Mushrooms - more than just a meal

Mushrooms are not just for eating. They have many other functions in nature. For example, as symbiotic partners of trees and plants, providing them with nutrients and water and thus making them more resistant to drought or pathogens.

Porcini mushrooms are highly sought-after edible mushrooms. But what we eat is only their fruiting body. What is less well known, however, is that their network of threads runs through the forest floor and that they are closely associated with oaks, spruces and pines. This is because their fungal network penetrates the fine roots of the partner trees. ‘Mycorrhiza’ is the name given to this fungus-root connection, which many other fungi also form.

If a tree or plant receives nutrients and water from deeper layers of soil through the fungus, the latter benefits from the sugar from the tree's roots for its growth. With or without - the differences are huge: thanks to the extensive root-fungus network, a tree can absorb up to 30 per cent more water than one without fungal symbiosis. This is a decisive advantage in times of climate change. Not only that: roots of a tree colonised with fungi also protect it better against pathogens in the soil. In addition, the soil becomes more stable thanks to the metre-long fungal network, because the mycelium holds organic and inorganic soil particles together. This creates a soil with stable pores that stores water and nutrients better.

These symbioses are not unusual: around 80 to 90 per cent of all plants are colonised by mycorrhiza. This also applies to many modern cultivated plants. For example:

• Wheat
• maize
• potatoes
• sunflowers
• soya beans
• Onions
• Pumpkin
• Lentils
• Flax

Protect soil and fungal nets

The fungal nets are mainly damaged by frequent digging or ploughing of the soil or by leaving it ‘naked’, says Andres Stucke, research assistant and lecturer at the Institute of Geography at the University of Hildesheim.

‘However, if you cover the soil and replant it after harvesting, most mycorrhizal fungi form a new fungal network with other plants.’ However, it is best to cultivate the soil in a ‘mycorrhiza-friendly’ way. ‘I'm a big fan of the Broadfork myself. This garden tool loosens the soil, allows the plants to root deeply and largely preserves the soil structure.’ Mixed cultivation, alternating between permanent crops and annual plants and avoiding pesticides that damage the fungal network also help.

Considering the poor condition of soils around the world, which are often eroded and have a low humus content, one could come up with the idea of inoculating them with mycorrhizal fungi. But what's the point? As recently as mid-2024, a research team from the Universities of Zurich and Basel, Agroscope and the Research Institute of Organic Agriculture (FIBL) conducted a trial in which mycorrhizal fungi were incorporated into the soil of 54 maize fields in northern and eastern Switzerland before sowing. The result? On a quarter of the fields, the crop yield increased by 40 per cent. However, this was not the case on a third of the fields. There, the harvest was the same or even lower.

‘Soil inoculation works particularly well when there are many fungal pathogens in the soil,’ explains co-first author Stefanie Lutz from Agroscope. ‘Mycorrhizal fungi act like a protective shield against pathogens in the soil.’ This is why the yield increased in soils contaminated with pathogens, but not in the other soils, because the plants were already strong there anyway and grew excellently. In future, however, it will be possible to determine whether or not it is worth inoculating the field with mycorrhizal fungi by analysing the soil fungi present. However, further research is needed to do this, according to co-study leader Klaus Schäppi from the University of Basel.

Instead of fields, you can also inoculate individual plants with mycorrhiza and then transplant them. Trees and shrubs can even be planted directly in the soil. ‘This procedure has been well researched,’ says Andres Stucke from the University of Hildesheim. ‘In 60 to 80 per cent of cases, mycorrhizal fungi then settle on a plant.’ The advantages are the same as with a natural symbiosis: ‘Inoculated plants are significantly more resistant to pathogens and adapt better to drought.’ However, Magdalena Steiner from Andermatt Biogarten, a company that also produces mycorrhiza products, adds that this requires a number of prerequisites. ‘It needs a good soil structure.’ In compacted and waterlogged soils, it is rather difficult to establish a symbiosis with mycorrhizal fungi.

But how do you achieve a better soil structure? ‘The best way to improve the soil is with compost,’ says Andres Stucke. This has many advantages: ‘It is easy to produce, is inexpensive and contains many mycorrhizal spores.’ These are spread by snails, woodlice, earthworms and other soil organisms. However, Andres Stucke says that this requires a little patience. This process can be accelerated with mycorrhiza products, adds Magdalena Steiner. This is because they contain different strains of fungi. This means that plants find a suitable fungus-plant more easily. ‘This is how we give nature a helping hand.’

Ecto-mycorrhiza (ecto = outside).

• The roots penetrate the roots of the trees or plants and form an exchange network around the plant cells. About two per cent of plants have this symbiosis. These are mainly trees and shrubs.

Endo mycorrhiza (endo = inside)

• Here too, the roots penetrate the trees and plants. However, they go one step further and channel their exchange network into the cells of the plants. It is estimated that around 80 per cent of plants form such a connection, especially herbs, grasses and most crops

Evidence: Stefanie Lutz, Natascha Bodenhausen et al. Soil microbiome indicators can predict crop growth response to large-scale inoculation with arbuscular mycorrhizal fungi. Nature Microbiology, 29 November 2023.

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