title: Molybdenum and phosphorus limitation of moss-associated nitrogen fixation in boreal ecosystems.
abstract: Biological nitrogen fixation (BNF) performed by moss-associated cyanobacteria is one of the main sources of new nitrogen (N) input in pristine, high-latitude ecosystems. Yet, the nutrients that limit BNF remain elusive. Here, we tested whether this important ecosystem function is limited by the availability of molybdenum (Mo), phosphorus (P), or both. BNF in dominant mosses was measured with the acetylene reduction assay (ARA) at different time intervals following Mo and P additions, in both laboratory microcosms with mosses from a boreal spruce forest and field plots in subarctic tundra. We further used a15N2tracer technique to assess the ARA to N2fixation conversion ratios at our subarctic site. BNF was up to four-fold higher shortly after the addition of Mo, in both the laboratory and field experiments. A similar positive response to Mo was found in moss colonizing cyanobacterial biomass. As the growing season progressed, nitrogenase activity became progressively more P limited. The ARA :15N2ratios increased with increasing Mo additions. These findings show that N2fixation activity as well as cyanobacterial biomass in dominant feather mosses from boreal forests and subarctic tundra are limited by Mo availability.
authors.affiliation: Department of Biology, Terrestrial Ecology Section, University of Copenhagen, Universitetsparken 15, 2100, Copenhagen, Denmark.|Center for Permafrost (CENPERM), University of Copenhagen, Øster Voldgade 10, 1350, Copenhagen, Denmark.
authors.forename: Kathrin
authors.identifier: http://orcid.org/0000-0003-3140-9864
authors.initials: K
authors.lastname: Rousk
authors.affiliation: Centre Sève, Département de Chimie, Université de Sherbrooke, Sherbrooke, J1K 2R1, QC, Canada.
authors.forename: Jefferson
authors.initials: J
authors.lastname: Degboe
authors.affiliation: Department of Biology, Terrestrial Ecology Section, University of Copenhagen, Universitetsparken 15, 2100, Copenhagen, Denmark.|Center for Permafrost (CENPERM), University of Copenhagen, Øster Voldgade 10, 1350, Copenhagen, Denmark.
authors.forename: Anders
authors.initials: A
authors.lastname: Michelsen
authors.affiliation: Département de Biologie, Université de Sherbrooke, Sherbrooke, J1K 2R1, QC, Canada.
authors.forename: Robert
authors.initials: R
authors.lastname: Bradley
authors.affiliation: Centre Sève, Département de Chimie, Université de Sherbrooke, Sherbrooke, J1K 2R1, QC, Canada.
authors.forename: Jean-Philippe
authors.initials: JP
authors.lastname: Bellenger
mesh_terms: D000114:Acetylene / Q000378:metabolism; D018533:Biomass; D044002:Bryophyta / Q000502:physiology*; D000458:Cyanobacteria / Q000187:drug effects / Q000378:metabolism; D017753:Ecosystem; D008982:Molybdenum / Q000494:pharmacology*; D009586:Nitrogen Fixation / Q000187:drug effects*; D009587:Nitrogen Isotopes; D010758:Phosphorus / Q000494:pharmacology*
chemical_list: D009587:Nitrogen Isotopes; D010758:Phosphorus; D008982:Molybdenum; D000114:Acetylene
keywords: 
  Hylocomium splendens
  ; 
  Pleurozium schreberi
  ; biological nitrogen fixation; boreal forests; cyanobacteria; molybdenum (Mo); phosphorus (P); subarctic tundra