Impacts of N fertilization on tidal creek N cycling: Denitrification vs. DNRA

Salt marshes are critical ecosystems for removing reactive nitrogen (N) through denitrification.  We examined the influence of long-term fertilization on N transformation and removal in a salt marsh tidal creek ecosystem fringing the Plum Island Sound estuary in northern Massachusetts, USA.  Sediment oxygen demand was within the range of other marsh systems (1271.9 to 7855.0 mmol m^-2 hr^-1) and was not significantly different between the fertilized and reference creek. Net N₂ fluxes ranged from net N fixation of -402.7 mmol N₂-N m^-2 hr^-1 in the reference creek to net denitrification of 524.9 mmol N₂-N m^-2 hr^-1 in the fertilized creek. Net N₂ flux and nitrate uptake were significantly higher in the fertilized creek and in both creeks net denitrification appeared to be nitrate limited. Large sediment ammonium effluxes were strongly correlated (r²=0.79, p<0.001) with sediment nitrite influxes. This relationship, combined with high NO₃^- uptake rates was indicative of dissimilatory nitrate reduction to ammonium (DNRA).   We calculated rates of DNRA, and found them to be significantly higher under N fertilization representing 45% and 11% of the total nitrate uptake in the fertilized and reference creeks, respectively. These results suggest that DNRA may outcompete denitrification at high nitrate concentrations. Thus, increased anthropogenic nutrient loading may have a detrimental effect on the nitrogen removal capacity of salt marsh ecosystems.