.57 mg/kg, with SRFHG10 achieving the highest values under both 100% (46.57 mg/kg) and 40% irrigation (38.77 mg/kg); only SRFHG10 met the ≥40 mg/kg biofortification target under optimal irrigation, and no treatment reached the target under drought. Stover Zn was higher (25.80-59.40 mg/kg), again maximized by SRFHG10 (59.40 and 52.00 mg/kg under 100% and 40% irrigation). Nutrient treatment significantly affected Zn in seeds (F=9.32, p<0.001) and stover (F=756.26, p<0.001); irrigation had a smaller effect on seeds (F=7.16, p=0.01) but a strong effect on stover (F=277.71, p<0.001), with a significant treatment×irrigation interaction for stover (p<0.001) but not seeds (p=0.57). SRFHG10 also showed the highest Zn use efficiency (38.68 and 37.04) and agronomic Zn use efficiency (11.00 and 10.82), whereas Zn, Fe produced negative agronomic efficiency, indicating poor agronomic response. Overall, cellulose-based slow-release fertilizer hydrogel, particularly SRFHG10, enhanced Zn accumulation and fertilizer efficiency and partially mitigated drought-associated declines, supporting hydrogel-mediated micronutrient delivery as a promising strategy for improving mung bean nutritional quality, although further optimization is needed for consistent target attainment under water limitation.