13.12 Gt CO₂e a year goes to mycorrhizal mycelium

In 2023 the authors of a global synthesis published in Current Biology estimated that land plants pass to underground mycorrhizal mycelium the equivalent of about 13.12 gigatonnes of CO₂ a year. That total comprised 3.93 Gt CO₂e for arbuscular mycorrhiza, 9.07 Gt for ectomycorrhiza and 0.12 Gt for ericoid mycorrhiza. The result was produced by combining maps of plant productivity with published measurements of the share of carbon directed to fungal partners.

This is a global estimate, not a direct measurement of all hyphae. Individual studies used different methods and covered unevenly distributed ecosystems. The authors had to adopt values representative of mycorrhizal types and apply them to less well studied areas. For that reason the figure carries uncertainty and should not be quoted with a precision that suggests a laboratory counter.

The most important limitation concerns time. Carbon passed to mycelium does not automatically become a durable soil resource. Some is used in respiration, some passes to other organisms, and some returns to the atmosphere after decomposition. The study describes the size of an annual flow through an important biological channel, not the quantity of carbon dioxide removed permanently.

The value does, however, help to see the scale of the plant–fungus relationship. Models of the carbon cycle, soil protection and the planning of habitat restoration should not treat mycelium as a minor addition to roots. To assess a climate effect, the residence time of carbon in biomass, aggregates and organic matter has to be measured separately. The methodology of the statistic deliberately retains the word "temporarily", because it removes the most common mistaken simplification.

Three components of one flux

The global synthesis estimated annual transfers of 3.93 Gt CO₂e to arbuscular-mycorrhizal mycelium, 9.07 Gt to ectomycorrhizal mycelium and 0.12 Gt to ericoid mycorrhiza. Together they make 13.12 Gt CO₂e. The large ectomycorrhizal component follows from the adopted plant-productivity and carbon-allocation values, not from directly weighing the world’s mycelium.

The authors describe a flow from plants into fungi. Some carbon returns to the atmosphere, some enters soil food webs, and some may persist in soil. The statistic is therefore neither a measurement of permanent storage nor a ready-made climate-benefit balance.