The fate of assimilated nitrogen in streams: an in situ benthic chamber study

Abstract

Summary1. Nitrogen (N) processing in streams has been investigated using whole‐stream 15N addition experiments that, in general, have found that a large proportion of added nitrate removed from the water column appears to be assimilated by the stream benthos. The long‐term fate of this retained N is unknown, and of particular interest is the possibility that it becomes denitrified through coupled mineralisation–nitrification–denitrification processes (indirect denitrification).2. We used in situ chambers to produce highly 15N‐enriched benthic biofilms and removed the chambers to allow biofilms to interact with ambient stream conditions. Nitrogen assimilation and direct denitrification were estimated from the first chamber deployment. Chambers were periodically reinstalled over 4 weeks to measure tracer 15N in ammonium (), nitrate () and dinitrogen (N2), from which we estimated subsequent rates of biotic N transformations, including N mineralisation (ammonification), nitrification and indirect denitrification. We also estimated rates of depuration of 15N tracer from benthic biomass compartments.3. Nitrate uptake was roughly equivalent in the sand and cobble habitats that dominated the stream. Direct denitrification (denitrification of from the water column) was an order of magnitude higher in cobble habitats than in sand habitats, accounting for c. 26 and 2% of total nitrate uptake in cobble and sand, respectively.4. Mean residence times of actively cycling organic N in stream benthos (algae and microbes) were 16 days in cobble habitats and 9 days in sand habitats. The difference between habitat types was driven by the influence of N residence time in epilithic biofilms (18 days) on cobbles.5. Release of enriched 15 was the primary flux of remineralised N, while release of enriched 15 was an order of magnitude less. We detected slight 15N enrichment in dissolved nitrogen gas (N2) in post‐enrichment sampling, indicating that indirect denitrification was taking place. However, indirect denitrification accounted for <0.1% of the assimilated N.6. These experiments agree with results of whole‐stream 15N additions, in that most added N was assimilated rather than directly denitrified. Assimilation was primarily a short‐term N retention mechanism in this stream, and indirect denitrification of assimilated N accounted for only a minor proportion of the observed 15N loss over time.7. Remaining possible fates include export of N as particulate organic matter, which may lead to additional storage of assimilated N in downstream habitats, and consumption by grazers.