Impact of chronic nitrogen loading on greenhouse gas fluxes in coastal wetlands

摘要

Coastal wetlands are valuable ecosystems that historically have not been protected and have been lost at rapid rates. Recently, they have gained attention for their potential role in climate change mitigation given that they have the ability to sequester and store large amounts of atmospheric carbon dioxide (CO2). While other ecosystems can sequester and store carbon as well, salt marshes have the unique ability to store vast amounts of carbon while emitting relatively negligible amounts of methane (CH4) and nitrous oxide (N2O). These are additional greenhouse gases (GHGs) that can be emitted from ecosystems, particularly CH4 in large quantities from freshwater wetlands. These gases are 45 and 270 times, respectively, greater at trapping heat in the atmosphere than CO2.;However, anthropogenic nitrogen (N) inputs into coastal estuaries have the potential to shift biogeochemical cycling within coastal wetlands, possibly switching salt marshes from CO2 sinks to being sources of one or more of the three major GHGs. Excessive anthropogenic N inputs are a threat to overall ecosystem health on local, regional, and global scales. Salt marshes are natural and efficient filters of excess N entering into these systems, although they cannot filter out unlimited quantities. Excess N in coastal systems can lead to a suite of negative consequences including poor water quality as a result of over stimulation of primary productivity and overall habitat degradation. The threat of anthropogenic N to coastal areas is only increasing as populations grow and concentrate along desirable coastal locations. For coastal wetlands that already face threats of habitat loss from increased rates of sea level rise (SLR) and urban development, N inputs can exacerbate rates of marsh loss. As efforts expand to protect these valuable ecosystems through development of financial incentives, such as carbon trading markets, it is important to quantify how N loading impacts GHG fluxes within wetlands and rates of N transformation. Most research to date in salt marsh systems has focused on impacts from short-term N additions on GHG fluxes.;The goal of this research was to examine the role of chronic N loading on GHG fluxes in Spartina alterniflora-dominated marshes and to assess quantities of N available for transformation through measurement of denitrification enzyme activity (DEA). To accomplish these goals, we first examined the role of chronic N loading on GHG fluxes using three salt marshes located along a historic N gradient (high, medium, low) within Narragansett Bay, RI. Narragansett Bay is an ideal location for this work since it has received chronic N loading, mainly from wastewater inputs, since the late 1800s. To asses impacts of N loading on GHG fluxes, CO2, CH4 and N2O were measured for one field season in 2016. Along with measured fluxes, plant properties, edaphic parameters, and nutrient availability were measured. Relationships of fluxes to these additional parameters were then explored. We then compared rates of DEA at the opposite ends (high, low) of the N gradient within Narragansett Bay in 2017, focusing on four marsh zones (creekbank, mudflat, low marsh, high marsh) at two sites to assess any differences in N availability and rates of N transformation. Additionally, GHG fluxes were measured at the high N site in 2017 to explore relationships with DEA rates.;As a result of this work, we found that the site receiving the highest N loading experienced the highest CO2 uptake as well as the highest emissions of CH4 and N2O compared to the other two sites along the N gradient. However, these emissions were not on an order of magnitude to significantly offset CO2 uptake. This was as expected, however, the other measured parameters (plant properties and edaphic variables) and DEA did not necessarily fall along expected trends of the N gradient. There were no significant differences in DEA among sites or zones, suggesting each site had similar amounts of N available for transformation and that soil in each zone had equal ability to transform N. At the marsh with the highest historic N loading, GHG fluxes fell along expected trends among zones with increased uptake of CO2 within vegetated zones contrasting with CO2 emission in non-vegetated zones. CH4 fluxes were highest in the bare creekbank zone, but were similar among the three remaining zones. Surprisingly, no significant N2O fluxes were measured in any of the four zones, suggesting along with DEA results that most N inputs are completely reduced to N2 via denitrification.