Nitrogen fixation can exceed inorganic nitrogen uptake fluxes in oligotrophic streams

Abstract

Nitrogen fixation can be a dominant flux of nitrogen (N) input providing up to 97 % of new N into some terrestrial and up to 82 % into some aquatic ecosystems, yet N2 fixation is rarely considered in the context of other N cycling fluxes. We compared N2 fixation with dissolved inorganic N (DIN) uptake fluxes in several streams. We measured N2 fixation in nine streams in Grand Teton National Park, Wyoming, USA and surrounding areas and we compared our estimates to the ammonium ( $$ {\text{NH}}_{4}^{ + } $$ ) uptake, nitrate ( $$ {\text{NO}}_{3}^{ - } $$ ) uptake, and denitrification estimates from the literature for those streams. N2 fixation was negligible or below detection in the four streams with $$ {\text{NO}}_{3}^{ - } $$ concentrations >20 μg $$ {\text{NO}}_{3}^{ - } $$ -N L−1. N2 fixation exceeded $$ {\text{NO}}_{3}^{ - } $$ uptake in two of the nine streams and $$ {\text{NH}}_{4}^{ + } $$ uptake in one stream. To further examine the relationship between N2 fixation and DIN uptake, we chose Ditch Creek, which is a low-N stream (<5 µg DIN-N L−1) with high rates of N2 fixation. We measured N2 fixation, $$ {\text{NH}}_{4}^{ + } $$ uptake, and $$ {\text{NO}}_{3}^{ - } $$ uptake biweekly throughout one summer. In Ditch Creek, DIN uptake exceeded N2 fixation at the beginning and end of the summer, but from July to the beginning of September N2 fixation was up to eight times greater than DIN uptake. The epilithic biofilm in Ditch Creek accumulated 1.5 g N m−2 throughout the summer, and N2 fixation may have contributed up to 73 % of that accumulation. Ditch Creek N2 fixation surpassed denitrification for both Ditch Creek and many streams. N2 fixation can be a dominant flux in low-N stream ecosystems.