The impact of nutrient loading on carbon sequestration and nitrogen removal capacity in a saltmarsh dominated by Juncus romerianus and Spartina alterniflora collocated at the same elevation

We conducted a one year field manipulation to investigate the impact of elevated nutrient loading on N-removal and C-fluxes in in a salt marsh located on Dauphin Island, AL. We increased N and phosphorus (P) inputs by 20 g N m^-2yr^-1/ 1.25 g P m^-2yr^-1and 40 g N m^-2yr^-1/2.5 g P m^-2yr^-1 in plots dominated by either Juncus roemerianus or Spartina alterniflora that were collocated at the same elevation with respect to sea level. Denitrification, measured with the isotope pairing technique, was 5X higher in unammended J. roemerianus plots than S. alterniflora. However, denitrification increased ten-fold with nutrient addition in S. alterniflora plots, while denitrification in J. roemerianus plots did not respond to nutrient additions. Gross primary productivity (GPP) was marginally higher (~5%) in control plots of J. roemerianus compared to S. alterniflora plots. However, high nutrient addition resulted in enhanced (27%) GPP in S. alterniflora plots, but GPP did not change in J. roemerianus plots in response to higher nutrient inputs. Net ecosystem exchange was similar in J. roemerianus and S. alterniflora control plots and did not change in response to nutrient additions for either vegetation type. Our results illustrate that while both J. roemerianus and S. alterniflora marshes have the capacity to to contribute to N removal via denitrification at higher nutrient loading in the Gulf of Mexico S. alterniflora dominated marshes may have a greater capacity to mitigate N inputs. Additionally, in a world with higher nutrient inputs and despite higher GPP in S. alterniflora, both vegetation types will continue to sequester C, but at rates similar to current levels.