Award: OCE-1635099

Bioavailable nitrogen (N) is an important controlling nutrient of primary production in the oceans. There is great interest in understanding the N cycle to predict how ocean ecosystems will respond to future environmental changes and human activities. At present it is not clear whether or not inputs and outputs in the global N cycle are in balance. This makes understanding how the oceanic N cycle will respond to future changes difficult. Continental shelves and oxygen minimum zones are the major sites of N loss in the oceans, through the microbial reduction of nitrate to dinitrogen gas (N2). Until recently, N loss was attributed solely to denitrification. More recently, a great deal of attention has been paid to the importance of anaerobic ammonium oxidation (anammox) as an alternative N loss pathway to denitrification. A third nitrate reduction mechanism, dissimilatory nitrate reduction to ammonium (DNRA), has been relatively ignored in studies of the global N cycle. Predicting how N loss in the oceans may change requires a deeper understanding of all nitrate reduction pathways, not just denitrification and anammox because the fate of N differs. DNRA is distinct from denitrification and anammox, as DNRA leads to the retention of bioavailable N in ecosystems instead of N loss. This project aimed to provide a quantitative predictive understanding of controls on anammox, denitrification, and DNRA in marine sediments. All three processes occur under anoxic conditions in marine sediments but controls on which process might dominate over another are not well understood. We used sediment thin disc reactors to reproduce an anoxic marine sediment layer to measure controls on rates on these processes and achieve the following goals: 1) determine relative importance of key controlling factors, particularly organic carbon and nitrate loading as a direct test of our previous Maximum Entropy Production (MEP) biogeochemical model 2) manipulate and measure these controls to represent realistic conditions in the environment, and 3) test and further develop our theoretical biogeochemical model. We used subtidal fine-grained sediments in Thin Disc Sediment Reactor (TDSR) experiments to directly measure key controlling factors of nitrate reduction. Anammox remained low (≤1 % of total nitrate consumption) or was not detected across experiments and denitrification was by far the dominant nitrogen loss process. DNRA rapidly increased at the beginning experiments corresponding to rapid increases in the remineralization of organic carbon. Denitrification increased after the initial pulse in DNRA and became much higher than DNRA later on across experiments, despite relatively high enrichments with organic carbon and differences in the nitrate inflow rates. The results overall suggest that DNRA is favored during the initial breakdown of organic carbon. The broader impacts of this research were that one PhD student, two Master?s students, two research technicians, and nine undergraduate students have been trained in research as a result of this research. Rich participated in an Institute for Broadening Participation (IBP) webinar on career and summer research opportunities for minority undergraduate students. Giblin worked with the Gulf of Maine Institute to teach students about watershed issues, especially eutrophication. Rich, Giblin, and Algar shared their research with the public through training citizen scientists, public seminars, visiting K-12 school groups, social media, and print articles in the local newspaper. Last Modified: 12/21/2021 Submitted by: Anne E Giblin