Concentration‐Dependent Sulphate Enrichment Restructures Microbial Communities and Associated Sulphur–Iron–Nitrogen Dynamics in Freshwater Sediments

Sulphate input from industrial discharge, mining, and agricultural runoff can significantly alter biogeochemical functions in freshwater systems. However, the concentration‐ and time‐dependent effects of sulphate on geochemistry and microbial communities of lake sediments remain insufficiently characterized. Here, we incubated lake sediment microcosms amended with 1, 5, and 10 mM sulphate under suboxic conditions for 45 days and monitored geochemical and microbial community changes. Sulphate enrichment induced distinct, dose‐ and time‐responsive changes. Nitrate increased progressively with sulphate concentration (10 > 5 > 1 mM), reflecting treatment‐specific shifts in nitrogen dynamics under changing redox conditions within sedimentary nitrogen pools. Microbial diversity declined initially across sulphate treatments, peaked at 5 mM, and remained suppressed at 10 mM. Dominant taxa shifted from Geobacter spp. at 1 mM to Citrifermentans spp. and Sulfurospirillum spp. at 10 mM, reflecting redistribution of electron flow among competing iron, sulphur, and nitrogen associated metabolisms at higher sulphate concentrations. Functional predictions suggested enhanced iron respiration at lower sulphate (1 mM), whereas higher concentrations were associated with increased chemoheterotrophy and fermentation, alongside suppression of methanogenesis. Our findings demonstrate that increasing sulphate inputs restructure sedimentary microbial networks and reconfigure redox‐linked carbon and nitrogen cycling, suggesting ecosystem‐level biogeochemical responses to rising sulphate stress in freshwater catchments.