Nitrogen uptake and transformation in a midwestern U.S. stream: A stable isotope enrichment study


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 μg N/L) and nitrate (17 μg 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 μg 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.

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© Kluwer Academic Publishers 2001