Nitrification and denitrification in a midwestern

Authors: Richard L Smith John Karl Böhlke Deborah A Repert Charles P Hart

Publish Date: 2009/08/15

Volume: 96, Issue: 1-3, Pages: 189-208

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Abstract

The extent to which instream processes alter or remove nutrient loads in agriculturally impacted streams is critically important to watershed function and the delivery of those loads to coastal waters In this study patchscale rates of instream benthic processes were determined using large volume openbottom benthic incubation chambers in a nitraterich first to third order stream draining an area dominated by tiledrained rowcrop fields The chambers were fitted with sampling/mixing ports a volume compensation bladder and porewater samplers Incubations were conducted with added tracers NaBr and either 15NNO3 − 15NNO2 − or 15NNH4 + for 24–44 h intervals and reaction rates were determined from changes in concentrations and isotopic compositions of nitrate nitrite ammonium and nitrogen gas Overall nitrate loss rates 220–3560 μmol N m−2 h−1 greatly exceeded corresponding denitrification rates 34–212 μmol N m−2 h−1 and both of these rates were correlated with nitrate concentrations 90–1330 μM which could be readily manipulated with addition experiments Chamber estimates closely matched wholestream rates of denitrification and nitrate loss using 15N Chamber incubations with acetylene indicated that coupled nitrification/denitrification was not a major source of N2 production at ambient nitrate concentrations 175 μM but acetylene was not effective for assessing denitrification at higher nitrate concentrations 1330 μM Ammonium uptake rates greatly exceeded nitrification rates which were relatively low even with added ammonium 35 μmol N m−2 h−1 though incubations with nitrite demonstrated that oxidation to nitrate exceeded reduction to nitrogen gas in the surface sediments by fivefold to tenfold The chamber results confirmed earlier studies that denitrification was a substantial nitrate sink in this stream but they also indicated that dissolved inorganic nitrogen DIN turnover rates greatly exceeded the rates of permanent nitrogen removal via denitrificationWe thank Jenny Baeseman for field assistance Lesley Smith for MIMS analysis Judson Harvey for collecting the minipoint porewater samples and John Duff Craig Tobias and Andrew Laursen for manuscript reviews This study was supported by the US Geological Survey Water Resources Discipline National Research Program and by a US Dept of Agriculture Cooperative State Research Education and Extension Service Competitive Grant 20013510209870 The use of trade or product names in this report is for identification purposes only and does not constitute endorsement by the US Geological Survey

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