Alder (Alnus spp.) is a woody plant with bacterial symbionts that fix atmospheric N 2 into bioavailable N. We studied 12 North American boreal headwater streams spanning a steep gradient of catchment alder cover (0-27%) to test the hypothesis that increasing inputs of inorganic N associated with increasing alder cover would reduce or eliminate in-stream benthic N 2 fixation. We measured N 2 fixation rates, chlorophyll a, and ash-free dry mass (AFDM) of periphyton in early (May) and late (August) summer 2011. Dissolved inorganic N (DIN) concentrations, composed almost entirely of NO 3 /NO 2-N, ranged from below detection limits to nearly 2 mg/L and were strongly predicted by catchment alder cover in both months. Higher N 2 fixation rates were observed in August than in May. N 2 fixation rates declined sharply when alder cover exceeded~2% of catchment cover, corresponding to 20 to 40 lg/L DIN. This pattern also was evident among 3 streams with contrasting catchment alder cover sampled approximately every 2 wk during the study. The stream with no catchment alder cover exhibited a steady increase in N 2 fixation rates over the summer, whereas the streams with low and high alder in their catchments had very low N 2 fixation rates that did not vary over time. The influence of alder cover on periphyton biomass was not clear. The strong regulation of alder cover on in-stream N availability leads us to suggest that alder presence may be an important terrestrial regulator of stream N cycling.
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