Winter climate or plant functional type? Controls on tundra winter nitrogen uptake along an Arctic climate gradient in East Greenland

Limited Nitrogen (N) availability causes competition in the warming Arctic tundra, where shrubs are spreading. Soil microbial N mobilization can occur year-round, and plants that better acquire N during winter may compete better during warmer winters. We therefore explored the influence of plant functional type on plant access to and retention of N during freeze-in to late winter in tundra heaths. We applied 15 N tracer at freeze-in to 34 tundra mesocosms from a High- to Subarctic East Greenlandic climatic gradient and analyzed N uptake and retention species-specifically in stems and leaves of 12 vascular plant species and bryophytes, and in roots, microbes and soil solution, four times during winter simulation. Roots and aboveground biomass took up and retained 3–8% of 15 N, but microbial N uptake dominated ecosystem N retention (50%). Roots acquired most N in Low arctic tundra, with uptake throughout winter. Least 15 N was recovered in the High arctic ecosystem, whereas Subarctic plants had largest 15 N recovery, particularly in deciduous shrub stems. While evergreen shrubs contained most winter-released N, the deciduous Betula nana and Salix arctica were most effective per unit biomass. The largest N reservoir during winter was in microbial biomass, and plant functional type, as well as climate, controlled tundra plant winter N uptake. Subarctic vegetation appears adapted to access increased winter N, while High arctic plants were most sensitive to a winter warming event. Across climates, we conclude that species-specific strategies must be considered when explaining Arctic tundra shrubification.