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Boreal forest is one of the world's major biomes, dominating the subarctic northern latitudes of Europe, Asia, and America. The soils of boreal forest function as a net sink in the global carbon cycle and, hitherto, it has been thought that organic matter in this sink primarily accumulates in the form of plant remains. Clemmensen et al. (p. 1615; see the Perspective by Treseder and Holden) now show that most of the stored carbon in boreal forested islands in Sweden is in fact derived from mycorrhizal mycelium rather than from plant litter. Biochemical and sequencing studies show that carbon sequestration is regulated by functional and phylogenetic shifts in the mycorrhizal fungal community. The results will need to be explicitly considered in models of the role of the boreal forest in the global carbon cycle.
Boreal forest soils function as a terrestrial net sink in the global carbon cycle. The prevailing dogma has focused on aboveground plant litter as a principal source of soil organic matter. Using 14C bomb-carbon modeling, we show that 50 to 70% of stored carbon in a chronosequence of boreal forested islands derives from roots and root-associated microorganisms. Fungal biomarkers indicate impaired degradation and preservation of fungal residues in late successional forests. Furthermore, 454 pyrosequencing of molecular barcodes, in conjunction with stable isotope analyses, highlights root-associated fungi as important regulators of ecosystem carbon dynamics. Our results suggest an alternative mechanism for the accumulation of organic matter in boreal forests during succession in the long-term absence of disturbance.