Collaborative Research: Deconstructing Nitrate Stable Isotope Ratios in Aquifers

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Anthropogenic nutrient loading and the consequent eutrophication of aquatic systems pose a substantial environmental threat. Excessive nitrogen can have profound detrimental effects on the diversity and function of ecosystems, and on human health. The proposed study is an important venture to establish a robust rubric in nitrogen isotope systematic, to allow for accurate and more quantitative interpretations of nitrate isotope distributions in the environment, and thus a sound understanding of inherent nitrogen dynamics. The goal of the study is to investigate the isotope dynamics of nitrogen species in the West Falmouth Wastewater Plume in Falmouth, Massachusetts, in order to resolve differences in isotope discrimination observed between studies of mono-cultures of nitrifying and of denitrifying bacterial isolates and patterns ascribed to analogous transformations in aquatic environments. For one, nitrate 18O/16O values produced by nitrifiers in cultures reveal isotope effects that are largely overlooked when interpreting environmental trends. Second, the fractionation of 18O/16O relative to 15N/14N has a characteristic and consistent slope of ~1 in cultures of denitrifiers, in contrast to a slope of ~0.6 observed for nitrate attenuation by denitrification in aquifers. The investigators seek to (1) constrain the nitrate 18O/16O values produce by nitrification in relation to environmental conditions and (2) investigate the organism-level 18O/16O versus 15N/14N enrichment imparted on nitrate during denitrification in situ as compared to culture observations. They propose to test hypotheses with a set of in-lab incubations of environmental samples from the site, as well as an in situ tracer test. The project is in collaboration with the US Geological Survey and will support a graduate student. The investigation is important because the discordance of culture to field observations raises the possibility that isotope trends in the environment are misinterpreted, possibly overlooking important fluxes in the nitrogen mass balance of aquifers.