title: Symbiotic N2-fixation in alpine tundra: ecosystem input and variation in fixation rates among communities.
abstract: Annual inputs of symbiotic N2-fixation associated with 3 species of alpine Trifolium were estimated in four alpine communities differing in resource supplies. We hypothesized that fixation rates would vary according to the degree of N, P, and water limitation of production, with the higher rates of fixation in N limited communities (dry meadow, moist meadow) and lower rates in P and water limited communities (wet meadow, fellfield). To estimate N2-fixation rates, natural abundance of N isotopes (δ15N) were measured in field collected Trifolium and reference plants and in Trifolium plants grown in N-free medium in a growth chamber. All three Trifolium species relied on a large proportion of atmospherically-fixed N2to meet their N requirements, ranging from 70 to 100%. There were no apparent differences in the proportion of plant N derived from fixation among the communities, but differences in the contribution of the Trifolium species to community cover resulted in a wide range of annual N inputs from fixation, from 127 mg m-2year-1in wet meadows to 810 mg m-2year-1in fellfields. Annual spatially integrated input of symbiotic N2-fixation to Niwot Ridge, Colorado was estimated at 490 mg m-2year-1(5 kg ha-1year-1), which is relatively high in the context of estimates of net N mineralization and N deposition.
authors.affiliation: Mountain Research Station Institute of Arctic and Alpine Research, University of Colorado, 80309-0334, CO, Boulder, USA.|Department of Environmental Population, and Organismic Biology, University of Colorado, 80309-0334, Boulder, CO, USA.
authors.forename: William D
authors.initials: WD
authors.lastname: Bowman
authors.affiliation: Department of Environmental Population, and Organismic Biology, University of Colorado, 80309-0334, Boulder, CO, USA.
authors.forename: James C
authors.initials: JC
authors.lastname: Schardt
authors.affiliation: Department of Environmental Population, and Organismic Biology, University of Colorado, 80309-0334, Boulder, CO, USA.
authors.forename: Steven K
authors.initials: SK
authors.lastname: Schmidt
keywords: Alpine ecosystem; Nitrogen cycling; Symbiotic N2-fixation; Trifolium; δ15N