Nitrogen cycling in a forest stream determined by a n-15 tracer addition

• Main Authors: Mulholland, P. J., Tank, J. L., Sanzone, D. M., Wollheim, W. M., Peterson, B. J., Webster, Jackson R., Meyer, J. L.
• Other Authors: Biological Sciences
• Format: Article in Journal/Newspaper
• Language: English
• Published: Ecological Society of America 2000
• Subjects: ammonium uptake length; n-15; gross primary production; nitrate uptake; length; nitrification; nitrogen cycling; respiration; stable isotope; stream ecosystem; tracer addition; ammonia diffusion method; oxygen change technique; woodland stream; chloroform fumigation; leaf decomposition; coniferous forest; headwater; stream; soil-nitrogen; fresh-waters; tundra river; Tundra
• Online Access: http://hdl.handle.net/10919/46856; http://www.esajournals.org/doi/pdf/10.1890/0012-9615%282000%29070%5B0471%3ANCIAFS%5D2.0.CO%3B2; https://doi.org/10.1890/0012-9615(2000)070[0471:nciafs]2.0.co;2

Nitrogen uptake and cycling was examined using a six-week tracer addition of N-15-labeled ammonium in early spring in Walker Branch, a first-order deciduous forest stream in eastern Tennessee. Prior to the N-15 addition, standing stocks of N were determined for the major biomass compartments. During and after the addition, 15N was measured in water and in dominant biomass compartments upstream and at several locations downstream. Residence time of ammonium in stream water (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 mu g N.m(-2).min(-1) and varying directly with changes in streamwater ammonium concentration (2.7-6.7 mu g/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 a nitrification/nitrate uptake model to the longitudinal profiles of N-15-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 one-half as large as nitrification of streamwater ammonium. Nitrate uptake lengths were longer and more variable than those for ammonium, ranging from 101 m to infinity. Nitrate uptake rate varied from 0 to 29 mu g.m(-2).min(-1) and was similar to 1.6 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 N-15 was accounted for, either in biomass within the 125-m stream reach (33-48%) or as export of N-15-NH4 (4%), ...