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The excessive input of anthropogenic nutrients to coastal waters has impacted estuarine ecosystems worldwide, resulting in low oxygen conditions, increases in the severity and frequency of nuisance and harmful algal blooms, the loss of submerged aquatic vegetation, and changes to community diversity and structure. Salt marshes are ecologically important estuarine ecosystems that provide habitat for marine and terrestrial species, provide protection from storm surge, and transform nutrients at high rates. Because of these qualities there is much interest from scientific and management communities to understand the impacts of nutrient enrichment on salt marshes, as well as the potential for marshes to remove excess nutrients from estuarine systems.

Nitrogen (N) is the limiting nutrient in most coastal ecosystems and therefore studies on nutrient enrichment in marshes have largely focused on N. While decades of research have characterized the exchange of nitrogen between marshes and adjacent tidal waters, the net impact of the microbial-mediated fluxes of nitrogen gas (N2) is less understood. Nitrogen fixation and denitrification serve as important pathways for sources and sinks of N within the ecosystem. Nitrogen fixation is the process by which N2 gas is fixed into a biologically-available form and can be important in enhancing marsh primary production. Denitrification transforms nitrate into N2 gas, effectively removing N from the marsh. Both processes are controlled by various factors, including dissolved inorganic nitrogen (DIN) levels. Prior research has demonstrated that N fixation can be suppressed by high levels of ammonium and nitrate while denitrification is often enhanced by an increased availability of nitrate.

Many studies have examined the impact of N enrichment on denitrification and/or N fixation in salt marshes and have found varying results. While some have reported that higher DIN levels stimulated denitrification and suppressed N fixation, opposite or no relationships have also been observed. The variation in findings demonstrates that more investigation is needed, particularly because of the spatial heterogeneity of salt marshes and the methodological difficulties in measuring denitrification and N fixation. Even less is known regarding the impact of nutrient reductions on salt marsh biogeochemistry and N cycling. In many estuaries and coastal watersheds management actions to reduce nutrient inputs from wastewater treatment facilities and septic systems have been or will soon be implemented. Therefore it is increasingly important to better understand the response of salt marsh nutrient cycling to both nutrient enrichment and reduction.

To examine the impact of changes in N regime on salt marsh N cycling, we measured denitrification and N fixation in two marshes with varying degrees of longterm N enrichment from tidal waters. We conducted our work in Narragansett Bay, Rhode Island, which has an established down-bay gradient in estuarine nutrient concentrations. Our highly N enriched marsh was located in the Providence River Estuary, where the majority of anthropogenic N enters the Bay, and our low N marsh was located near the mouth of the Bay. To compare N cycling activity between the two marshes and to understand how activity differs seasonally, we measured denitrification and N fixation in separate sediment incubations on a monthly basis over an annual cycle (excluding winter months) from June 2011 to June 2012. Our measurements were made in intact sediment cores collected from the tidally influenced low marsh zone dominated by short-form Spartina alterniflora. While this was meant to capture differences between the marshes with long-term exposure to high or low tidal N inputs, we also aimed to understand how N cycling activity would respond to changes in N regime. Therefore we additionally conducted an experiment in which sediment cores were extracted and transplanted between the marshes, along with cores that were re-planted within the same marsh (serving as experimental controls). After three months (July to October 2011) we collected the cores and in two separate incubations measured denitrification and N fixation rates.

For all of our denitrification measurements, we employed the isotope pairing technique (IPT) in which a heavy isotope nitrate (15N-NO3 -) tracer is added to the overlying water to track the production of N2 gas. The IPT method allowed us to measure ambient denitrification, including differentiating direct denitrification from coupled nitrification-denitrification. We were also able to measure the capacity for denitrification. By measuring and distinguishing the different types of denitrification using IPT, we could comprehensively characterize the role of marsh sediments in removing tidal N (via direct denitrification) and the total capacity to denitrify when nitrate was not limiting (i.e. very high nitrate concentrations).

(For full abstract, please see document.)