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Nitrogen uptake and transformation in a midwestern U.S. stream: A stable isotope enrichment study
Authors:Stephen K Hamilton  Jennifer L Tank  David F Raikow  Wilfred M Wollheim  Bruce J Peterson  Jackson R Webster
Institution:(1) Kellogg Biological Station and Department of Zoology, Michigan State University, 3700 E. Gull Lake Dr., Hickory Corners, MI, 49060, USA (Author for correspondence; e-mail;(2) Department of Biological Sciences, University of Notre Dame, Notre Dame, IN 46556, USA;(3) Kellogg Biological Station and Department of Zoology, Michigan State University, 3700 E. Gull Lake Dr., Hickory Corners, MI 49060, USA;(4) Marine Biological Laboratory, Ecosystems Center, Woods Hole, MA 02543, USA;(5) Department of Biology, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061, USA
Abstract:This study presents a comprehensive analysis ofnitrogen (N) cycling in a second-order forestedstream in southern Michigan that has moderatelyhigh concentrations of ammonium (mean,16 mgrg N/L) and nitrate (17 mgrg N/L). Awhole-stream 15NH4 + addition wasperformed for 6 weeks in June and July, and thetracer 15N was measured downstream inammonium, nitrate, and detrital and livingbiomass. Ancillary measurements includedbiomass of organic matter, algae, bacteria andfungi, nutrient concentrations, hydrauliccharacteristics, whole-stream metabolism, andnutrient limitation assays. The resultsprovide insights into the heterotrophic natureof woodland streams and reveal the rates atwhich biological processes alter nitrogentransport through stream systems.Ammonium uptake lengths were 766–1349 m anduptake rates were 41–60 mgrg N m–2min–1. Nitrate uptake could not bedetected. Nitrification rates were estimatedfrom the downstream increase in15N-enriched nitrate using a simulationmodel. The ammonium was removed bynitrification (57% of total uptake),heterotrophic bacteria and fungi associatedwith detritus (29%), and epilithic algae(14%). Growth of algae was likely limited bylight rather than nutrients, and dissolvedO2 revealed that the stream metabolism washeterotrophic overall (P:R = 0.2). Incubationsof detritus in darkened chambers showed thatuptake of 15N was mostly heterotrophic.Microbial N in detritus and algal N inepilithon appeared to reach isotopic steadystate with the dissolved ammonium, but theisotopic enrichment of the bulk detritus andepilithon did not approach that of ammonium,probably due to a large fraction of organic Nin the bulk samples that was not turning over. The actively cycling fraction of total N inorganic compartments was estimated from theisotopic enrichment, assuming uptake ofammonium but not nitrate, to be 23% forepilithon, 1% for fine benthic organic matter,5% for small woody debris, and 7% for leaves. These percentages agree with independentestimates of epilithic algal biomass, whichwere based on carbon:chlorophyll ratios in bulksamples and in algal fractions separated bydensity-gradient centrifugation in colloidalsilica, and of microbial N in the detritus,which were based on N released by chloroformfumigations.
Keywords:ammonium  15N  nitrification  nitrogen  stable isotopes  streams
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