Is in-stream N-2 fixation an important N source for benthic communities and stream ecosystems?

We evaluate the current state of knowledge concerning the ecosystem- and community-level importance of N-2 fixation in streams. We reviewed the literature reporting N-2-fixation contributions to stream N budgets and compared in-stream N-2-fixation rates to denitrification and dissolved inorganic N (DIN)-Uptake rates. In-stream N-2 fixation rarely contributed >5% of the annual N input in N budgets that explicitly measured N-2 fixation, but could contribute higher proportions when considered over daily or seasonal time scales. N-2-fixation rates were statistically indistinguishable from denitrification and DIN-uptake rates from the same stream reach. However, published N-2-fixation rates compiled from a wide variety of streams were significantly lower than denitrification or DIN-uptake rates, which were indistinguishable from one another. The data set we compiled might be biased because the number of published N-2-fixation measurements is small (9 studies reporting rates in 22 streams), the range of stream conditions (NO3--N concentration, discharge, season) under which N-2-fixation and other N-processing rates have been measured is limited, and all of the rate estimates have associated methodological artifacts. To broaden our understanding of how N-2 fixation contributes to stream ecosystems, studies must measure all rates concurrently across a broad range of stream conditions. In addition, focusing on how N-2 fixation supports food webs and contributes to benthic community dynamics will help us understand the full ecological ramifications of N-2 fixation in streams, regardless of the magnitude of the N flux into streams from N-2 fixation.