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Impacts of Thawing Permafrost on Nitrogen in Subarctic Peatlands
Keuper, F., van Bodegom, P.M., Dorrepaal, E., Weedon, J.T., van Hal, J., van Logtestijn, R.S.P. and Aerts, R. 2012. A frozen feast: thawing permafrost increases plant-available nitrogen in subarctic peatlands. Global Change Biology 18: 1998-2007.

The authors write that "many of the world's northern peatlands are underlain by rapidly thawing permafrost," and they say that "because plant production in these peatlands is often nitrogen (N)-limited, a release of N stored in permafrost may stimulate net primary production or change species composition if it is plant-available," which phenomenon they thus proceed to assess by means of direct measurements.

What was done
Keuper et al., as they describe it, "compared plant-available N-pools and N-fluxes in near-surface permafrost (0-10 cm below the thawfront) to those taken from a current rooting zone layer (5-15 cm depth) across five representative peatlands in subarctic Sweden." This they did by means of a range of complementary methods, including "extractions of inorganic and organic N, inorganic and organic N-release measurements at 0.5 and 11°C (over 120 days, relevant to different thaw-development scenarios) and a bioassay with Poa alpina test plants."

What was learned
The seven scientists report that "all extraction methods, across all peatlands, consistently showed up to seven times more plant-available N in near-surface permafrost soil compared to the current rooting zone layer," and they say that "these results were supported by the bioassay experiment, with an eightfold larger plant N-uptake from permafrost soil than from other N-sources such as current rooting zone soil or fresh litter substrates." In addition, they note that "net mineralization rates were much higher in permafrost soils compared to soils from the current rooting zone layer (273 mg N/m2 and 1348 mg N/m2 per growing season for near-surface permafrost at 0.5°C and 11°C, respectively, compared to -30 mg N/m2 for current rooting zone soil at 11°C)."

What it means
The six Swedes and their lone Dutch collaborator conclude that their results "demonstrate that near-surface permafrost soil of subarctic peatlands can release a biologically relevant amount of plant available nitrogen, both directly upon thawing as well as over the course of a growing season through continued microbial mineralization of organically bound N." Therefore, given the nitrogen-limited nature of northern peatlands, they confidently conclude that "this release may have impacts on both plant productivity and species composition," which we presume would likely consist of significant increases in productivity and ecosystem species richness.

Reviewed 17 October 2012