Short-term nitrogen additions can shift a coastal wetland from a sink to a source of N₂O

Coastal salt marshes sequester carbon at high rates relative to other ecosystems and emit relatively little methane particularly compared to freshwater wetlands. However, fluxes of all major greenhouse gases (N₂O, CH₄, and CO₂) need to be quantified for accurate assessment of the climatic roles of these ecosystems. Anthropogenic nitrogen inputs (via run-off, atmospheric deposition, and wastewater) impact coastal marshes. To test the hypothesis that a pulse of nitrogen loading may increase greenhouse gas emissions from salt marsh sediments, we compared N₂O, CH₄ and respiratory CO₂fluxes from nitrate-enriched plots in a Spartina patens marsh (receiving single additions of NaNO₃ equivalent to 1.4 g N m⁻²) to those from control plots (receiving only artificial seawater solutions) in three short-term experiments (July 2009, April 2010, and June 2010). In July 2009, we also compared N₂O and CH₄ fluxes in both opaque and transparent chambers to test the influence of light on gas flux measurements. Background fluxes of N₂O in July 2009 averaged −33 μmol N₂O m⁻² day⁻¹. However, within 1 h of nutrient additions, N₂O fluxes were significantly greater in plots receiving nitrate additions relative to controls in July 2009. Respiratory rates and CH₄ fluxes were not significantly affected. N₂O fluxes were significantly higher in dark than in transparent chambers, averaging 108 and 42 μmol N₂O m⁻² day⁻¹ respectively. After 2 days, when nutrient concentrations returned to background levels, none of the greenhouse gas fluxes differed from controls. In April 2010, N₂O and CH₄ fluxes were not significantly affected by nitrate, possibly due to higher nitrogen demands by growing S. patens plants, but in June 2010 trends of higher N₂O fluxes were again found among nitrate-enriched plots, indicating that responses to nutrient pulses may be strongest during the summer. In terms of carbon equivalents, the highest average N₂O and CH₄ fluxes observed, exceeded half the magnitude of typical daily net carbon sequestration rates by salt marshes. Thus, anthropogenic additions of nitrate to coasts can substantially alter N₂O fluxes from marshes, although substantial temporal variation in these fluxes was observed. To better assess the climatic roles of salt marshes, greenhouse gas emissions need to be studied in the context of chronic nitrogen loads that impact many coastal ecosystems.