Foraging the Thaw Front: Increased nutrient uptake at the permafrost surface enhances biomass production of deep- rooting subarctic peatland species

1. Plant production in subarctic peatlands is nitrogen (N)-limited. Climate warming increases N mineralization in superficial peat layers and recent results additionally show that permafrost thawing in these peatlands may substantially increase plant-available N at the thaw front. This might stimulate net primary production and affect species composition. However, the ability of individual peatland plant species to take up N from the permafrost thaw front has never been studied before. 2. We aimed to identify the potential impact of increased N-availability due to thawing permafrost on subarctic peatland plant productivity and species composition. We compared this impact with the effect of increased nutrient availability in shallower layers (e.g. through enhanced N-mineralization due to climatic warming). Therefore, we supplied 15 N-labeled nitrogen at the thaw front and performed a 3-year full-factorial belowground fertilization experiment with deep-fertilization at the thaw front at 45 cm depth and shallow-fertilization at 10 cm depth. 3. We found that only particular species (e.g. Rubus chamaemorus) are present with active roots at the thaw front. Further, if supplied with nitrogen at the thaw front, these species had higher aboveground biomass and N-content, whereas this was not the case for shallower-rooting species (e.g. Empetrum hermaphroditum and Andromeda polifolia). Moreover, the effects of increased nutrient availability at the thaw front on total aboveground biomass production were similar in magnitude to the effects of increased nutrient availability in shallower layers. Additionally, nutrient limitation of plant growth in subarctic peatlands appeared to be sufficiently strong for the effects of increased deep and shallow nutrient-availability on biomass production to be additive. 4. Synthesis Altogether, our results show that plant-available N released from thawing permafrost can be considered a true 'new' N source for deep-rooting sub-arctic plant species, which will increase their biomass production. As this is not the case for shallow-rooting species, the release of plant-available N from thawing permafrost has the potential to alter species composition on the long-term by benefitting specific deep-rooting species only.