Mulholland, P. J. Oak Ridge National Laboratory, mulhollandpj@ornl.gov
Tank, J. J. University of Illinois / Dept. of Natural Resources,
Sanzone, D. M. University of Georgia / Institute of Ecology, spider@sparc.ecology.uga.edu
Wollheim, W. M. Ecosystems Center / Marine Biological Laboratory, wollheim@lupine.mbl.edu
Peterson, B. J. Ecosystems Center / Marine Biological Laboratory, peterson@lupine.mbl.edu
Meyer, J. L. University of Georgia / Institute of Ecology, meyer@sparc.ecology.uga.edu

 
NITROGEN CYCLING IN A STREAM ECOSYSTEM DETERMINED BY A TRACER-LEVEL NITROGEN-15 ADDITION EXPERIMENT
 
We examined nitrogen uptake and cycling in a low DIN (< 0.05 mgN/L) forested stream by adding 15N-ammonium continuously for six weeks in early spring to Walker Branch in eastern Tennessee. Ammonium residence times in water and uptake lengths were short (5-6 min and 20-26 m, respectively), indicating tight cycling. Ammonium uptake rate varied with ammonium concentration and was dominated by bryophytes, decomposing leaves, and fine benthic organic matter. Nitrification accounted for 20% of ammonium uptake. Nitrate uptake length, determined by applying a nitrification/nitrate uptake model to the longitudinal distribution of 15N-nitrate, was longer and more variable (102-625 m) than for ammonium. A 6-fold decline in stream primary productivity over the experimental period due to reduction in light as leaves emerged on riparian vegetation had little effect on ammonium uptake rate but reduced nitrate uptake rate by 50%. Ecosystem N turnover time, based on uptake rates and N standing stocks, was 108 d. In summary, our results showed that ammonium retention and nitrification were relatively high. Downstream loss of DIN was primarily as nitrate and was controlled ultimately by nitrification, assimilatory demand for N, and the availability of ammonium to meet that demand.
 
Day: Friday, Feb. 5
Time: 08:45 - 09:00am
Location: Sweeney Center
 
Code: CS55FR0845S