Mycorrhizal mycelium as a global carbon pool

Hawkins et al, 2024

This analysis of 200 datasets is the first attempt at a global quantitative estimate of carbon allocation from plants to mycorrhizal fungi.

Their conservative estimate is that 13Gt CO2 equivalent of carbon fixed by terrestrial plants is, at least temporarily, allocated to mycelium. This is approximately 36% of current annual CO2 emissions from fossil fuels

The authors describe the different types of mycorrhizal fungi associated with different plant types and landscapes. They then explore 3 main mechanisms by which fungi increase carbon in soils, and discuss some of the ways it is lost.

  1. Carbon is used to build hyphae and support active mycelial networks: as networks grow, they move carbon away from the rhizosphere to areas of lower respiratory activity. They also become attached to soil particles, increasing stability of soil aggregates. Arbuscular mycorrhizae (one type) constitutes 20-50% of total living microbial biomass.
  2. Mycelium have a short life span. The necromass of dead mycelium forms a complex scaffold of organic matter. This is no longer drawing down carbon, but forms and stabilises soil aggregates. These soil aggregates increase in size and the organic matter is therefore increasingly protection from decomposition, stabilising the carbon storage. The turnover of mycorrhizal necromass affects how labile (easily decomposed) or recalcitrant (decomposes slowly) carbon storage is – this is important but poorly understood.
  3. Mycorrhizae exude compounds that help retain soil carbon. They release carbon and nitrogen to be used and immobilised by other microbes, subsequently forming mineral-associated organic matter, the most stable form (attached to mineral particles and can’t be degraded). There is “increasing evidence that fungal residues play an important role in forming stable soil organic matter that may contribute more to mineral associated organic matter than plants”. Plants allocate 20-40% of recently fixed photosynthates to root exudates into rhizosphere, stimulating bacterial growth, increasing mineralisation and nutrient availability.
  4. However, fungal respiration and decomposition causes soil carbon losses, with arbuscular mycorrhizae responsible for 6-14% of soil respiration. This flux is a trade off between soil carbon storage and loss. It is a major research challenge to account for all carbon fluxes and pools.

The authors state their results should be interpreted cautiously, especially as datasets are based on a snapshot and do not take into consideration how allocations evolve over plant cycles. However, they have only taken into account carbon biomass in fungi external to roots. Biomass within roots may be 4 times higher.

They therefore believe mycorrhizal fungi should be included in climate and carbon cycling models and within conservation policy and practice

This is a fascinating review into mycorrhizae, including descriptions of the different functional types and how they contribute to global soil carbon pools. It is definitely worth setting time aside to read. Much of the language is accessible to non-scientists.