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- Biological nitrogen (N) fixation, the microbial conversion of N2 gas to ammonia, makes N available to food webs. Low-N streams often have a high relative abundance of N-fixing taxa, suggesting that N fixation is an important N source in these systems. Despite this potential, stream N fixation has not been well-characterised, particularly compared to lakes and marine environments. One unknown is the relative contributions of various N-fixing organisms, particularly heterotrophic microbes.
- In low-N streams in the Cascade Mountains (Washington, USA), three groups of N-fixers predominate: cyanobacteria (Nostoc paramelioides) colonies that house a midge symbiont (Cricotopus spp.), cyanobacteria without a midge symbiont, and heterotrophic sediment microbes. In seven streams, we measured N fixation rates in each group with the acetylene reduction assay and a 15N2 calibration.
- Cyanobacteria N fixation rates were relatively low (7.9 ± 8.9 μg N m−2 hr−1, mean ± SD) compared to other mountain streams. Although rates were comparable among types of N-fixers, our sediment conversion ratio (moles of ethylene produced:moles of N fixed) was 0.16:1, much lower than our cyanobacteria conversion ratio of 1.72:1 and the commonly used theoretical ratio of 3:1. Sediment N fixation rates (5.7 ± 4.0 μg N m−2 hr−1) were higher than previously reported rates measured only with acetylene reduction.
- The midge symbiosis did not greatly impact N fixation rates; however, owing to their prevalence, colonies with the midge probably contributed more total N to streams than colonies without the midge. Additionally, N fixation by sediment heterotrophs was comparable to that of cyanobacteria colonies on an areal basis.
- Our study demonstrated that the contribution of sediment heterotrophs previously may have been underestimated in streams, especially considering that sediment heterotrophs are probably present for a longer portion of the growing season than cyanobacteria in temperate and boreal ecosystems.
Bakker, E. A. H., Vizza, C., Arango, C. P., & Roley, S. S. (2022). Nitrogen fixation rates in forested mountain streams: Are sediment microbes more important than previously thought? Freshwater Biology, ahead of print. https://doi.org/10.1111/fwb.13925
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