Denitrification in fringing salt marshes of Narragansett Bay, Rhode Island, USA

Abstract

In the past century, loading of terrestrial inorganic nitrogen to coastal receiving waters has increased dramatically. Salt marshes, because of their location between upland regions and coastal waters and their recognized role as nutrient transformers, have the potential to ameliorate some of this loading. In the current study, we used core incubations in the laboratory to investigate denitrification rates in high marsh soils from five fringing salt marshes in Narragansett Bay, Rhode Island, USA. The marshes showed a wide variety of terrestrial N loading, with rates ranging from 2 to 6037 kg N ha−1 yr−1. Field-collected cores were selected to include both vegetated and bare soils at each marsh, and the six-hour incubations were designed to approximate natural tidal rhythms. Total dissolved nitrogen flux in these marshes ranged between +1255 and −710 μmol N m−2 hr−1, with N2 gas accounting for the majority of the total N flux (average 76%). Nitrogen gas flux ranged between −375 (nitrogen fixation) and +420 (denitrification) μmol N2 m−2 hr−1. While N2 gas fluxes were significantly correlated (r=+0.64, p<0.05) with marsh organic carbon content, we also detected a significant inverse relationship (r=−0.91, P<0.05) between average N2 gas fluxes and terrestrial nitrogen loads. Comparison of N2 gas fluxes in vegetated vs. bare soils indicated a significant (p <0.05) but variable effect of vegetation on N2 flux. This field survey shows the potential of New England fringe salt marshes to intercept and transform land-derived nitrogen loads; however, sediment characteristics (e.g., percent of labile organic matter) and plant community structure can significantly affect the capacity of the marsh to process inorganic nitrogen loads. In order to understand the role of salt marshes in buffering coastal N loading, we need a better understanding of the natural and anthropogenic factors controlling denitrification and net N losses.