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

Abstract

Abstract 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 N2 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 N2 fixation rates were measured in the laboratory using the acetylene reduction assay and trace metal concentrations in the moss were analyzed using NO3 + H2O2 digestion. Results : We found levels of N2 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, suggesting N2 fixation is sensitive to the bioaccumulation of heavy metals. In A. macrophyllum canopies, I. stoloniferum was found to yield 1.13 kg N ha-1 yr-1 in the Hoh Rainforest and only 0.009 kg N ha-1 yr-1 in Seattle. Conclusions : These results highlight a rarely explored source of biological N2-fixation in temperate rainforests and suggest that epiphytic N2-fixation may contribute bio-available nitrogen in secondary successional A. macrophyllum stands.