Soils contain two-thirds of the world’s terrestrial carbon (3,000 Pg C). The total annual soil CO2 efflux yearly exceeds the current rate of anthropogenic CO2 emissions from deforestation and burning of fossil fuels by a factor of 10. Subtle changes in soil organic carbon (SOC) processing (formation and decomposition) are, therefore, highly relevant to the global carbon cycle as soils have the potential to enhance or mitigate current increases in atmospheric CO2. Globally, the boreal forest biome covers 11% of the land surface and contains 16% of the carbon stock sequestered in soils, forming a substantial net sink in the global carbon cycle.
To date, the mechanisms by which carbon enters soil and becomes stabilized (resistant to decomposition) are unclear, limiting our capacity to effectively manage this carbon sink. The prevailing model assumes that photosynthetically fixed carbon enters the soil as plant leaf litter. Although most is decomposed, some remains, eventually forming the principal source of soil organic carbon. However, a large proportion of plant-derived carbon is directed below ground to roots, and taken up by microorganisms (predominantly fungi). In a recent study, Clemmensen et al. (2013) show that 50 to 70% of stored carbon in a chronosequence of boreal forested islands derives from root-associated fungi. In essence, the fungi acquire labile carbon directly from plant roots which they use for growth before dying. This forms a huge store of dead microbial necromass, deep under the soil, which is resistant to decomposition by other microbes. The size of this carbon sink is significantly larger in old, undisturbed islands than in more recently disturbed soils. This alternative mechanism for the accumulation of organic matter in boreal forests highlights the importance of managing soil microbes, as well as trees, in order to prevent the loss of carbon into the atmosphere. Although plants are responsible fixing atmospheric carbon, directing it into soil terrestrial sinks, it is soil fungi that are responsible for stabilizing this carbon and retaining it in the soil.
K. E. Clemmensen, A. Bahr, O. Ovaskainen, A. Dahlberg, A. Ekblad, H. Wallander, J. Stenlid, R. D. Finlay, D. A. Wardle, B. D. Lindahl (2013) Roots and Associated Fungi Drive Long-Term Carbon Sequestration in Boreal Forest. Science, 339, 1615-1618.