Mycorrhizal fungi: the underrated heroes of carbon capture?

Mycorrhizal fungi have been supporting life on land for at least 450 million years by helping to supply most land plants with water and soil nutrients essential to their growth. In exchange, plants allocate some of their carbohydrates to these fungi. My colleague on the paper, Prof. Toby Kiers (from SPUN), describes this relationship in economic terms, as a reciprocal exchange.

Findings

In recent years, scientists have begun to consider the consequences of this symbiotic relationship for the transport and burial of carbon into soil ecosystems. While several rough approximations have been made in the past, none were quantitative or global in extent. In a meta-analysis published on June 5 in the journal Current Biology, my colleagues and I estimated that 13.12 gigatons of carbon dioxide equivalents (CO2e) fixed by terrestrial plants are allocated to mycorrhizal fungi annually. This equates to a staggering 36% of yearly global fossil fuel emissions.

Understandably, much focus has been placed on protecting and restoring forests as a natural way to mitigate climate change. But little attention has been paid to the fate of the vast amounts of carbon dioxide that are moved from the atmosphere during photosynthesis by those plants and sent belowground to mycorrhizal fungi.

How does this work?

“Mycorrhiza” comes from the Greek words “mykes” meaning fungus and “rhiza” meaning root, so literally “fungus-root”. This gives a clue to the astonishing fact that about 90% of plant species have roots that do not exist as a single organ but as a combination of both plant and fungal parts. From the fossil record, we know that early land plants had rudimentary root systems, something similar to the rhizoids of moss today, where these organs mostly function to anchor the plant to the substrate. Early plants evolved from aquatic plants that had free access to nutrients and water. On land, the soil would have presented a relatively harsh environment to these early plants.

This is where the fungi came in. Hyphae (the branching thread-like filaments that make up the fungal mycelium) enter into roots and receive carbohydrates and other carbon-containing compounds. Fueled by this carbon, the mycelium can spread out into the soil, rapidly extending the area of soil available to the root and entering fine pores of the soil not accessible to the plant, thereby providing the plant partner with water and certain nutrients. This relationship was successful in the past and appears just as successful today. 

The carbon flux we refer to in the paper is based on an estimation of plant-fixed carbon entering the living mycelium only. Due to a lack of data, we could not account for the carbon in the mycorrhizal mycelium inside the root, dead mycelium (necromass) or fungal exudates, all of which surely contribute to soil organic carbon (fungal exudates include sticky compounds that, together with necromass, form a structural scaffold for soils, which increases soil aggregation and protects soil from erosion and carbon loss). We also do not know how much of the allocated carbon is subsequently lost as the respiration of CO2 back to the atmosphere. Therefore major gaps in our knowledge are the full extent of carbon allocation to the fungus but also the permanence of carbon within mycorrhizal structures. 

“Mycorrhizal fungi represent a blind spot in carbon modeling, conservation, and restoration,” says co-author Katie Field (@KatieField4), a professor of plant-soil processes at the University of Sheffield. “Many human activities destroy underground ecosystems. Besides limiting the destruction, we need to radically increase the rate of research,” says co-author Merlin Sheldrake (@MerlinSheldrake). “Organizations like SPUN, the Fungi Foundation, and GlobalFungi are leading a massive global sampling effort to create open-source maps of Earth’s fungal networks. These maps will help chart the properties of underground ecosystems, such as carbon sequestration hotspots, and document new fungal species able to withstand drought and high temperatures.” These maps could one day assist in designing nature-based solutions, e.g., Conservation International’s Natural Climate Solutions. 

The relevance of our results

What is clear, is that our results highlight the importance of protecting soil as our major terrestrial carbon sink. The UN Food and Agriculture Organization warns that 90% of soils could be degraded by 2050, and fungi are left out of most conservation and environmental policies. Without the fertility and structure that soil provides, the productivity of both natural and crop plants will rapidly decline. Our paper is part of a global push to understand the role that fungi play in Earth’s ecosystems. 

Meanwhile, we can protect soils and their associated ecosystems by protecting natural lands like forests, grasslands and shrublands while restoring degraded natural areas wherever possible. Likewise in agricultural lands, there is much we can do to conserve not only soil carbon but also biodiversity. Avoiding excessive disturbance of soil by e.g., ploughing and overgrazing, are ways to protect these fungal networks. Other practices like no-till, cover cropping, planting of perennial versus annual plants will also support mycorrhizal fungal networks. People can apply these principles in their own gardens by e.g., planting diverse, indigenous plants and covering soil with groundcover plants and mulch.

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