AquaSteady seaweed-derived alginate hydrogel alters soil chemistry and biological indicators in a rate- and soil-dependent manner

Abstract AquaSteady (AQST) is a biodegradable, alginate-based, seaweed-derived hydrogel used in agriculture to enhance soil water retention, but its effects on soil physicochemical and biological indicators across contrasting soils remain poorly understood. We evaluated four AQST rates (0, 2, 5, and 10 mg/g soil) in six contrasting soils over a 56-day incubation, and measured soil chemistry and phospholipid fatty acid (PLFA)-based biological indicators. Soil pH rose from 7.38 (control) to 8.23 (high rate; p < 0.001), shifting several soils toward alkaline, while cation exchange capacity declined from 25.32 to 23.83 cmolc kg − 1 ( p < 0.001 ). Exchangeable K, Ca, and Mg fell by ~ 13%, 8%, and 12%, and sulfate and nitrate-N decreased by ~ 29% and 62%, at high AQST rates versus control, while Na roughly doubled (271 to 548 mg kg − 1 ; p < 0.001), indicating salinization risk in Na-sensitive soils. Total living microbial, fungal, protozoa, and undifferentiated microbial biomass increased mainly at medium and high rates; bacterial biomass and functional diversity showed no dose response. Mantel analysis found no statistically robust physicochemical-biological association after correction for multiple comparisons in any soil. Biological indicators were generally highest at the high rate, though medium (5 mg g⁻¹) did not differ statistically from high for most indicators, while sodium rose less and pH rose comparably at both rates. The medium rate may offer a better trade-off between biological benefit and salinity risk for sodicity-prone soils, pending calibration. These findings show soil-dependent trade-offs between biological stimulation and chemical shifts, indicating field validation is needed before broader AQST recommendations.