Wetlands are important in mediating the effects of global climate change by sequestering CO2 in recalcitrant plant biomass or anoxic soils. Alternatively, these ecosystems may be sites of greenhouse gas (CH4, CO2, N2O) production. Using existing gradients in nitrogen loading and elevation among salt marshes in Waquoit Bay, Mass., on the south side of Cape Cod, Woods Hole Sea Grant funded investigators examined the hypotheses that net N2O emissions and ecosystem respiration from coastal marshes increase under conditions of nitrogen loading, with sufficient changes to potentially offset a substantial portion of the net CO2 uptake from the atmosphere; that plant productivity in each marsh is related to nitrogen loading rate; and that greenhouse gas emissions from salt marsh sediments vary as a function of soil elevation. In the final comparison of greenhouse gas fluxes across the anthropogenic nitrogen gradient in Waquoit Bay, permanently inundated ponds were consistently a net source of CO2 to the atmosphere, whereas vegetated marsh zones were a sink of CO2. In addition, neither condition emitted significant quantities of methane compared to less saline habitats. This suggests that sea level rise may at least transiently decrease the carbon sequestration potential of coastal marshes.