Isotopic insights into iron and sulfur cycling in acid sulfate soils: Implications of seasonal redox fluctuations

Acid sulfate soils (AS-soils), common in coastal, estuarine, and mining-impacted areas, form under reducing conditions and can cause severe environmental degradation by releasing acidity and mobilizing heavy, trace, and toxic metals when sulfide minerals oxidize under changing hydrological conditions. AS-soils exhibit redox gradients that govern iron (Fe) and sulfur (S) cycling, affecting their mobility, mineral forms, and isotopic compositions. This study combines Fe/S ratios, δ 56 Fe and δ 34 S isotope systematics, sequential extractions, and groundwater monitoring to assess redox-driven processes in two AS-soil profiles from northern Sweden. In reduced zones, negative δ 56 Fe and δ 34 S values signal microbial Fe(III) and sulfate reduction, producing Fe(II) and sulfides. In oxidized zones, secondary Fe (oxy)hydroxides and sulfates form, inheriting isotopic signals from precursor sulfides. Groundwater δ 56 Fe enrichment reflects Fe oxidation, while high δ 34 S values (up to +45.2 ‰) indicate ongoing sulfate reduction. Groundwater Fe/S ratios (0.07–8.24) reveal redox interactions but are unreliable as sole redox indicators. Sequential extractions show that redox-sensitive pools—water-soluble, exchangeable, and organic-bound phases—exhibit strong isotope fractionation and drive short-term cycling, despite their small mass. Isotopic signals from different Fe phases within each profile likely offset each other when measured in bulk (∼0.06–0.11 ‰), diluting any clear redox-related patterns. Similarly, δ 34 S values trace a shift from oxidized to stable sulfide-bound forms across depths and redox zones. The results emphasize the value of combining isotopic and phase-specific analyses to unravel redox heterogeneity, trace element fluxes, and identify acidification-prone zones. Environmentally, maintaining saturated conditions, encouraging reducing environments, and monitoring reactive Fe and S fractions can help limit the mobilization of heavy, trace, and toxic metals in AS-soils under shifting hydrology. • Integrated Fe/S ratio and isotope analysis reveals redox heterogeneity in AS-soils. • Reactive Fe and S pools dominate short-term biogeochemical cycling. • Seasonal and depth-resolved isotope shifts indicate microbial and hydrological control. • Phase-specific methods enhance risk assessment for acidification in AS-soils.