Nitrous oxide production in groundwater in the Newer Volcanics basalt aquifer system, Western Victoria, Australia

While soil-derived nitrous oxide emissions and their impacts on greenhouse gas budgets have been relatively well studied, much less is known about how much nitrous oxide is produced from groundwater and the mechanisms by which it is produced or consumed. In this study, we used gas chromatography and clumped isotope analyses to quantify nitrate, ammonia and nitrous oxide levels in historically nitrate-contaminated agricultural soils in the Newer Volcanics province of Western Victoria. We also applied quantitative polymerase chain reaction experiments to conduct functional gene assays from whole community microbial DNA extracted from groundwater and sediments. Samples of groundwater were obtained at different depths down to 37 meters below the surface within different but hydrologically linked aquifers using a low-flow pumping strategy. Nitrate levels of up to about 40 mg/L, ammonia levels of up to about 0.85 mg/L, and nitrous oxide levels of up to roughly 770 nM were observed in several groundwater samples. δ¹⁵N and δ¹⁸O values from clumped isotope analyses ranged from -2.68% to 68.19% and -3.37% to 26.83%, respectively. Nitrate and nitrous oxide concentrations tended to decrease with depth through the unconfined aquifer, while TOC generally increased. Higher ammonia levels were observed where aquifer materials contained more heavily ferruginized sandstone. Interestingly, higher nitrate and nitrous oxide values were associated with samples obtained from within the basaltic aquifer (the principle rock component of the Newer Volcanics region of Western Victoria). DNA concentrations extracted from groundwater samples ranged from 2-88 ng/μL, and quantitative PCR results show variable concentrations of amplified nir, nar, nos and amo genes associated with different redox transformations along the nitrification and denitrification pathways, indicating potential nitrous oxide formation via both pathways within different depths in the aquifer.