Identifying Cross-talk between Nitrogen and Manganese Redox Cycling

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Aquatic environments such as wetlands are natural settings in which many chemical elements undergo transformations that play critical roles in Earth's biogeochemical cycles. This research will focus on developing a more detailed understanding of natural reactions between two important elements that undergo such chemical transformations: nitrogen and manganese. Reactions between these elements in aquatic environments have implications for the mobilization of nutrients and metal contaminants in natural waters, the production of gases that play important roles in Earth's atmosphere-climate system, as well as implications for reactions that occurred during early Earth history before the formation of the modern atmosphere. The research will provide training and mentoring opportunities for a graduate student and postdoctoral scholar, and will contribute to the education of undergraduate summer student fellows at the Woods Hole Oceanographic Institution. The researchers will collaborate with the Boston Green Academy to provide science presentations and teaching materials to science teachers, giving a variety of students from underrepresented groups opportunities to participate in research experiences in the Earth sciences.This research will shed new light on the relatively poorly understood coupling of redox reactions between nitrogen (N) and manganese (Mn) in aquatic ecosystems. Although thermodynamic conditions are generally favorable for coupled reactions between N and Mn, evidence for the occurrence of such reactions has remained enigmatic. This research will revisit previously hypothesized reactions between Mn and N, armed with new tools for both Mn and N speciation and isotopic behavior such as advanced soluble and solid-phase Mn-speciation, multi-isotope analyses and flow-through sediment incubations. These techniques will be used to examine abiotic oxidation dynamics of N species by Mn(III)-ligand complexes and other environmentally relevant forms of Mn in naturally dynamic redox zones. The research team will examine the role of reactive forms of Mn such as Mn(III)-ligand complexes and disordered Mn(III/IV) oxides in abiotic transformation of N, and how these abiotic reactions are controlled by the biological production of reactive intermediate species of both N and Mn. The specific objectives are to quantify reaction kinetics and associated isotope effects during oxidation of reduced N species, determine the influence of natural dissolved organic carbon on these reactions, identify reactions between N and Mn intermediates formed by microbes, and examine coupled Mn and N reactions under natural conditions. By bringing new tools to understand these reactions in naturally dynamic redox zones, these results will expand our burgeoning understanding of these reactions, with important implications for aquatic biogeochemistry, fate and transport of natural and anthropogenic nitrogen, impacts of dual isotope signatures of nitrate, mobilization of contaminants and nutrients, production of N-bearing greenhouse gases, and catalysis of early Earth reactions before the Great Oxidation Event.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.