Salt marsh microbial response to chronic nitrogen enrichment

The aim of this study was to understand how chronic nitrogen exposure influences organic matter decomposition in response to elevated nitrate. We collected salt marsh sediments from three sites that vary in nitrogen exposure, both by extent and duration: a reference marsh, a marsh exposed to 13-years of nitrate enrichment, and a marsh exposed to 40 years of nitrogen from wastewater effluent. We took cores at 25 cm depth from S. alterniflora habitat, characterized organic matter and the microbial community, and then used a flow through reactor experiment to expose sediments from each site to either 500 uM nitrate in seawater or seawater only (unamended control). Throughout the ~3-month incubation, we monitored biogeochemical rates, changes to organic matter, and shifts in the active microbial community with 16S rRNA sequencing. Our results suggest that chronic exposure to elevated nitrate may lead to residual pools of organic matter that may be less biologically available for decomposition. Thus, it is important to consider the legacy of nutrient exposure when examining the carbon cycle of salt marsh sediments.