Isotopic analysis of cyanobacterial nitrogen fixation associated with subarctic lichen and bryophyte species

Abstract

Dinitrogen fixation by cyanobacteria is of particular importance for the nutrient economy of cold biomes, constituting the main pathway for new N supplies to tundra ecosystems. It is prevalent in cyanobacterial colonies on bryophytes and in obligate associations within cyanolichens. Recent studies, applying interspecific variation in plant functional traits to upscale species effects on ecosystems, have all but neglected cryptogams and their association with cyanobacteria. Here we looked for species-specific patterns that determine cryptogam-mediated rates of N 2 fixation in the Subarctic. We hypothesised a contrast in N 2 fixation rates (1) between the structurally and physiologically different lichens and bryophytes, and (2) within bryophytes based on their respective plant functional types. Throughout the survey we supplied 15 N-labelled N 2 gas to quantify fixation rates for monospecific moss, liverwort and lichen turfs. We sampled fifteen species in a design that captures spatial and temporal variations during the growing season in Abisko region, Sweden. We measured N 2 fixation potential of each turf in a common environment and in its field sampling site, in order to embrace both comparativeness and realism. Cyanolichens and bryophytes differed significantly in their cyanobacterial N 2 fixation capacity, which was not driven by microhabitat characteristics, but rather by morphology and physiology. Cyanolichens were much more prominent fixers than bryophytes per unit dry weight, but not per unit area due to their low specific thallus weight. Mosses did not exhibit consistent differences in N 2 fixation rates across species and functional types. Liverworts did not fix detectable amounts of N 2 . Despite the very high rates of N 2 fixation associated with cyanolichens, large cover of mosses per unit area at the landscape Plant Soil (2010) 333:507-517

Methanotrophy induces nitrogen fixation during peatland development

“…Our areal estimations for fen stages and fen-bog transition are within the wide range of the admittedly few estimates available for Sphagnum-associated N 2 fixation in temperate, boreal, and subarctic ecosystems. These range from 0.1-6.4 g of N·m −2 ·y −1 for fens (11,16,24) and 0.1-1 g·m −2 ·y −1 for bogs (9,25). Nevertheless, our estimates may be rough because of the large SEs and the extrapolation over time:…”

“…Because of this methodological problem, the role of methanotrophic N 2 fixation and the relationship of N 2 fixation with C cycling have not previously been evaluated at an ecosystem scale. The elucidation of the linkage between methanotrophy and the overall N cycle in peatlands becomes feasible by the application of stable isotope ( 15 N 2 ) techniques (8,16).…”

Across-Habitat Comparison of Diazotroph Activity in the Subarctic

“…lichen (<20 %) and mosses (50-100 %) [13][14][15], their input via N 2 fixation at ecosystem level is likely to differ substantially. For instance, although total N 2 fixation rates in some lichens commonly exceed those measured in moss-associated N 2 fixers, N 2 fixation rates converted to area may be magnitudes higher in mosses than in lichens due to high moss ground cover [16][17][18].…”

Cyanobacteria in Terrestrial Symbiotic Systems

“…Cyanolichens are generally less diverse in arid climates, but can be important components of biological soil crusts in semideserts, where they help to stabilize the soil and contribute to its fertility. The N 2 -fixing cyanobionts of cyanolichens contribute significant amounts of nitrogen to the ecosystem (Gavazov et al 2010;Elbert et al 2012). Flexibility between alternative nitrogen fixation pathways in Nostoc can be significant for cyanolichens that grow on nutrient-poor substrates and get most of their mineral nutrition through aerial deposition (Darnajoux et al 2014;Hodkinson et al 2014).…”

“…10). While most of these associations are undoubtedly facultative, they definitely play an important role in N 2 fixation, especially in boreal and arctic and in some temperate ecosystems (Solheim et al 1996;Zackrisson et al 2004;Gavazov et al 2010;Lindo and Whiteley 2011;Turetsky et al 2012;Rousk et al 2013;Arróniz-Crespo et al 2014). Feather mosses in boreal forests commonly house heterocystous cyanobacteria (e.g., Nostoc, Calothrix, Stigonema) in addition to other N 2 -fixing bacteria (Houle et al 2006;Cornelissen et al 2007;Ininbergs et al 2011).…”