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- Creators: Arizona State University
Constructed treatment wetlands (CTW) have been a cost-efficient technological solution to treat different types of wastewater but may also be sources of emitters of methane (CH4) and nitrous oxide (N2O). Thus, my objective for this dissertation was to investigate CH4 and N2O fluxes via multiple pathways from the Tres Rios CTW located in Phoenix, AZ, USA. I measured gas fluxes from the CTW along a whole-system gradient (from inflow to outflow) and a within-marsh gradient (shoreline, middle, and open water sites). I found higher diffusive CH4 release in the summer compared to spring and winter seasons. Along the whole-system gradient, I found greater CH4 and N2O emission fluxes near the inflow compared to near the outflow. Within the vegetated marsh, I found greater CH4 emission fluxes at the vegetated marsh subsites compared to the open water. In contrast, N2O emissions were greater at the marsh-open water locations compared to interior marsh. To study the plant-mediated pathway, I constructed small gas chambers fitted to Typha spp. leaves. I found plant-mediated CH4 fluxes were greater near the outflow than near the inflow and that CH4 fluxes were higher from lower sections of plants compared to higher sections. Overall, Typha spp. emitted a mean annual daily flux rate of 358.23 mg CH4 m-2 d-1. Third, using a 30-day mesocosm experiment I studied the effects of three different drydown treatments (2, 7, 14 days) on the fluxes of CH4 and N2O from flooded CTW soils. I found that CH4 fluxes were not significantly affected by soil drydown events. Soils that were dry for 7 days shifted from being N2O sources to sinks upon inundation. As a result, the 7-day drydown soils were sinks while the 14-day drydown soils showed significant N2O release. My results emphasize the importance of studying ecological processes in CTWs to improve their design and management strategies so we can better mitigate their greenhouse gas emissions.
The global transport and deposition of anthropogenic nitrogen (N) to downwind ecosystems are significant and continue to increase. Indeed, atmospheric deposition can be a significant source of N to many watersheds, including those in remote, unpopulated areas. Bacterial denitrification in lake sediments may ameliorate the effects of N loading by converting nitrate (NO3-) to N2 gas. Denitrification also produces nitrous oxide (N2O), a potent greenhouse gas. The ecological effects of atmospheric N inputs in terrestrial ecosystems and the pelagic zone of lakes have been well documented; however, similar research in lake sediments is lacking. This project investigates the effects N of deposition on denitrification and N2O production in lakes. Atmospheric N inputs might alter the availability of NO3- and other key resources to denitrifiers. Such altered resources could influence denitrification, N2O production, and the abundance of denitrifying bacteria in sediments. The research contrasts these responses in lakes at the ends of gradients of N deposition in Colorado and Norway. Rates of denitrification and N2O production were elevated in the sediments of lakes subject to anthropogenic N inputs. There was no evidence, however, that N deposition has altered sediment resources or the abundance of denitrifiers. Further investigation into the dynamics of nitric oxide, N2O, and N2 during denitrification found no difference between deposition regions. Regardless of atmospheric N inputs, sediments from lakes in both Norway and Colorado possess considerable capacity to remove NO3- by denitrification. Catchment-specific properties may influence the denitrifying community more strongly than the rate of atmospheric N loading. In this regard, sediments appear to be insulated from the effects of N deposition compared to the water column. Lastly, surface water N2O concentrations were greater in high-deposition lakes compared to low-deposition lakes. To understand the potential magnitude of deposition-induced N2O production, the greenhouse gas inventory methodology of Intergovernmental Panel on Climate Change was applied to available datasets. Estimated emissions from lakes are 7-371 Gg N y-1, suggesting that lakes could be an important source of N2O.
diverse range of anaerobic microbes, but its sources and sinks are poorly understood. The overarching goal of my dissertation is to explore abiotic N2O formation and microbial N2O consumption across reducing environments of the early and modern Earth. By combining experiments as well as diffusion and atmospheric modeling, I present evidence that N2O production can be catalyzed on iron mineral surfaces that may have been present in shallow waters of the Archean ocean. Using photochemical models, I showed that tropospheric N2O concentrations close to modern ones (ppb range) were possible before O2 accumulated. In peatlands of the Amazon basin (modern Earth), unexpected abiotic activity became apparent under anoxic conditions. However, care has to be taken to adequately disentangle abiotic from biotic reactions. I identified significant sterilant-induced changes in Fe2+ and dissolved organic matter pools (determined by fluorescence spectroscopy). Among all chemical and physical sterilants tested, γ - irradiation showed the least effect on reactant pools. Targeting geochemically diverse peatlands across Central and South America, I present evidence that coupled abiotic and biotic cycling of N2O could be a widespread phenomenon. Using isotopic tracers in the field, I showed that abiotic N2O fluxes rival biotic ones under in-situ conditions. Moreover, once N2O is produced, it is rapidly consumed by N2O-reducing microbes. Using amplicon sequencing and metagenomics, I demonstrated that this surprising N2O sink potential is associated with diverse bacteria, including the recently discovered clade II that is present in high proportions at Amazonian sites based on nosZ quantities. Finally, to evaluate the impact of nitrogen oxides on methane production in peatlands, I characterized soil nitrite (NO2–) and N2O abundances along soil profiles. I complemented field analyses with molecular work by deploying amplicon-based 16S rRNA and mcrA sequencing. The diversity and activity of soil methanogens was affected by the presence of NO2– and N2O, suggesting that methane emissions could be influenced by N2O cycling dynamics. Overall, my work proposes a key role for N2O in Earth systems across time and a central position in tropical microbial ecosystems.