Soil warming opens the nitrogen cycle at the alpine treeline

Dawes Melissa A. 1,2, Schleppi Patrick 1, Hättenschwiler Stephan 3, Rixen Christian 2, Hagedorn Frank 1

1 Swiss Federal Institute for Forest, Snow, and Landscape Research (WSL), Zürcherstrasse 111, CH-8903 Birmennsdorf, Switzerland
2 WSL Institute for Snow and Avalanche Research (SLF), Flüelastrasse 11, CH-7260 Davos Dorf, Switzerland
3 Centre d’Ecologie Fonctionnelle & Evolutive (CEFE UMR 5175), CNRS - Université Paul-Valéry - EPHE, 1919 route de Mende, FR-34293 Montpellier, France

Glob. Change Biol. 23 (2017): 421-434

DOI: 10.1111/gcb.13365


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 analyzed. 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, non-vascular 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.

Keywords: dissolved organic nitrogen (DON), European larch, Larix decidua, mountain pine, Pinus uncinata, stable isotope, Vaccinium gaultherioides, Vaccinium myrtillus