Modification of gelatin hydrogel for soil water retention: Physical blending or chemical combination with humic substance extracted from compost

Water scarcity is a critical problem around the world, and superabsorbent hydrogels has attracted growing attention in water management for handling water deficiency during agricultural and forestry practices. Herein, intending to apply gelatin hydrogel as soil conditioner, humic substances (HS) extracted from Chinese medicine residue compost were used to modify gelatin hydrogel through either physical mixing or chemical cross-linking. The results demonstrated that low level of HS could improve the hardness and rheological properties of the hydrogels, however, the gel strength significantly decreased when the concentration of HS rose up to 16 g/L. As revealed by TEM and XRD, chemical cross-linking reaction promoted the development of denser network structures, thereby improving the hardness and rheological properties of the hydrogels. Subsequently, applying HS at a concentration of 3 g/L was found to be preferable for enhancing the swelling ratio of the gelatin hydrogels, and lightweight substrates amended with the resultant hydrogels displayed superior water retention ratio (17.23 ± 0.79 % for GelHS3 and 17.74 ± 1.31 % for GelHS3-EDC). Furthermore, it was proved that HS-incorporated hydrogels can effectively keep moisture for the growth of Melaleuca alternifolia (Maiden & Betche) Cheel saplings under drought stress. These findings suggest that humic substances can be utilized to modify hydrogels for use as soil conditioners.