Nitrogen uptake and transformation in a midwestern US stream: A stable isotope enrichment study

This study presents a comprehensive analysis of nitrogen (N) cycling in a second-order forested stream in southern Michigan that has moderately high concentrations of ammonium (mean, 16 mug N/L) and nitrate (17 mug N/L). A whole-stream (NH4+)-N-15 addition was performed for 6 weeks in June and July, and the tracer N-15 was measured downstream in ammonium, nitrate, and detrital and living biomass. Ancillary measurements included biomass of organic matter, algae, bacteria and fungi, nutrient concentrations, hydraulic characteristics, whole-stream metabolism, and nutrient limitation assays. The results provide insights into the heterotrophic nature of woodland streams and reveal the rates at which biological processes alter nitrogen transport through stream systems. Ammonium uptake lengths were 766-1349 m and uptake rates were 41-60 mug N m(-2) min(-1). Nitrate uptake could not be detected. Nitrification rates were estimated from the downstream increase in N-15-enriched nitrate using a simulation model. The ammonium was removed by nitrification (57% of total uptake), heterotrophic bacteria and fungi associated with detritus (29%), and epilithic algae (14%). Growth of algae was likely limited by light rather than nutrients, and dissolved O-2 revealed that the stream metabolism was heterotrophic overall (P:R = 0.2). Incubations of detritus in darkened chambers showed that uptake of N-15 was mostly heterotrophic. Microbial N in detritus and algal N in epilithon appeared to reach isotopic steady state with the dissolved ammonium, but the isotopic enrichment of the bulk detritus and epilithon did not approach that of ammonium, probably due to a large fraction of organic N in the bulk samples that was not turning over. The actively cycling fraction of total N in organic compartments was estimated from the isotopic enrichment, assuming uptake of ammonium but not nitrate, to be 23% for epilithon, 1% for fine benthic organic matter, 5% for small woody debris, and 7% for leaves. These percentages agree with independent estimates of epilithic algal biomass, which were based on carbon:chlorophyll ratios in bulk samples and in algal fractions separated by density-gradient centrifugation in colloidal silica, and of microbial N in the detritus, which were based on N released by chloroform fumigations.