Despite the fundamental role of N fixation in the global N cycle, there is clear disagreement among freshwater scientists regarding the importance of this process for modern ecosystems. On one hand, repeated studies have shown that N fixation rarely comprises more than 5% of the annual N inputs to freshwater ecosystems. On the other hand, many assume that N fixation rates, although low, are adequate to overcome N limitation and therefore P should be the primary nutrient limiting production in these ecosystems. Both of these arguments, ironically, have been used to justify limited measurements or study of N fixation in freshwater ecosystems. These measurements have been most limited in streams, where N fixation rates have been published for only a handful of streams despite that ecologists often recognize N-fixing cyanobacteria as key members of stream communities. Our goal is to quantify N fixation relative to denitrification across gradients of reactive N in streams to evaluate how these processes shift or balance with increasing N loads, and also to consider alternative frameworks for evaluating the importance of N fixation to stream ecosystems and communities.
We measured rates N fixation and denitrification in 7 southeastern Idaho streams encompassing a nitrate gradient from 2.5 to 580 µg/L nitrate-N during Jul-Aug 2015. N fixation rates measured using acetylene reduction were between 0-198 μg/m2/hr and while denitrification rates measured using acetylene block ranged from 0.59-5.66 μg/m2/hr. The greatest N fixation and lowest denitrification rates occurred in the stream with the lowest nitrate concentration. The stream with the highest nitrate concentration, however, had intermediate rates of both processes. Streams with intermediate nitrate concentrations (~100-300 μg /L) exhibited low rates of N fixation and high rates of denitrification. These results suggest that stream nitrate concentration alone cannot explain the distribution of N fixation and denitrification. Our next step is to scale up our study to conduct a similar survey in streams across the US, and also to conduct experimental nutrient enrichments in a subset of streams in different regions. Finally, we will discuss N fixation within an ecological framework as a novel input of N carried out by specialized groups of organisms that reside in ecological communities. Such a framework demonstrates how N fixation can alter ecological interactions and ecosystem trajectories, highlighting the vital need to understand the fundamental effects of N fixation on ecological systems.