Nitrogen fixation rates in forested mountain streams: Are sediment microbes more important than previously thought?

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