Mycorrhiza-mediated competition between plants and decomposers drives soil carbon storage

Ecosystem mycorrhizal type is shown to have a stronger effect on soil carbon storage than temperature, precipitation, clay content and primary production; ecosystems dominated by ectomycorrhizal and ericoid mycorrhizal fungi contain 70% more soil carbon per unit nitrogen than do ecosystems dominated by arbuscular mycorrhizal fungi. Ecosystems differ in the type of plant-associated mycorrhizal fungi (root symbionts associated with nearly all land plants) that dominate. Ectomycorrhiza and ericoid mycorrhizal (EEM) fungi produce nitrogen-degrading enzymes, whereas arbuscular mycorrhiza do not, leading to the prediction that plants in the EEM ecosystems will compete with decomposers for soil nitrogen and therefore increase soil carbon storage. These authors assemble a global data set to show that this is indeed the case, with 70% more carbon storage in EEM ecosystems than in ecosystems dominated by arbuscular mycorrhiza, and that mycorrhizal type is more important than other determinants of soil carbon storage levels. Soil contains more carbon than the atmosphere and vegetation combined1. Understanding the mechanisms controlling the accumulation and stability of soil carbon is critical to predicting the Earth’s future climate2,3. Recent studies suggest that decomposition of soil organic matter is often limited by nitrogen availability to microbes4,5,6 and that plants, via their fungal symbionts, compete directly with free-living decomposers for nitrogen6,7. Ectomycorrhizal and ericoid mycorrhizal (EEM) fungi produce nitrogen-degrading enzymes, allowing them greater access to organic nitrogen sources than arbuscular mycorrhizal (AM) fungi8,9,10. This leads to the theoretical prediction that soil carbon storage is greater in ecosystems dominated by EEM fungi than in those dominated by AM fungi11. Using global data sets, we show that soil in ecosystems dominated by EEM-associated plants contains 70% more carbon per unit nitrogen than soil in ecosystems dominated by AM-associated plants. The effect of mycorrhizal type on soil carbon is independent of, and of far larger consequence than, the effects of net primary production, temperature, precipitation and soil clay content. Hence the effect of mycorrhizal type on soil carbon content holds at the global scale. This finding links the functional traits of mycorrhizal fungi to carbon storage at ecosystem-to-global scales, suggesting that plant–decomposer competition for nutrients exerts a fundamental control over the terrestrial carbon cycle.