Winter climate or species-specific traits? Controls on tundra winter nitrogen uptake along an Arctic climate gradient

Nitrogen (N) availability is a point of competition between tundra plants in a warming Arctic, where shrubs are spreading. Soil N mobilization from microbial mineralization can happen year round, and if some plants are able to access N during winter, they will have competitive advantages during warming winters with more N turnover. We therefore compared access to and retention of N during freeze-in to late winter in tundra plants from different tundra sites and explored how this may be linked to root biomass and plant functional type specific traits.

 We used ¹⁵N tracers in mesocosms from around Greenland, spanning a climate gradient from High- to Subarctic climates, and analyzed N uptake and retention species-specifically in stems and leaves, and in roots, microbes and in soil solution at four different winter stages along the climate gradient. We further measured the N uptake during a simulated late winter warming event.

We found that roots and aboveground biomass took up and retained 3-8 % of the early winter-released tracer N, but that early microbial N recovery of up to 50% dominated the ecosystem N retention. Most root N uptake was found in the Low Arctic, where continuous uptake was indicated. Least winter-released ¹⁵N overall was recovered in the High Arctic ecosystem, whereas the Subarctic ecosystem had the highest ¹⁵N recovery in plant biomass, especially in stems of deciduous shrubs. While evergreen shrubs, especially Empetrum nigrum, were overall most successful at acquiring and retaining winter-released  N with the current vegetation composition, the deciduous shrub Salix arctica stood out as most effective per unit biomass. 

We conclude that plant-specific traits and strategies, as well as climate, controlled tundra plant N access and retention during winter. Our results reveal that tundra plants access N during winter, but that plants in the Subarctic could be better adapted to access future increased winter N compared to the High Arctic. Across climates, species-specific winter N acquisition must be considered when explaining the expansion of shrubs in the Arctic tundra.

tundra plant N access and retention during winter is controlled by plant-specific traits and strategies as well as climate. Furthermore,  species-specific winter N access and acquisition must be considered when explaining the expansion of shrubs in the Arctic tundra.