Hydrologic history and flooding pulses control iron and sulfur metabolic composition with variable associations to greenhouse gas production in a coastal wetland mesocosm experiment

Coastal wetlands can store carbon by sequestering more carbon through primary production than they release though biogenic greenhouse gas production. The joint effects of saltwater intrusion and sea level rise (SWISLR) and changing precipitation patterns alter sulfate and oxygen availability, challenging estimates of biogenic greenhouse gas emissions. Iron-rich soils have been shown to buffer soil sulfidization by sequestering sulfide into iron-sulfide. But as SWISLR increases soil sulfate concentrations, sulfide produced via sulfate reduction will likely exceed the buffering capacity of soil iron, allowing toxic sulfide levels to accumulate. We used a soil mesocosm approach to examine the influence of hydrology (wet, dry, interim) and plant presence (with or without plants) on wetland soils sourced from different hydrologic histories at a restored coastal wetland. We hypothesized that reducing conditions (i.e., flooded, no plants) impact anaerobic metabolisms similarly, whereas oxidizing conditions (i.e., dry, plant presence) disrupt coupled sulfate reduction and iron reduction. Over eight weeks of hydrologic manipulation, 16S rRNA amplicon sequencing and shotgun metagenomic sequencing were used to characterize microbial communities, while greenhouse gas fluxes, soil redox potential, and physicochemical properties were measured. Results showed that contemporary hydrologic treatment affected assimilatory sulfate reduction gene composition, and hydrologic history influenced dissimilatory sulfate reduction and iron reduction gene composition. Sulfate and iron reduction genes were correlated, and dissimilatory sulfate reduction genes explained variance in methane fluxes. These findings highlight the role of historical hydrology, potential saltwater exposure, and soil iron in shaping microbial responses to future changes in soil moisture and salinity.