PARENT SESSION
Thursday, August 10, 1:30-5:00 pm
COS 92 - Nitrogen cycling II
Sultana, Mezzanine Level, Cook Convention Center
Presiders: J Ely and S Baer
Whole-stream estimates of dissimilatory nitrate reduction to ammonium (DNRA): a 15N approach.
Burgin, Amy*,1, Hamilton, Stephen1, 1 W.K. Kellogg Biological Station, Hickory Corners, MI, USA
ABSTRACT- Nitrate disappearance in freshwaters usually is assumed to be due to respiratory denitrification or microbial assimilation. There is, however, a growing amount of evidence that alternative pathways, including dissimilatory nitrate reduction to ammonium (DNRA), can account for a significant proportion of the removal. These experiments were done in conjunction with the Lotic Intersite Nitrogen eXperiment (LINX2), a cross-site comparison study which involved adding 15N-nitrate (to∼20,000‰) and a conservative tracer to a stream over 24-hours and assessing the various pools into which the 15N appears. The LINX2 protocol required sampling for biotic assimilation and 15N2 to estimate denitrification, but with more intensive, time-series sampling, we were able to estimate 15N appearance in the ammonium pool (indicative of DNRA) in six streams across three sites (MI, MA, and PR). Preliminary analysis shows that in most streams, the 15N label appeared in the ammonium pool quickly and closely tracks the appearance of the conservative tracer. Typically, the amount of label in the ammonium pool reached a plateau enrichment of 120-250‰ within 3-5 hours. Once the addition was stopped, the label in the ammonium pool quickly diminished, leaving the samples taken 24-hours after the addition at less than half of the plateau enrichment. This rapid appearance and disappearance of 15N in the ammonium pool suggests that the pathway is an instantaneous conversion, rather than biotic assimilation and subsequent mineralization. Further analysis will indicate how important DNRA is to whole-stream nitrate removal, and comparatively how important DNRA is to denitrification in nitrate removal. Nitrate removed via DNRA remains in a biologically available form, in contrast to respiratory denitrification, possibly leading to profound implications for N cycling and availability in streams.
Key words: nitrogen biogeochemistry, streams, stable isotopes