Relative contribution of iron reduction to sediments organic matter mineralization in contrasting habitats of a shallow eutrophic freshwater lake

Iron reduction is one of the important organic matter (OM) mineralization pathway in sediments. Here we investigated the rates and the relative contribution of iron reduction to OM mineralization in Zhushan bay (ZSB, cyanobacterial bloom biomass (CBB)-dominated habitats) and East Taihu Lake (ETL, submerged macrophypes (SM)-dominated habitats) of Lake Taihu, China. Anaerobic microcosm incubation revealed that the rate of iron reduction at ZSB (4.42 μmol cm⁻³ d⁻¹) in summer was almost 1.5 times higher than at ETL (3.13 μmol cm⁻³ d⁻¹). Iron reduction accounted for 66.5% (ZSB) and 31.8% (ETL) of total anaerobic carbon mineralization, respectively. No detectable methanogenesis was found at ZSB, while methanogenesis was responsible for 16.7% of total anaerobic respiration in sediments of ETL. Geochemical analysis of solid phase constituents indicated that ZSB surface sediments experienced highly oxidizing conditions with much higher amorphous Fe(III) (71 mmol m⁻²) than ETL (11 mmol m⁻²). Conversely, AVS inventories at ETL (38 mmol m⁻²) were up to 30 times higher than at ZSB (1.27 mmol m⁻²), indicating significant sulfate reduction in sediments of ETL. Overall results suggested that varying carbon sources and distinct geochemical characterizations of the sediments in contrasting habitats significantly influenced the rate of iron reduction and the pathway of C mineralization in a large freshwater lake. - Highlights: • Iron reduction dominated anaerobic carbon metabolism in algae-dominated habitats. • Significant sulfate reduction was observed in Elodea Nuttallii-dominated habitats. • Obvious methanogenesis was observed in Elodea Nuttallii-dominated habitats. • Relatively high rate of iron reduction was found in algae-dominated habitats. • Elodea Nuttallii-dominated habitats were relative hotspots of cycling of elements. - Iron reduction played a more important role in organic matter mineralization in CBB-dominated habitats than in SM-dominated habitats.