Did you know that fungi in the soil are a major carbon sink? Amidst the complex web of life beneath our feet, mycorrhizal fungi stand as unsung heroes in the battle against climate change. These remarkable organisms have been forming symbiotic partnerships with plants for more than 400 million years – ever since mycorrhizal fungi first supported plants when they emerged on land. Mycorrhizal fungi forage in the soil for minerals and nutrients, exchanging them for the carbohydrates and fats that plants capture through photosynthesis.

The symbiotic relationship with fungi allowed plants to diversify and become more complex over time. Climate models suggest that it was this that reduced atmospheric CO2 levels in the atmosphere – from 300 to 3000ppm – during the Paleozoic Era (500-250 million years ago), corresponding to a decline in global temperatures and oxygenation of the atmosphere. It is already understood that about 75% of terrestrial carbon is stored belowground at any one time. New research presented in Current Biology alludes to the significance that mycorrhizal fungi play in storing carbon underground.

Carbon storage depends on vegetation type

Different types of mycorrhizal fungi associate with different plants. Arbuscular mycorrhiza, for example, is associated with 55% of global vegetation, including herbaceous plants like grasses, herbs, and most crop species. Arbuscular mycorrhiza is so called because they form structures within roots called “arbuscules”, meaning “a branded tree-like organ”. They are the main location for carbon-nutrient exchange between plants and their fungal partners In temperate grasslands, a healthy network of arbuscular mycorrhizal fungi can make up to 50% of the total living microbial biomass.

Commonly associated with trees growing in forest ecosystems, ectomycorrhiza makes up more than 30% of the total soil living microbial biomass. These fungi create extensive underground networks, stretching up to 2000 meters per cubic centimeter of soil. These release carbon from the organic matter in the soil as a by-product when they are getting nitrogen. Their hyphae – long thread-like structures – are mostly made up of carbon from the plants they are connected to. You might have seen (or eaten!) some of their fruiting bodies – like the poisonous Fly Agaric (Amanita muscaria) and delicious Porcini (Boletus edulis) and Chanterelle (Cantharellus cibarius).

Different mycorrhizal fungi sequester different amounts of carbon and play a role against climate change.
Different plants and different mycorrhizal fungi absorb different amounts of carbon (Source: Current Biology).

Carbon remains in the soil even when fungi die

Even when mycorrhizal fungi die, their legacy lives on. Their mycelium transforms into “fungal litter” or ‘necromass.’ Although these networks no longer absorb carbon from plants, they help create and stabilise soil aggregates. As a result, organic matter becomes more protected from breaking down, maintaining stable soil organic carbon levels. Some scientists believe that fungal necromass might contribute significantly more to soil organic carbon pools than living fungal biomass, and even surpassing the amount contributed by plant litter.

Mycorrhiza release compounds that retain soil carbon

Mycorrhizal fungi play another crucial role in carbon storage. They release special substances called exudates as they grow in the soil. These exudates contain sugars and organic acids with carbon and nitrogen. Other tiny soil organisms, like bacteria and other fungi, utilise and trap these exudates. The carbon from the exudates becomes part of mineral-associated organic matter, a stable carbon pool in the soil. It attaches to mineral surfaces, protecting it from being broken down by other microbes.

Implications for climate mitigation

While our understanding of mycorrhizal fungi’s role in the global carbon cycle is still developing, it is becoming evident that they are more vital than previously thought. Understanding their role in how different vegetation systems absorb carbon is crucial to developing more nuanced climate and carbon models. It is essential for a comprehensive understanding of carbon fluxes and effective environmental practices – especially when it comes to managing and promoting soil health.

Recognising the significance of mycorrhizal fungi could enable us to devise efficient land management strategies to maximise carbon sequestration potential and develop climate change mitigation approaches, as well as delivering other ecological benefits. These, in turn, could help to deliver social and economic benefits for those living in landscapes, who are reliant upon healthy ecosystems – and their foundational mycorrhizal fungi – to survive and thrive.

Embracing the complex relationships beneath our feet may hold the key to preserving a healthier and balanced world for generations to come!