title: Leaf- and cell-level carbon cycling responses to a nitrogen and phosphorus gradient in two Arctic tundra species.
abstract: OBJECTIVE
  Consequences of global climate change are detectable in the historically nitrogen- and phosphorus-limited Arctic tundra landscape and have implications for the terrestrial carbon cycle. Warmer temperatures and elevated soil nutrient availability associated with increased microbial activity may influence rates of photosynthesis and respiration. •
  
  
  METHODS
  This study examined leaf-level gas exchange, cellular ultrastructure, and related leaf traits in two dominant tundra species, Betula nana, a woody shrub, and Eriophorum vaginatum, a tussock sedge, under a 3-yr-old treatment gradient of nitrogen (N) and phosphorus (P) fertilization in the North Slope of Alaska. •
  
  
  RESULTS
  Respiration increased with N and P addition-the highest rates corresponding to the highest concentrations of leaf N in both species. The inhibition of respiration by light ("Kok effect") significantly reduced respiration rates in both species (P < 0.001), ranged from 12-63% (mean 34%), and generally decreased with fertilization for both species. However, in both species, observed rates of photosynthesis did not increase, and photosynthetic nitrogen use efficiency generally decreased under increasing fertilization. Chloroplast and mitochondrial size and density were highly sensitive to N and P fertilization (P < 0.001), though species interactions indicated divergent cellular organizational strategies. •
  
  
  CONCLUSIONS
  Results from this study demonstrate a species-specific decoupling of respiration and photosynthesis under N and P fertilization, implying an alteration of the carbon balance of the tundra ecosystem under future conditions.
authors.affiliation: Department of Ecology, Evolution, & Environmental Biology, 1200 Amsterdam Avenue, Columbia University, New York, New York 10027, USA. mah2207@columbia.edu
authors.forename: Mary A
authors.initials: MA
authors.lastname: Heskel
authors.forename: O Roger
authors.initials: OR
authors.lastname: Anderson
authors.forename: Owen K
authors.initials: OK
authors.lastname: Atkin
authors.forename: Matthew H
authors.initials: MH
authors.lastname: Turnbull
authors.forename: Kevin L
authors.initials: KL
authors.lastname: Griffin
mesh_terms: D000704:Analysis of Variance; D001110:Arctic Regions; D029662:Betula / Q000166:cytology / Q000254:growth & development / Q000378:metabolism* / Q000648:ultrastructure; D018533:Biomass; D057486:Carbon Cycle / Q000528:radiation effects; D002245:Carbon Dioxide / Q000378:metabolism; D019069:Cell Respiration / Q000528:radiation effects; D002736:Chloroplasts / Q000378:metabolism / Q000528:radiation effects / Q000648:ultrastructure; D029785:Cyperaceae / Q000166:cytology / Q000254:growth & development / Q000378:metabolism* / Q000648:ultrastructure; D005306:Fertilization / Q000528:radiation effects; D008027:Light; D058503:Mesophyll Cells / Q000166:cytology / Q000528:radiation effects / Q000648:ultrastructure; D008928:Mitochondria / Q000378:metabolism / Q000528:radiation effects / Q000648:ultrastructure; D009584:Nitrogen / Q000378:metabolism*; D010101:Oxygen Consumption / Q000528:radiation effects; D010758:Phosphorus / Q000378:metabolism*; D010788:Photosynthesis / Q000528:radiation effects; D018515:Plant Leaves / Q000033:anatomy & histology / Q000166:cytology* / Q000378:metabolism* / Q000528:radiation effects; D012621:Seasons; D013696:Temperature
chemical_list: D002245:Carbon Dioxide; D010758:Phosphorus; D009584:Nitrogen