Nitrogen cycling in a forest stream determined by a 15N tracer addition

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%), N-15-NO3 (23%), and fine particulate organic matter (4%) from the reach, Much of the N-15 not accounted for was probably lost downstream as transport of particulate organic N during a storm midway through the experiment or as dissolved organic N produced within the reach. Turnover rates of a large portion of the N-15 taken up by biomass compartments were high (0.04-0.08 per day), although a substantial portion of the N-15 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, and availability of ammonium to meet that demand. Our results are consistent with recent N-15 tracer experiments in N-deficient forest soils that showed high rates of nitrification and the importance of nitrate uptake in regulating losses of N. Together these studies demonstrate the importance of N-15 tracer experiments for improving our understanding of the complex processes controlling N cycling and loss in ecosystems.