In the soil, thousands of species of fungi are quietly playing their part in the soil ecosystem. Most soil fungi don’t produce obvious or large above-ground structures like mushrooms, which are the fruiting bodies of some fungi. Soil fungi are much less conspicuous, living almost entirely below ground as mycelial networks of small, thread-like strands called hyphae.
Many soil fungi support nutrient cycling through decomposition, improving soil structure and water retention at the same time. Between 80 to 90% of all plants form symbiotic relationships with some species of fungi, with both organisms benefiting from the association. A further 72% of flowering plants form associations with arbuscular mycorrhizal fungi (AMF), which are among the most common fungi found in agricultural systems. While AMF are best known for helping plants get soil phosphorus, that’s not all they do. These fungi and their branching hyphae provide a whole host of benefits to plants and the soil, ultimately benefiting growers too.
With expertise provided by Anne Kakouridis, Research Scientist at Sound
What Are AMF?
Arbuscular mycorrhizal fungi (AMF) exist in most soils as spores until they encounter plant roots. AMF are unable to produce their own food because they can’t fix their own carbon and since they’re not predators or decomposers, they can’t eat other organisms. Instead, AMF must consume the carbohydrates that plants create through photosynthesis in exchange for helping plants access not just phosphorus but other nutrients like nitrogen, magnesium, and potassium as well. This makes them obligate symbiotes — they are unable to grow from spores unless they encounter plant roots. As the result of a chemical dialogue between plants and AMF, the fungi are able to sense chemicals from the plant to tell when to wake up.
“Spores are sort of like eggs, and they don’t hatch without a plant.” says Anne Kakouridis, Research Scientist at Sound. Anne has a Ph.D. in environmental biology with a focus on microbiology, mycology, and plant-soil-microbe interactions.
Anne explains that inside the spore are most of the building blocks and resources needed for AMF to thrive and spread long enough to encounter a root. If a spore were to “hatch” without roots nearby to provide food, the resources and the energy needed to grow would quickly be used up and wasted.
The relationship between AMF and plants is an old one. In fact, Anne says there is evidence that AMF helped plants colonize land from the ocean approximately 450 million years ago; many old plants already contain some of the genes for this symbiosis.
“The first plants to move to land didn’t really have roots,” says Anne. “There’s a hypothesis that AMF acted much like roots for those early plants, and while plants did evolve roots, they also kept the relationship with AMF.”
AMF are made up of networks of tiny, thread-like strands of hyphae.
AMF in the Soil
Decomposed organic matter plays a foundational role in soil health, making important minerals and nutrients available to soil organisms and plants, improving the physical properties of the soil including water infiltration and soil aeration and increasing the soil’s CEC, or its nutrient holding capacity. Many fungi, including other mycorrhizal fungi, create enzymes outside of their bodies to help with decomposition.
“It’s kind of like a stomach outside of their bodies,” says Anne.
AMF, however, do very little decomposition themselves. Instead, they pass on a portion of the sugars they get from the plant to other microbes or bacteria, outsourcing decomposition functions and acquiring solubilized phosphorus or fixed nitrogen in exchange.
“The bacteria do the actual job of decomposition and then the AMF can just pick up the resulting nutrients and transport them back to the plant,” explains Anne. “I think of them as managers — they assign all the tasks and pick up things when they’re done.”
AMF are obligate symbiotes—they are unable to grow from spores unless they encounter plant roots.
Plant roots are already incredibly efficient at picking up available nutrients in the rhizosphere; so efficient in fact, that sometimes they can actually create a depletion zone. The primary role of AMF is to increase the roots’ surface area; they grow much further than a plant’s roots alone, effectively extending the reach of the root system and where nutrients and water can be accessed.
“AMF act like an extension of the root system, except smaller,” says Anne. “They’re about the diameter of a root hair, but much longer: while a root hair may be only a few millimeters long, AMF hyphae can reach up to 20 centimeters. This allows them to get into very small pores in the soil that roots may not be able to access.”
Because of this reach, AMF aid the interactions between bacteria and plants within the soil. Soil is often heterogeneous, with micro pockets of air and areas of higher or lower bacterial activity. Water in particular isn’t distributed evenly throughout the soil, which can present a problem for bacteria that travel through water.
