Plant roots are also hungry for nitrogen in winter

In the Arctic, much of the year is cold and dark and often snow cover is present. Due to these limitations for active photosynthetic growth during such extended period, plants are challenged. However, while their aboveground parts have a clear seasonal bound by available light, the same may not be the case belowground. Here, temperature, moisture, and nutrient availability may be more important for their activity, which may benefit from a thick snow cover. Recent studies have shown that even after senescence aboveground, plant roots continue to grow in arctic ecosystems, but it is not known for how long into the winter they can remain active in nutrient uptake.

To better understand the year-round variation in potential plant N-uptake during a full year in the Arctic, we set up an experiment with non-fertilising ¹⁵N-addition (applied as NH₄NO₃) each month, followed by destructive harvests the month after. ¹⁵N-recovery was then measured in aboveground and (attached) belowground parts (separated into plant functional groups), in (unattached) coarse and fine roots, and in microbial biomass as well as in extractable inorganic N. This was done for two sites in northern Sweden differing in precipitation regime and thus snow cover thickness and duration.

Overall, our results show that there is clear potential for plants to take up N all year round, with winter potential for N-uptake matching or exceeding summer levels for evergreen and deciduous shrub as well as graminoid species. Shrubs have slightly reduced uptake of ¹⁵N during the autumn (beginning of snowfall) and spring (snowmelt). Difference in ¹⁵N-uptake between the sites differing in snow depth was smaller than expected, possibly because snow fall was high for both sites during the measurement year causing the snow depth to reach a critical threshold for decoupling soil from air temperature fluctuations.

Our results suggest that if N is available in the soil and mobile enough to reach the roots, arctic plants will be able to acquire these resources irrespective of the season. It is therefore important to also consider the period outside of the “active growing” season for understanding plant activity and N-relations. Furthermore, the arctic winters are more sensitive to climate change and increasing temperatures will especially impact this season. Without understanding belowground activity in winter, it is therefore hard to predict future outcome of a changing planet.