Arctic plants can take up inorganic nitrogen year-round

Arctic tundra experiences strong climatic seasonality, with cold and long winters, but effective insulation by snow might enable plants and microbes to remain active in winter. We investigated year-round seasonality of plant and microbial inorganic nitrogen (N) uptake, within-plant allocation, and their dependence on snow depth in low-Arctic tundra, using monthly in situ ¹⁵N-pulse-labelling. Plants and microbes took up the ¹⁵N-label throughout the year, likely due to sufficiently deep and early snow that stabilised soil temperatures c. 0°C in both study sites. Surprisingly, we found only a few indications for higher plant and microbial ¹⁵N-uptake and (estimated) plant total inorganic N-uptake during the growing season compared to the cold season (i.e. the non-growing season). Instead, N-acquisition strongly varied within the cold season, with clearly higher uptake during the cold deep-winter (November-March) than the warmer spring-winter (April-May/June). Taken together, our results suggest that 78-82% of the annual plant inorganic N-uptake may take place during the long cold season and as much as 50-56% during deep-winter. This demonstrates the importance of nutrient dynamics during the cold season in the Arctic and challenges the general assumption that arctic plants remain dormant during the winter.