Denitrification removes nitrogen from watersheds under reducing conditions, but N2O and CH4, both greenhouse gases, can also be produced. The overarching hypothesis of my thesis was that hydric environments accumulate N2O and CH4 in groundwater and the vadose zone. To test the hypothesis, groundwater samples were taken monthly during 2007-2009 at 64 piezometers in 10 wetlands for analysis of excess N2 , N2O, CH4, and CO2. Vadose zone gas and groundwater samples were taken during 2008-2010 at two riparian buffers and a hydrologically restored wetland.
The hydrology of the 10 locations was complex. A hydrologic connection across a transect was determined at one location where NO3 - significantly decreased, excess N2 significantly increased, and moderate concentrations of N2O and CH4 accumulated. Within these 10 locations, three N2O and four CH4 hot spots were identified, and hot moments accounted for a large percentage of total accumulated N2O and CH4. I found evidence of CH4 ebullition, the production of CH4 bubbles in the vadose zone that strip other dissolved gases. The locations that accumulated the most dissolved CH4 and N2O were natural wetlands and riparian areas, respectively.
I measured both positive and negative excess N2 concentrations in the vadose zone. Flux estimates ranged from -600 to 880 kg N ha -1 yr-1, which brackets missing N estimates at the watershed scale. These concentrations were calculated using N2/Ar, and both gases are affected by physical processes. These calculated excess N2 profiles could have been produced through either biological and/or physical mechanisms, and these processes currently cannot be distinguished. Less than 1% of the missing N on the transect scale, measured as the difference in N concentration between two piezometers, was accounted for by calculated diffusional fluxes from groundwater to the vadose zone.
The primary mechanism transporting gases from the vadose zone to the atmosphere was diffusion, but convection transported 20% of the calculated median CO2 yearly flux. Increased production of N2O and CO2 was observed in the vadose zone after rainfall events. Overall, large concentrations of N2O, CH4, CO2 , and excess N2 accumulated in the groundwater and vadose zone of these locations, supporting the overarching hypothesis.