title: Soil warming opens the nitrogen cycle at the alpine treeline.
abstract: Climate warming may alter ecosystem nitrogen (N) cycling by accelerating N transformations in the soil, and changes may be especially pronounced in cold regions characterized by N-poor ecosystems. We investigated N dynamics across the plant-soil continuum during 6 years of experimental soil warming (2007-2012; +4 °C) at a Swiss high-elevation treeline site (Stillberg, Davos; 2180 m a.s.l.) featuring Larix decidua and Pinus uncinata. In the soil, we observed considerable increases in the NH4+ pool size in the first years of warming (by >50%), but this effect declined over time. In contrast, dissolved organic nitrogen (DON) concentrations in soil solutions from the organic layer increased under warming, especially in later years (maximum of +45% in 2012), suggesting enhanced DON leaching from the main rooting zone. Throughout the experimental period, foliar N concentrations showed species-specific but small warming effects, whereas δ15N values showed a sustained increase in warmed plots that was consistent for all species analysed. The estimated total plant N pool size at the end of the study was greater (+17%) in warmed plots with Pinus but not in those containing Larix, with responses driven by trees. Irrespective of plot tree species identity, warming led to an enhanced N pool size of Vaccinium dwarf shrubs, no change in that of Empetrum hermaphroditum (dwarf shrub) and forbs, and a reduction in that of grasses, nonvascular plants, and fine roots. In combination, higher foliar δ15N values and the transient response in soil inorganic N indicate a persistent increase in plant-available N and greater cumulative plant N uptake in warmer soils. Overall, greater N availability and increased DON concentrations suggest an opening of the N cycle with global warming, which might contribute to growth stimulation of some plant species while simultaneously leading to greater N losses from treeline ecosystems and possibly other cold biomes.
authors.affiliation: Swiss Federal Institute for Forest, Snow and Landscape Research (WSL), Zürcherstrasse 111, CH-8903, Birmensdorf, Switzerland.|WSL Institute for Snow and Avalanche Research - SLF, Flüelastrasse 11, CH-7260, Davos Dorf, Switzerland.
authors.forename: Melissa A
authors.identifier: 0000-0003-4919-0151
authors.initials: MA
authors.lastname: Dawes
authors.affiliation: Swiss Federal Institute for Forest, Snow and Landscape Research (WSL), Zürcherstrasse 111, CH-8903, Birmensdorf, Switzerland.
authors.forename: Patrick
authors.initials: P
authors.lastname: Schleppi
authors.affiliation: Centre d'Ecologie Fonctionnelle & Evolutive (CEFE UMR 5175), CNRS - Université de Montpellier - Université Paul-Valéry Montpellier - EPHE, 1919 route de Mende, F-34293, Montpellier Cedex 5, France.
authors.forename: Stephan
authors.initials: S
authors.lastname: Hättenschwiler
authors.affiliation: WSL Institute for Snow and Avalanche Research - SLF, Flüelastrasse 11, CH-7260, Davos Dorf, Switzerland.
authors.forename: Christian
authors.initials: C
authors.lastname: Rixen
authors.affiliation: Swiss Federal Institute for Forest, Snow and Landscape Research (WSL), Zürcherstrasse 111, CH-8903, Birmensdorf, Switzerland.
authors.forename: Frank
authors.initials: F
authors.lastname: Hagedorn
mesh_terms: D000531:Altitude; D017753:Ecosystem; D028221:Larix; D009584:Nitrogen; D058458:Nitrogen Cycle; D012987:Soil / Q000737:chemistry*; D013696:Temperature
chemical_list: D012987:Soil; D009584:Nitrogen
keywords: 
  Larix decidua
  ; 
  Pinus uncinata
  ; 
  Vaccinium gaultherioides
  ; 
  Vaccinium myrtillus
  ; European larch; dissolved organic nitrogen; mountain pine; stable isotope