Soil contains more carbon than the atmosphere and all the planet's vegetation combined. A large part of the explanation for why this carbon remains stored—instead of decomposing and being released back into the atmosphere—involves a microscopic actor: the type of mycorrhizal fungus that dominates each forest ecosystem.
An Invisible Competition for Nitrogen
A study published in Nature analyzed global soil databases and found that ecosystems dominated by trees associated with ectomycorrhizal and ericoid fungi store approximately 70% more carbon per unit of nitrogen than ecosystems dominated by trees associated with arbuscular mycorrhizal fungi. The proposed explanation: ectomycorrhizal fungi produce enzymes that degrade organic nitrogen, directly competing for that nitrogen with free-living decomposers that would otherwise release soil carbon. The effect of mycorrhizal type on soil carbon was found to be independent of—and of greater magnitude than—variables such as net primary productivity, temperature, or precipitation.
A Global Map of Symbiosis
A second study constructed the first explicit global map of the dominant mycorrhizal symbiosis type in the world's forests, using over 1.1 million forest inventory plots covering more than 28,000 tree species. The central finding: ectomycorrhizal trees, representing just 2% of plant species, constitute around 60% of tree stems on Earth, and their distribution is primarily controlled by the effect of climate on the rate of soil decomposition.
Why This Accounting Matters
Understanding which type of fungus dominates a forest is not anecdotal information: it predicts how much carbon that forest is capable of retaining underground in the long term, and therefore directly informs global climate models. The next time you walk through a conifer forest, it's worth remembering that a large part of the reason why that soil functions as a carbon reservoir is decided in a microscopic negotiation between roots, fungi, and decomposing microorganisms.
References
- Averill C, Turner BL, Finzi AC. Mycorrhiza-mediated competition between plants and decomposers drives soil carbon storage. Nature. 2014;505(7484):543-545. PMID: 24402225. DOI: 10.1038/nature12901
- Steidinger BS, Crowther TW, Liang J, et al. Climatic controls of decomposition drive the global biogeography of forest-tree symbioses. Nature. 2019;569(7756):404-408. PMID: 31092941. DOI: 10.1038/s41586-019-1128-0