Arctic plants can take up inorganic nitrogen year‐round

Summary 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 15 N‐pulse‐labelling. Plants and microbes took up the 15 N‐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 15 N‐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.