Arbuscular Mycorrhizal Fungi Increase Organic Carbon Decomposition Under Elevated CO2

A Fungal Culprit to Carbon Loss In some ecosystems, such as in the layer of soil containing plant roots, fungi, and bacteria, increased levels of CO2 should stimulate more efficient aboveground photosynthesis, which in turn should promote increased sequestration of organic carbon in soil through the protective action of arbuscular mycorrhizal fungi. However, in a series of field and microcosm experiments performed under elevated levels of CO2 thought to be consistent with future emissions scenarios, Cheng et al. (p. 1084; see the Perspective by Kowalchuk) observed that these fungi actually promote degradation of soil organic carbon, releasing more CO2 in the process. Counter to expectations, fungi associated with plant roots diminish the carbon pool in soil ecosystems under elevated levels of carbon dioxide. The extent to which terrestrial ecosystems can sequester carbon to mitigate climate change is a matter of debate. The stimulation of arbuscular mycorrhizal fungi (AMF) by elevated atmospheric carbon dioxide (CO2) has been assumed to be a major mechanism facilitating soil carbon sequestration by increasing carbon inputs to soil and by protecting organic carbon from decomposition via aggregation. We present evidence from four independent microcosm and field experiments demonstrating that CO2 enhancement of AMF results in considerable soil carbon losses. Our findings challenge the assumption that AMF protect against degradation of organic carbon in soil and raise questions about the current prediction of terrestrial ecosystem carbon balance under future climate-change scenarios.