01/08/2014

Symbiotic Fungi Inhabiting Plant Roots Have Major Impact on Atmospheric Carbon

Summary

Of central concern to climate change science is the potential for natural feedbacks to the warming currently under way as a result of anthropogenic CO₂ emissions. Soil is the largest reservoir of carbon in the terrestrial biosphere, containing more than that already found in the atmosphere and biomass combined. If soils were to lose even a small fraction of their carbon, climate could change rapidly with important repercussions for U.S. policy on topics as disparate as food security and coastal inundation. To date, it is has been difficult to identify the factors controlling gains of soil carbon on local to global scales. In recent study, researchers show that mycorrhizal fungi—symbiotic fungi on plant roots—control the quantity of carbon in today’s soils. Using global datasets, they found that the soil in ecosystems dominated by ecto- and ericoid mycorrhizal fungi contains ~70% more carbon than those dominated by arbuscular mycorrhizal fungi. In their analysis, the effect of mycorrhizal type on soil carbon pools was of far larger consequence than the effects of an ecosystem’s productivity, its climate (i.e., temperature and precipitation) or the physical properties of its soil (e.g., clay content). While the mechanism accounting for the difference in soil carbon storage is still debated, it appears that competition for nitrogen in the soil provides the best answer. Ecto- and ericoid mycorrhizal fungi produce many different types of enzymes that they release into the soil in an effort to unlock the nitrogen bound to carbon pools in soil. These fungi also are very effective competitors for nitrogen, making it very scarce to other decomposers in the soil, reducing their biomass and hence the rate of decomposition. By contrast, arbuscular mycorrhizal fungi lack many of these enzyme systems and decomposition rates are rapid. Importantly, this research links the traits of mycorrhizal fungi to carbon storage at the global scale—from tropical forests to the far northern reaches of the boreal forest—suggesting that decomposer competition for nutrients exerts fundamental control over the terrestrial carbon cycle. Whether climate change alters the distribution of these different fungal species remains to be seen, but increases in the abundance or geographical spread of arbuscular mycorrhizal may portend a significant, biologically controlled positive feedback to the climate system.

References

Averill, C., B. L. Turner, and A. C. Finzi. 2014. “Mycorrhiza-Mediated Competition Between Plants and Decomposers Drives Soil Carbon Storage,” Nature 505, 543-45. DOI:10.1038/nature12901.