Collaborative Research: Deconstructing Nitrate Stable Isotope Ratios in Aquifers

人为的营养负荷和随之而来的水生系统的富营养化对环境构成了巨大的威胁。过量的氮会对生态系统的多样性和功能以及人类健康产生深远的有害影响。拟议的研究是一个重要的冒险，建立一个强大的标题在氮同位素系统，允许准确和更定量的解释硝酸盐同位素分布在环境中，从而合理理解固有的氮动态。本研究的目的是调查氮物种的同位素动力学在法尔茅斯，马萨诸塞州，在西法尔茅斯废水羽，以解决在单培养物的硝化和反硝化细菌菌株和模式归因于在水生环境中的类似转换的研究之间观察到的同位素歧视的差异。首先，培养物中硝化菌产生的硝酸盐18 O/16 O值揭示了在解释环境趋势时很大程度上被忽视的同位素效应。第二，相对于15 N/14 N的18 O/16 O分馏在微生物培养物中具有~1的特征和一致的斜率，与在含水层中通过反硝化作用观察到的硝酸盐衰减的~0.6的斜率相反。研究人员试图（1）限制硝化作用产生的硝酸盐18 O/16 O值与环境条件的关系，（2）调查生物体水平的18 O/16 O与15 N/14 N富集在原位反硝化过程中赋予硝酸盐相比，培养观察。他们建议用一组来自现场的环境样本的实验室内孵育以及原位示踪剂测试来测试假设。该项目与美国地质调查局合作，将资助一名研究生。调查是重要的，因为文化的不一致性，实地观察提出的可能性，同位素的趋势在环境中被误解，可能忽略了重要的通量在氮质量平衡的含水层。

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.