A growing body of research suggests that mycorrhizal fungi, which penetrate or attach to plant roots and help to exchange nutrients, could have a significant role to play in increasing soil carbon stocks.
An international team of researchers used around 200 datasets to calculate the carbon moved from plants to their symbiotic fungi. They estimated that a total of 13.12 gigatonnes of CO2 equivalent is allocated from plants to mycorrhizae, at least temporarily — roughly the same as 36% of current emissions from fossil fuels.
The estimates are likely to be conservative, they said, and justify the inclusion of mycorrhizae in climate and carbon cycling models, as well as policymaking.
The carbon allocation of crops to mycorrhizae averaged 3.3%, with the cover crops medic and ryegrass allocating the most, and carrot also showing high levels of transfer is some studies. Wheat and barley allocated the least, at less than 1%, which the scientists speculated may be due to them having fine roots, or due to selective breeding for other traits not favouring the formation of dense mycorrhizal communities.
They also noted that the amount of CO2 allocated from plants to mycorrhizae depends on the species involved in the mutual relationship.
Proven effects
A recent review of evidence on the impact of both arbuscular mycorrhiza and ectomycorrhiza (fungi that do and do not penetrate plant roots) by researchers in China echoed the call for greater attention to be placed on interactions between plants, fungi and soil.
They observed that the fungi add to the soil carbon sink in several ways. These include through dead fungal matter and secretions from hyphae, the filamentous structure of fungi. The symbiotic relationships between plants and mycorrhizae also increase plant biomass, even under stressful conditions, allow carbon transport and decrease carbon output from the soil.
Making land more productive
They also made a case for the use of combining crops suitable for carbon farming, which have characteristics including well-developed root systems and high nitrogen efficiency, with compatible mycorrhizae as a means of increasing the viability of marginal lands.
“Mycorrhizal seedlings have the advantages of a high survival rate, high biomass, and resistance to stress, making them ideal for application in agriculture and forestry to increase carbon sink. However, due to the wide range of mycorrhizal fungal hosts, it is difficult to select the best combination of mycorrhizal fungi and plants in practice,” they wrote in the journal Eurasian Soil Science.
They suggested that combining plants selected for high light efficiency with mycorrhiza could maximise the impact on the soil carbon sink.
Despite these findings, the scientists did note that ectomycorrhiza have been observed to slow the decay of organic matter by outcompeting other microorganisms. However, this has yet to be seen for arbuscular mycorrhiza species, which have been linked with accelerated decomposition.
Measurement questions
Both sets of researchers agree that the role of mycorrhizae in transporting carbon into soil is not only under-represented in the bigger soil carbon picture, but also under-studied — with substantial knowledge gaps needing to be filled.
“Mycorrhizal fungi are currently lacking in the description of carbon sink models, which may be due to the “black box” nature of the soil and the fact that mycorrhizal fungal hyphae not only exist in soil but also in plant bodies, so it is impossible to quantify mycorrhizal fungi accurately,” the Chinese team wrote.
“In the future, we should focus on the cross-fertilization of multiple disciplines to break the bottleneck of mycorrhizal fungal metrology.”
