Nitrogen Cycling In A Forest Stream Determined By A 15N Tracer Addition. Mulholland, P. J., Tank, J., Sanzone, D. M., Wollheim, W. M., Peterson, B., Webster, J., & Meyer, J. Ecological Monographs, 2000.
Nitrogen Cycling In A Forest Stream Determined By A 15N Tracer Addition. [pdf]Paper  abstract   bibtex   
Nitrogen uptake and cycling was (5-6 min) and ammonium uptake lengths (23-27 m) were short and relatively constant during the addition. Uptake rates of NH4 were more variable, ranging from 22 to 37 mg/L). The highest rates of ammonium uptake per unit area were by the liverwort Porella pinnata, decomposing leaves, and fine benthic organic matter (FBOM), although epilithon had the highest N uptake per unit biomass N. Nitrification rates and nitrate uptake lengths and rates were determined by fitting nitrification/nitrate uptake model to the longitudinal profiles of 15N-NO3 flux. Nitrification was an important sink for ammonium in stream water, accounting for 19% of the total ammonium uptake rate. Nitrate production via coupled regeneration/nitrification of organic N was about on-half as large as nitrification of streamwater ammonium. Nitrate uptake lengths were longer and more variable that those for ammonium, ranging from 101 m to infinity. Nitrate uptake rate varied from 0 to 29 mg+m-2+min-1 and was \textasciitilde1.6 times greater than assimilatory ammonium uptake rate early in the tracer addition. A sixfold decline in instream gross primary production rate resulting from a sharp decline in light level with leaf emergence had little effect on ammonium uptake rate but reduced nitrate uptake rate by nearly 70%. At the end of the addition, 64-79% of added 15N was accounted for either in biomass within the 125-m stream reach (33-48%) or as export of 15N-NH4 (4%), 15N-NO3 (23%), and fine particulate organic matter (4%) from the reach. Much of the 15N not accounted fro was probably lost downstream as transport of particulate organic N during a storm midway through the experiment or as dissolved organic N produced with the reach. Turnover rates of a large portion of the 15N taken up by biomass compartments are high (0.04-0.08 per day), although a substantial portion of the 15N in Porella (34%), FBOM (21%), and decomposing wood (17%) at the end of the addition was retained 75 d later, indicating relatively long-term retention of some N taken up from water. In total, our results showed that ammonium retention and nitrification rates were high in Walker Branch, and that the downstream loss of N was primarily as nitrate and was controlled largely by nitrification, assimilatory demand for N, . of the complex processes controlling N cycling and loss in ecosystem.

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