The other half of the spiral: determining the fate of biologically assimilated N in stream and river systems

Poster Disciplines/Format:
Final Report (Required, .pdf format only) : 
Jonathan O'Brien
Stephen Hamilton
William McDowell

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.


Session Info

Working Group Session 3

Tue, 09/15/2009 - 10:00am - 12:00pm
Reusch Auditorium Billhiemer