“AMF hyphae provide a surface that can act like little waterslides and bridge these gaps in the soil,” says Anne. She and several colleagues actually conducted an experiment, published in New Phytologist, demonstrating AMF ability to act as extensions of the roots to increase access to water and nutrients, even across small air gaps in the soil.
In addition to being water and nutrition couriers, AMF offer some protection against certain pathogens and help plants deal with stresses like heavy metals or excess salt. AMF hyphae also benefit soil structure, helping form and hold together soil aggregates, which are hugely important for soil and plant health.
“The physical structures of the soil are better, but the aggregates also act like little sponges, storing more carbon, water and nutrients,” explains Anne. “It even helps with soil carbon storage.”
Through photosynthesis, plants create carbohydrates to fuel their own biologic processes and that soil microbes need to survive.
Carbon Storage and Sequestration
On a global scale, soil contains three times as much carbon as either the atmosphere or plants. Soil carbon comes from plant matter, roots and organisms in the soil. Anne explains that
“There are different pools of carbon in the soil where carbon is more or less protected from biochemical and microbial degradation, so it can persist for shorter or longer periods before returning to the atmosphere,” Anne explains. “One pool of carbon, often called ‘light carbon,’ is plant matter and organisms that still mostly look like the plants and organisms, as if they just died, That’s usually being cycled through pretty fast.”
Some of the light carbon moves into the longer lasting protected carbon pools, though. This carbon is within soil aggregates where it’s physically protected from degradation; microbes aren’t able to access it as easily, eat it and then respire it as carbon dioxide.
“Carbon in the protected pools also has chemical interactions with minerals that make it more stable,” says Anne. “As carbon is transformed by microbial processes, you can’t tell where it came from anymore. In addition to the physical changes, it also has a different chemistry that helps it retain water and nutrients, both of which are very important to plants.”
In the root zone, AMF transport nutrients and water to crops in exchange for some of the sugars the plant makes.
Supporting AMF Growth
While most soils have some AMF spores, Anne says there are things growers can do to encourage the growth of AMF and their symbiotic relationship with plants. Cover cropping can support AMF growth, as can reducing tillage, which disrupts the fungal networks throughout the soil. It’s especially important to avoid excess fertilizer use, however.
“Whether the plant decides to keep associating with AMF seems to mostly be based on phosphorus, even though the fungi provide many benefits beyond phosphorus transport,” says Anne.
When plants get enough phosphorus through synthetic fertilizer, for example, plants stop signaling to the AMF. Without the sugars from the plant, AMF can’t survive, and the symbiotic relationship between plant and fungi ends. This may not matter much in the short term, but in the long term, it can actually be detrimental.
“It seems that when excess phosphorus is added to the soil, the plants stop associating with the AMF,” says Anne. “But the plants actually suffer in the longer term, even though they have enough nutrition right now: they don’t get the benefits of improved soil, extra water or protection from stresses like heavy metals or salt.”
AMF increase the roots’ surface area, extending the reach of the root system and where nutrients and water can be accessed.
Without a soil environment where AMF can thrive, growers will struggle to reap the benefits of their symbiotic relationship with plants. Some growers may want to add AMF to their soils, and while there may be some benefit to that, it’s no substitute for maintaining an AMF-friendly soil environment.
“It can be beneficial in some soils to add spores, but if you don’t create a hospitable environment, they won’t really have any effect,” says Anne. “If you’re going to add AMF to your soil, you still need to watch how much phosphorus you’re applying and how much you’re tilling. Just adding a bunch of AMF isn’t going to help.”
What growers are maintaining is really the communication between AMF and the plants, which is based on chemical signals sent out through plant roots as exudates. This means creating a healthy environment for the plants that will encourage them to signal microbes like AMF and a hospitable soil habitat where the microbes can thrive. Then, the plants and the fungi should take it from there.
“I’m a big fan of AMF,” says Anne. “I love to talk about them and all the good things they do; I truly believe that overall AMF are incredibly beneficial.”