Burgin, A. J. 2007. Alternative microbial pathways of nitrate removal from freshwater ecosystems. Ph.D. Dissertation, Michigan State University, East Lansing, Michigan, USA.

The removal of nitrogen (N) in aquatic ecosystems is of particular interest because excessive nitrate in ground and surface waters is a growing problem. Research on nitrate removal processes has emphasized biotic uptake (assimilation) or respiratory denitrification by bacteria. The increasing application of tracer techniques (e.g., stable isotopes) has yielded a growing body of evidence for alternative microbially mediated processes of nitrate transformation, including dissimilatory reduction of nitrate to ammonium (DNRA), chemoautotrophic denitrification via sulfur or iron oxidation, and anaerobic ammonium oxidation (Anammox). In Chapter 1, I review evidence for the importance of alternative nitrate removal pathways in aquatic ecosystems and discuss how the possible prevalence of these pathways may alter views of N cycling and its controls.

Anaerobic microbial processes are responsible for much of the nutrient cycling in freshwater systems. Nitrate disappearance in sediments is usually assumed to be due to respiratory denitrification. Push-pull tracer experiments entail adding nitrate and a conservative solute to sediment porewater, followed by in-situ incubation with periodic subsampling.

While performing such tracer experiments to quantify rates of nitrate removal in aquatic sediments of Michigan streams and wetlands, I found that nitrate removal coincided with sulfate production. Push-pull experiments in a diverse set of streams, lakes and wetlands revealed a persistent pattern of sulfate production during nitrate removal (Chapter 2). Push-pull experiments done with 15NO¬3- also indicate the importance of DNRA to overall nitrate removal in these sediments.

To compare the relative importance of alternative pathways of NO¬3- reduction (e.g., to NH4+ or N2), I used of stable isotopes with a flow-through cores (Chapter 3). Using a flow-through set up, treatment water (15NO3-, 15NH4+/14NO3-, or control) was pumped over cores from six different sites. Results indicate that conversion to N2 was the predominant nitrate loss across all six sites. I also found that conversion into the 15NH4+ pool, indicative of DNRA, can account for a variable fraction of the dissimilatory nitrate removal, but that anammox accounted for very little of the overall nitrate removal.

I tested the relative importance of carbon vs. sulfide in regulating DNRA using a laboratory assay (Chapter 4), by adding nitrate along with a gradient of organic carbon (as acetate) and free sulfide to anoxic sediments. I found that both carbon and sulfide were important in controlling nitrate removal rates and end-products in both sites. While denitrification tended to be the more important removal pathway in the low ambient sulfide site, DNRA was of equal importance in the high ambient sulfide site.

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