Foraging the frozen feast: Uptake of nitrogen from thawing permafrost in subarctic peatlands
Thawing of permafrost soils affects many northern peatlands, where one-third of the world’s soil organic carbon is stored due to their high ratio of carbon assimilation to carbon mineralization. Because plant production in these peatlands is often nitrogen (N)-limited, a release of permafrost N upon thawing may likely stimulate net primary production or change species composition. However, surprisingly little is known about plant-availability and plant-uptake of nitrogen stored in permafrost soils. We quantified plant-available N in thawing permafrost soils of subarctic peatlands and found up to seven times more plant-available N in near-surface permafrost soil compared to the current rooting zone layer. With the work presented here, we aimed to identify the potential impact of this increased N-availability due to thawing permafrost on subarctic peatland plant production and species composition. We compared this impact with the effect of increased nutrient availability in shallower layers (e.g. in relation to enhanced N-mineralization due to climatic warming). Therefore, we supplied 15N-labelled 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.
We found that only particular species (e.g. Rubus chamaemorus) have active roots at the thaw front. Further, if presented with increased 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. E. 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. Nutrient limitation of plant growth in subarctic peatlands appeared to be strong enough for the effects of increased deep and shallow nutrient availability on biomass production to be additive. Altogether, these 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.