Impaired sustainability of thawing permafrost peatland ecosystems by Siberian alder colonization

Anthropogenic climate warming causes thawing of permafrost soil in pan-Arctic areas of the Northern Hemisphere, thereby triggering changes in ecosystem biodiversity and biogeochemistry. Here, we analyzed the consequences of Siberian alder colonization for the thawing of permafrost soil, soil microbial biodiversity, and the performance of neighboring peatland vegetation. We show, for the first time, that heat dissipation from biological nitrogen fixation (BNF) by alder-Frankia symbiosis in numerous nodule clusters accelerates the thawing of permafrost soil in alder forests. On an areal basis, a rough estimate of heat dissipation from BNF amounts to 4,330-34,630 MJ year^-1 per hectare. The maximum value of this estimate is of the same order of magnitude as the reported areal heat dissipation from microbial organic matter decomposition and accounts for ∼7.6% of the heat dissipation from this decomposition. Colonization by Siberian alder trees strongly modified microbial biodiversity in the top peat and organic soil layers and had nursing effects on dominant peatland plant species neighboring alder forests, as indicated by carbon and nitrogen stable isotope signatures. These results reveal the mechanism of permafrost soil thawing attributed to BNF-mediated heat dissipation by Siberian alder forests at both the site-specific and ecosystem levels. They complement present knowledge on microbial-decomposition-driven soil heating and carbon release in permafrost regions under global warming. In addition, they show that colonization by Siberian alder has significant feedback on climate-change-mediated thawing of permafrost soil, thereby impairing the sustainability of pan-Arctic peatland ecosystems.