A multifunctional sodium alginate/chitosan composite hydrogel for saline-alkali soil remediation: Synthesis, characterization, and proof-of-concept for promoting crop growth.

Soil salinization threatens agriculture by restricting water uptake, inducing ionic stress, and destabilizing soil structure. We developed a vapor-phase acidification method to prepare a sodium alginate/chitosan (SA-CTS) dual-network hydrogel incorporating biochar (BC) and palygorskite (Pal) as functional fillers. The BC/Pal/SA-CTS composite absorbed 66.08 g/g in deionized water and 36.37 g/g in 0.9% NaCl, retaining approximately 55% of its DI-water swelling capacity under saline conditions. Component-resolved assays conducted at an equal total amendment rate suggested that the SA-CTS hydrogel control mainly supported water retention and aggregation, whereas BC- and Pal-amended soils showed stronger Na+-management and CEC-related responses. Because these controls were not matched to the exact component fractions in the composite, the results indicate apparent dominant roles rather than a quantitative component mass balance or formal synergy. Carbonate stability tests showed that the composite retained measurable swelling and visible integrity at 5-10 mM Na₂CO₃, showed destabilization onset at 25 mM, and disintegrated at 50-100 mM; however, it maintained measurable swelling in 1:5 Songyuan soil water extract under the present test conditions. In a 15-day maize seedling assay in saline-alkali soil, all amended treatments achieved 100% survival versus 53.3% in untreated soil. Among the composite dosages, 0.8% (w/w) gave the most balanced growth and stress-marker responses under controlled pot conditions. Overall, this polysaccharide-based composite provides a multifunctional proof-of-concept platform for saline-alkali soil amendment, with soil-burial dry-mass-loss potential under the tested conditions. Field performance, economic viability, carbonate-rich site specificity, degradation-product identity, long-term environmental fate, and economically realistic application strategies remain to be validated.