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Salt marsh ecosystems serve as critical nutrient filters by 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 μmol m−2 h−1) and was not significantly different between the fertilized and reference creek. Net N2 fluxes ranged from net N fixation of −402.7 μmol N2-N m−2 h−1 in the reference creek to net denitrification of 524.9 μmol N2-N m−2 h−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. We calculated rates of dissimilatory nitrate reduction to ammonium (DNRA) and found it to be significantly higher in the fertilized creek, representing 45 and 11% of the total nitrate uptake in the fertilized and reference creeks, respectively. Additionally, there was a strong relationship between ammonium and nitrite fluxes in both creeks. These results suggest that DNRA may outcompete denitrification at high nitrate concentrations. Increased anthropogenic nutrient loading may therefore have a detrimental effect on the N removal capacity of salt marsh ecosystems.

Impacts of long-term fertilization on salt marsh tidal creek benthic nutrient and N<sub>2 gas fluxes