Plant nutrient acquisition strategies in tundra species: at which soil depth do species take up their nitrogen?

The Arctic is warming at unprecedented rates. Increased thawing of permafrost releases nutrients locked up in
the previously frozen soils layers, which may initiate shifts in vegetation composition. The direction in which the
vegetation shifts will co-determine whether Arctic warming is mitigated or accelerated, making understanding
successional trajectories urgent. One of the key factors influencing the competitive relationships between plant
species is their access to nutrients, in particularly nitrogen (N).
We assessed the depth at which plant species took up N by performing a 15N tracer study, injecting 15(NH4)2SO4
at three depths (5, 15, 20 cm) into the soil in arctic tundra in north-eastern Siberia in July. In addition we explored
plant nutrient acquisition strategy by analyzing natural abundances of 15N in leaves.
We found that vascular plants took up 15N at all injection depths, irrespective of species, but also that
species showed a clear preference for specific soil layers that coincided with their functional group (graminoids,
dwarf shrubs, cryptogams). Graminoids took up most 15N at 20 cm depth nearest to the thaw front, with grasses
showing a more pronounced preference than sedges. Dwarf shrubs took up most 15N at 5 cm depth, with deciduous
shrubs displaying more preference than evergreens. Cryptogams did not take up any of the supplied 15N . The
natural 15N abundances confirmed the pattern of nutrient acquisition from deeper soil layers in graminoids and
from shallow soil layers in both deciduous and evergreen dwarf shrubs.
Our results prove that graminoids and shrubs differ in their N uptake strategies, with graminoids profiting
from nutrients released at the thaw front, whereas shrubs forage in the upper soil layers. The above implies that
graminoids, grasses in particular, will have a competitive advantage over shrubs as the thaw front proceeds and/or
superficial soil layers dry out. Our results suggest that the vertical distribution of nutrients over the soil will play
an important role in vegetation succession as permafrost thaw progresses.