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.