Final Report (Required, .pdf format only) : 

Final Workshop Report

One of the overarching conclusions from the recent LINX II stream ¹⁵N tracer experiments was that direct denitrification tends to explain only a minority of nitrate (NO₃^-) loss from the water column (median, 16%: Mulholland et al. 2008, Nature 452: 202). The balance of “retained” NO₃^- (measured as ¹⁵N-NO₃^- lost from stream water) appears to have been assimilated rather than denitrified, and may eventually be released back to the stream. This means the fate of most NO₃^- “retained” in streams is uncertain, as assimilated NO₃^- may or may not contribute to further nutrient loading.

In order to understand the long-term impact of “retained” NO₃^- on nutrient loading, the residence time and ultimate fate of ¹⁵NO₃^- removed by the stream bottom must be quantified. Potential fates include: 1) ¹⁵N assimilated into organic matter may be denitrified at the site of uptake through coupled remineralization-nitrification-denitrification; 2) ¹⁵N could be mineralized and released back to the water column as ammonium or nitrate; and 3) ¹⁵N-labeled organic matter may be transported, as particulates or DON, some distance downstream before becoming stored in a depositional environment, for example in lakes or reservoirs. Besides assimilation into organic N, dissimilatory nitrate reduction pathways (such as DNRA or Anammox) or nitrate storage by chemolithoautotrophs could also play a significant role in proximate N uptake.
 

The purpose of this working group is to consider hypotheses about the fate of assimilated N in streams and to discuss new approaches for examining N fates across gradients in stream type and stream size (from headwater streams to rivers). After a brief review of the literature and presentation of results from recent experiments to look at this question, we will open the floor to ideas and discussion.