I know, it sounds like some sort of disease, maybe something to do with feet and old funky boots or something, but that's what arbuscular (endo) mycorrhizae is sometimes called since it can be so beneficial for growers. I haven't talked about it much for a while though it was an enthusiasm a few years ago. It isn't that it is less interesting than before, there have just been other novelties and enthusiasms to speak of. My bad.

To review, it can be understood by analogy to rhizobia, the bacteria that nodulate in legume roots and live there in symbiosis swapping the nitrates that it fixes to the plant for the sugars the plant photo-synthesizes. The fungus grows inside plant roots but pushes threadlike protrusions called hyphae out into the soil. The hyphae are thinner than even roots hairs of the plant and can penetrate into every nook and cranny of the soil where they find nutrients, even ones too tightly bound to the soil for plants to access, and transport them back to the plant. Phosphorus is a key nutrient that is transported, and it is often tightly bound in soil and otherwise unavailable to plants, but nitrogen is transported too. Like rhizobia, they swap the nutrients to the plant for sugar.

The thinness and extent of the hyphae aren't the only reasons that they are so good at scavenging nutrients. They exude chemicals and enzymes that free bound nutrients and modify soil chemistry and structure in beneficial ways, helping form stable aggregates that improve soil porosity so that air and water can circulate. Importantly, the hyphae soak up water when it is abundant, and share it with the plant when it is not, helping plants to be drought tolerant. They even capture and strangle predatory soil nematodes.

In the past much of my enthusiasm was for glomalin, one the the exudates of hyphae that helps form stable soil aggregates, since it is a stable form of soil carbon, something like humus, which builds over time and does not easily recycle back to the atmosphere as a GHG due to bacterial decomposition. That is still so, but it is just one of the beneficial attributes of farmer's fungus.

It makes sense to inoculate farm land with mycorrhizae for many crops. Not all crop plants form associations, and there are many kinds of mycorrhizae so you need to be specific and use the right inoculant, just as you need the right rhizobia for your particular legume. Tillage, pesticides and high doses of fertilizer can kill off mycorrhizae. They don't spread on the wind so once you've killed them off they are very slow to come back unless you inoculate.

There's also a biochar angle.

The structure of the charcoal provide a refuge for small beneficial soil organisms, such as symbiotic mycorrhyzal fungi.

Bio-char is able to serve as a habitat for extraradical fungal hyphae that sporulate in their micropores due to lower competition from saprophytes (Saito and Marumoto, 2002 as reported in Lehmann, 2006)

Nishio (1996) states “the idea that the application of charcoal stimulates indigenous arbuscular mycorrhiza fungi in soil and thus promotes plant growth is relatively well-known in Japan, although the actual application of charcoal is limited due to its high cost”. The relationship between mycorrhizal fungi and charcoal may be important in realising the potential of charcoal to improve fertility. Nishio (1996) also reports that charcoal was found to be ineffective at stimulating alfalfa growth when added to sterilised soil, but that alfalfa growth was increased by a factor of 1.7-1.8 when unsterilised soil containing native mycorrizal fungi was also added. Warnock (2007) suggests four possible mechanisms by which biochar might influence mycorrhizal fungi abundance. These are (in decreasing order of currently available evidence supporting them): “alteration of soil physico-chemical properties; indirect effects on mycorrhizae through effects on other soil microbes; plant–fungus signalling interference and detoxification of allelochemicals on biochar; and provision of refugia from fungal grazers. (Woolf, 2006)

There seems to be an additive effect. Biochar is better when mycorrhizae are present, and mycorrhizae are better when biochar is present.