Nitrogen-sulfur coupling in freshwater sediments:Implications for nitrate removal by wetlands, lakes, andstreams

Burgin, A.J. and S.K. Hamilton

Presented at the All Scientist Meeting (2004-10-08 )

Nitrate disappearance in sediments is usually assumed to be due to respiratory denitrification, an important process for improving water quality. While performing tracer experiments in aquatic sediments of Michigan, we found that nitrate removal coincided with sulfate production, and that sulfate removal commenced only after nitrate was depleted. Because this activity appears to be biological, we hypothesize that the nitrate removal is due to sulfur oxidizing bacteria that use nitrate as an oxidant in the production sulfate. Sulfur oxidizers do not necessarily denitrify nitrate to dinitrogen, but may instead produce ammonium via a form of dissimilatory nitrate reduction to ammonium (DNRA). Push-pull experiments in streams, lakes and wetlands (23 sites) revealed a persistent pattern of sulfate production during nitrate removal. The nitrate introduction also yielded significant ammonium production and sulfide removal compared to controls (bromide introduction only). The sulfur-oxidizing pathway of nitrate removal has important implications for water quality by coupling nitrate removal to sulfur cycling. If this is a form of DNRA, nitrate removed via this pathway would remain in a biologically available form, in contrast to respiratory denitrification.

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