[L&O Featured Article] Vol. 49, Issue 3, May 2004

[lo-feature at aslo.org]

The Featured Articles in the May 2004 issue of L&O are:

Bohlke, John Karl, Judson W. Harvey, and Mary A. Voytek. 2004. Reach-
scale isotope tracer experiment to quantify denitrification and 
related processes in a nitrate-rich stream, midcontinent United 
States. Limnol. Oceanogr. 49: 821-838.

And 

Mulholland, Patrick J., H. Maurice Valett, Jackson R. Webster, Steven 
A. Thomas, Lee W. Cooper, Stephen K. Hamilton, and Bruce J. Peterson. 
2004. Stream denitrification and total nitrate uptake rates measured 
using a field 15N tracer addition approach. Limnol. Oceanogr. 49: 809-
820.

These papers are freely available at:

          http://aslo.org/lo/toc/vol_49/issue_3/0821.pdf
and
          http://aslo.org/lo/toc/vol_49/issue_3/0809.pdf

Instructions for reading PDF files are located on the ASLO web page: 

          http://aslo.org/help/loonline.html 

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 

Introductory comments by Stephen Hamilton and Elizabeth Canuel (L&O 
Associate Editors)

Human activities such as fossil fuel combustion, agricultural 
fertilizer application, and legume cultivation are adding large 
amounts of fixed (reactive) nitrogen (N) to the hydrosphere.  This 
added N causes a number of serious environmental problems, including 
acidification of soil and water, impairment of drinking water 
supplies, and stimulation of harmful algal blooms and consequent 
oxygen depletion in coastal waters such as where the Mississippi 
River discharges into the Gulf of Mexico.  Despite the importance of 
these problems, we have a poor understanding of the fate of the N we 
are adding to terrestrial and aquatic ecosystems.  Studies of large 
river watersheds show that most of the added N is not exported to the 
oceans but has disappeared from our accounting somewhere within these 
watersheds.  Recent research has indicated that uptake and removal by 
plants and microbes in streams and rivers might account for some of 
the missing N.  In particular, it appears that small streams in the 
headwaters of river basins are important in removing N, and bacterial 
denitrification in their sediments is a key process for permanent N 
removal. 

To gain a better understanding of the role that small streams play in 
removing N from water and preventing it from polluting downstream 
ecosystems, investigators are developing and refining techniques for 
whole-stream stable isotope additions.  Such tracer experiments offer 
the advantage of indicating in-situ fluxes and transformations of N 
at ambient concentrations, at the scale of an entire stream reach.  
This information on N cycling also provides fundamental insights into 
autotrophic and heterotrophic metabolism in streams and the role of N 
in regulating that biological activity.  The two articles featured 
here represent the state-of-the-art in this line of research.    

Both studies added 15N-labelled nitrate to small streams and studied 
the disappearance of the tracer nitrate and the accumulation of the 
15N in denitrification products as the water flowed downstream.  The 
studies concur in demonstrating that the two small stream channels 
under study--one in a N-rich agricultural landscape (Bvhlke et al.) 
and the other in a relatively pristine deciduous forest (Mulholland 
et al.)--do indeed modify the forms and amount of N carried through 
them, and they offer the first whole-stream estimates of N uptake and 
transformation rates.  The studies differ in the amount of available 
N in the streams as well as in their approaches to data analysis and 
modeling. The two papers are being published back-to-back to ensure 
that readers are aware of the alternative approaches as well as the 
results for these contrasting stream ecosystems.  The challenge 
remains to see whether these results can be applied to headwater 
streams in general, and to understand 
-- 
_________________________
Paul F. Kemp, ASLO Web Editor
Web: http://www.aslo.org/
Email: webeditor at aslo.org
Ph: 631-632-8796