Nitrogen-fixation in Acer macrophyllum canopy bryophytes in the Pacific Northwest, USA

Purpose Old-growth forests in the Pacific Northwest host a variety of epiphytes on their branches and stem. Given the common and often large epiphytic biomass associated with Acer macrophyllum (Pursh) in this region, we evaluated how seasonal weather changes and urbanization (metal and nitrogen deposition), affect canopy epiphytic N 2 fixation in the Hoh Rainforest of the Olympic Peninsula and in urban parks and forests in Seattle. Methods We collected Isothecium stoloniferum (Brid.) samples from both the Hoh Rainforest and Seattle at four periods from April 2016 through January 2017. Moss-associated N 2 fixation rates were measured in the laboratory using the acetylene reduction assay and trace metal concentrations in the moss were analyzed using NO 3 + H 2 O 2 digestion. Results We found levels of N 2 fixation were highest during the spring sampling period. Elevated levels of heavy metals were observed in I. stoloniferum samples collected in the urban canopies in Seattle where N 2 fixation rates were low, suggesting N 2 fixation is sensitive to the bioaccumulation of heavy metals. In A. macrophyllum canopies, I. stoloniferum was found to yield 0.1130 g N m −2 yr −1 in canopy branches within the Hoh Rainforest and only 0.0009 g N m −2 yr −1 on branches in Seattle. Conclusions These results highlight a rarely explored source of biological N 2 -fixation in temperate rainforests and suggest that epiphytic N 2 -fixation may contribute bio-available nitrogen in A. macrophyllum stands. N 2 -fixation in canopy bryophytes was found to be highly sensitive to urban pollution, possibly due to bioaccumulation of heavy metals in bryophyte tissue.