Mulholland P.J.

Control of Nitrogen Export from Watersheds by Headwater Streams

Peterson, B.J. et al. 2001. Control of Nitrogen Export from Watersheds by Headwater Streams.
Science 6 April 2001:
Vol. 292 no. 5514 pp. 86-90
DOI: 10.1126/science.1056874

A comparative 15N-tracer study of nitrogen dynamics in headwater streams from biomes throughout North America demonstrates that streams exert control over nutrient exports to rivers, lakes, and estuaries. The most rapid uptake and transformation of inorganic nitrogen occurred in the smallest streams. Ammonium entering these streams was removed from the water within a few tens to hundreds of meters. Nitrate was also removed from stream water but traveled a distance 5 to 10 times as long, on average, as ammonium. Despite low ammonium concentration in stream water, nitrification rates were high, indicating that small streams are potentially important sources of atmospheric nitrous oxide. During seasons of high biological activity, the reaches of headwater streams typically export downstream less than half of the input of dissolved inorganic nitrogen from their watersheds.

Can uptake length in strams be determined by nutrient addition experiments? Results from an interbiome comparison study

Mulholland, P. J; Tanks, J. L.; Webster, J. R.; Bowden, W. B.; Dodds, W. K; Gregory, S. V.; Grimm, N. B; Meriam, J. L.; Meyer, J. L.; Peterson, B. J.; Valett, H. M.; Wollheim, W. M. 2002. Can uptake length in strams be determined by nutrient addition experiments? Results from an interbiome comparison study. J. N. Am. Bethol. Soc. 2002, 21(4): 544-560.

Nutrient uptake length is an important parnmeter tor quantifying nutrient cycling in streams. Although nutrient tracer additions are the preierred method for measuring uptake length under ambient nutrient concentrations, short-term nutrient addition experiments have more irequently been used to estimate uptake length in streams. Theoretical analysis of the relationship between uptake length determined by nutrient addition experiments (Sw') and uptake length determined by tracer additions (Sw)predicted that Sw' should be consistently longer than 5,", and that the overestimate of uptake length by Sw( should be related to the level of nutrient addition above ambient concentrations and the degree of nutrient limitation. To test these predictions, we used data irom an interbiorne study of NH,- uptake length in which 15NH,- tracer and short-term NH,-a ddition experiments were performed in 10 streams using a uniform experimental approach. The experimental results largely contirmed the theoretical predictions: sw' was consistently longer than Sw and Sw':Sw ratios were directly related to the level of NH,- addition and to indicatvrs of N limitation. The experimentally derived Sw':Sw, ratios were used with the theoretical results to infer the N limitation status of each stream. Together, the theoretical and experimental results showed the tracer experiments should be used whenever possible to determine nutrient uptake length in streams. Nutrient addition experiments may be useful for comparing uptake lengths between different streams or cliiferent times in the same stream. however, provided that nutrient additions are kept as low as possible and of similar miagnitude.
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