Iron Reduction in Coastal Wetland Habitats: Kinetics, Pathways, and Controls

Iron(III) reduction is a critical biogeochemical process governing the dynamics of organic carbon (OC), nutrients, and contaminants under fluctuating redox conditions in coastal wetlands. However, the relative importance of dissimilatory and sulfur-mediated Fe(III) reduction pathways and their environmental controls remain poorly constrained across coastal habitats. Through large-scale anaerobic incubations of 50 surface soils from mangrove and saltmarsh wetlands along China's coastline, we showed that dissimilatory Fe(III) reduction was the dominant pathway, accounting for ∼80% of total Fe(III) reduction. Although both dissimilatory and sulfur-mediated Fe(III) reduction rates were higher in mangroves, dissimilatory pathway contributed a greater fraction in saltmarshes, whereas sulfur-mediated Fe(III) reduction was more pronounced in mangroves. Across all soils, poorly crystalline Fe minerals and OC availability were the dominant predictors of dissimilatory Fe(III) reduction. Substrate-addition experiments (glucose, ferrihydrite, and Fe(III)-reducing bacteria) further revealed that Fe(III) reduction was primarily limited by labile OC availability and microbial abundance in both habitats, with stronger OC limitation in saltmarshes, whereas reactive Fe(III) mineral limitation occurred exclusively in OC-rich mangroves. These results define habitat-specific controls on Fe(III) reduction at a continental scale and help predict the fate of Fe-bound OC, nutrients, and contaminants in coastal wetlands.