Linkages between denitrification in stream sediments and biodegradable dissolved organic carbon (BDOC)

Background/Question/Methods

Many streams export large amounts of nitrate due to increases in anthropogenic loadings, and these nitrate fluxes can cause eutrophication and drinking water violations.  Denitrification, a respiratory process in which heterotrophic microbes use nitrate as the electron acceptor and organic carbon as an electron donor, is a mechanism that can mitigate the amount of nitrate transported downstream through the permanent removal and reduction of nitrate to gaseous forms of nitrogen.  Denitrifers represent an important yet poorly understood link between carbon and nitrogen cycling in streams and we tested the influence of dissolved organic carbon (DOC) quality on rates of denitrification from intact sediment cores collected from White Clay Creek, a 3^rd-order stream in an agricultural basin.  Biodegradable DOC (BDOC) concentrations, our measure of DOC quality, was measured with plug-flow bioreactors colonized by stream microorganisms  and separated into multiple biological reactivity classes using bioreactors with different residence times. DOC amendments to stream water included glucose and storm-induced overland flow water.  Denitrification rates were measured by enrichment with¹⁵N-nitrate followed by measurement of ¹⁵N₂ production over time.

Results/Conclusions

Denitrification rates for unamended stream water were 136 ± 65 µmol/ m² hr under summer temperatures (18 to 21 °C) and 49 ±  7 µmol/ m² hr under autumn temperatures (9.1 to 9.5 °C).  Additions of labile DOC (0.4 mg glucose-C/L) under summer temperatures had no effect on denitrification rates, but additions of overland flow water (1.1 mg labile-C/L and 3.2 mg semi-labile-C/L) increased denitrification rates to 1001 ±  137 µmol/ m² hr. The organic matter content in individual sediment cores was also a significant covariate.  Our results suggest either a threshold concentration of labile DOC and/or that the semi-labile fraction of the DOC pool has a contributing influence on denitrification.  The measured sediment denitrification rates, when scaled to the stream ecosystem based on conservative estimates of surface area, could remove the entire nitrate flux in White Clay Creek under baseflow and stormflow conditions. However, White Clay Creek exports high nitrate loads indicating the difficulty in scaling rates derived from cores to the whole ecosystem. Addressing this challenge and the challenge of understanding the combined interactive effects of the various controlling factors on rates of denitrification is necessary for determining management efforts that optimize removal of nitrate.