| Summary |
Waterfowl impoundments are wetlands that are hydrologically managed to provide food and habitat for migratory bird populations. Certain impoundment types have recently been found to export nitrogen (N) loads during seasonally prescribed drawdowns. A recent study observed these high N loads in moist-soil managed (MSM) waterfowl impoundments, while knowledge of N export in other impoundment types, such as seasonally flooded agricultural fields (Ag), is lacking. Literature describing the N cycling properties of impoundment soils at the laboratory scale is also scarce. This study focused on (1) identifying the effects of storms and drawdowns on N dynamics at the field scale, (2) comparing seasonal N cycling differences between and within MSM and Ag impoundment soils, and (3) evaluating the potential for their soils to act as treatment wetlands. I monitored N dynamics at an Ag and MSM impoundment in the Lake Mattamuskeet (Hyde County, NC) watershed for 17-months. Storm events were identified during this period and their influence on N dynamics was examined. N concentrations were monitored at a frequency of 30-minutes using in situ spectroscopy. Twelve soil cores were collected at each site at three time points to represent Summer, Fall, and Winter conditions. Soil cores were assayed for nitrification, denitrification, and N-mineralization potential rates, and these rates were compared to soil physiochemical properties (soil moisture, pH, nitrate, ammonium, total soil carbon and nitrogen). Mesocosms were collected from each of the coring locations at each site, subjected to monthly N-amended simulated rainfall for 6 months and assessed for changes in N concentrations at 24 and 72 hours post rainfall. At the field scale, I observed substantial differences in N dynamics between the Ag and MSM systems when the Ag impoundment was not flooded, while similar N dynamics were observed in both systems during the fall and winter flooding period. I found that seasonal flooding from early October through late March resulted in greatly increased ambient NH4-N conditions. Storm events stimulated coupled nitrification denitrification processes in these open water systems. At the laboratory scale, I found substantial differences of seasonal N cycling dynamics between and within the Ag and MSM sites. Hydrologic management was identified to be the main driver of variability in N cycling dynamics at the Ag site, and a combination of low soil pH and hydrologic management drove variability in MSM N cycling dynamics. I identified nitrification and enitrification processes to be strongly coupled in both systems, resulting in low denitrification potential in both sites. In addition, soils from both sites showed limited potential to act as treatment wetlands under experimental mesocosm conditions. This study demonstrates that N dynamics of different waterfowl impoundment types can function similarly during seasonal flooding periods. As such, timing seasonal drawdowns of waterfowl impoundments shortly after storm events can be a strategy to enhance coupled nitrification-denitrification processes that result in less inorganic N being exported to downstream systems. I suggest changes in Ag and MSM hydrologic management, as well as efforts to increase soil pH at MSM, to stimulate nitrification potential to promote N removal via denitrification year-round. |
