Elevated nutrient inputs to marshes differentially impact carbon and nitrogen cycling in two northern Gulf of Mexico saltmarsh plants

Salt marsh biogeochemical processes are regulated by ecosystem structure (e.g. plant community composition). However, plant-specific responses to stressors such as elevated nutrient inputs can have differing impacts on nitrogen (N) removal and carbon (C) sequestration. We conducted a field manipulation to investigate the impact of elevated nutrient loading on ecosystem C dynamics and nitrate reduction pathways (denitrification and dissimilatory nitrate reduction to ammonium (DNRA)) in plots dominated by either Juncus roemerianus or Spartina alterniflora that were collocated in a northern Gulf of Mexico salt marsh. We increased N and phosphorus (P) inputs by two- and three-times current levels in the region. Nutrient enrichment had no effect on net ecosystem exchange. However, a three-fold increase in nutrient input resulted in nearly one-third increases in gross primary productivity (GPP) and ecosystem respiration in S. alterniflora plots, whereas there was no impact in J. roemerianus plots. Denitrification increased in S. alterniflora plots tenfold at both treatment levels relative to controls, but as with GPP, there was no response in J. roemerianus plots to higher nutrient inputs. In contrast, a three-fold increase in nutrients reduced DNRA by half in J. roemerianus plots. This work demonstrates that plant species-specific responses in marshes need to be considered for determining the impact of higher nutrient inputs on plant productivity and N-removal and retention.