Long-term nutrient addition increases respiration and nitrous oxide emissions in a New England salt marsh

Salt marshes may act either as greenhouse gas (GHG) sources or sinks depending on hydrological conditions, vegetation communities, and nutrient availability. In recent decades, eutrophication has emerged as a major driver of change in salt marsh ecosystems. An ongoing fertilization experiment at the Great Sippewissett Marsh (Cape Cod, USA) allows for observation of the results of over four decades of nutrient addition. Here, nutrient enrichment stimulated changes to vegetation communities that, over time, have resulted in increased elevation of the marsh platform. In this study, we measured fluxes of carbon dioxide (CO ₂), methane (CH ₄) and nitrous oxide (N₂O) in dominant vegetation zones along elevation gradients of chronically fertilized (1,572 kg N ha^-1 year^-1) and unfertilized (12 kg N ha^-1 year^-1) experimental plots at Great Sippewissett Marsh. Flux measurements were performed using darkened chambers to focus on community respiration and excluded photosynthetic CO ₂ uptake. We hypothesized that N-replete conditions in fertilized plots would result in larger N₂O emissions relative to control plots and that higher elevations caused by nutrient enrichment would support increased CO ₂ and N₂O and decreased CH ₄ emissions due to the potential for more oxygen diffusion into sediment. Patterns of GHG emission supported our hypotheses. Fertilized plots were substantially larger sources of N₂O and had higher community respiration rates relative to control plots, due to large emissions of these GHGs at higher elevations. While CH ₄ emissions displayed a negative relationship with elevation, they were generally small across elevation gradients and nutrient enrichment treatments. Our results demonstrate that at decadal scales, vegetation community shifts and associated elevation changes driven by chronic eutrophication affect GHG emission from salt marshes. Results demonstrate the necessity of long-term fertilization experiments to understand impacts of eutrophication on ecosystem function and have implications for how chronic eutrophication may impact the role that salt marshes play in sequestering C and N.