I had the pleasure of listening to a seminar given by Dr. Hafiz Maherali, Director of the University of Guelph Phytotron, about the latest mycorrhizal fungi (mycorrhizae) news. This post summarizes some of this fascinating new information about this important group of organisms that every gardener should understand.
Introduction to Mycorrhizae
These fungi form symbiotic relations with about 95% of the plants on earth. The plant provides the fungi with sugars and other carbohydrates while the fungi supplies the plant with nutrients. There are two basic types: AM and EM.
AM fungi (arbuscular mycorrhizae) make up the largest group and form bonds with the majority of plants. They provide mostly phosphorus to the plant. These are obligate symbiotic fungi, which means they can not survive for long without a host, although they can produce spores (a types of seed) that can survive until a host root shows up.
EM fungi (ectomycorrhiza) form associations with only about 2% of plants, mostly the woody plants like trees and shrubs. Since woody plants make up a large segment of the plant population, EM are more important than they might seem. Interestingly, EM provides both phosphorus and nitrogen to plants and it is believed that in some cases close to 100% of a plants nitrogen comes from fungi.
Plants Initiate Germination of Fungal Spores
Not all plants are able to form associations with fungi. Members of the mustard family do not possess the gene needed to attract fungi to its roots. In effect, the fungi is blind to this type of root and simple ignores them.
When spores are placed near plants that form mycorrhizal associations, a compound produced by the plant root stimulates them to germinate. This is then followed by a joining of the two organisms. If the same spore is not near a plant root or is only near roots of the mustard family, it never receives the chemical signal from the plant and does not germinate.
AM Fungi and Phosphate Levels
One of the main benefits of the symbiotic relationship is that the fungi are more efficient at extracting phosphorus from soil than plant roots, and phosphate is normally low in soil. The diagram below is from a study that looked at root growth in two annual seedlings (only black medick is pictured) to see the effect of low and high phosphorus fertilization. This was a pot culture done using soil with low natural P levels.
Inoculation increased plant growth at low P levels which is expected since the fungi make it easier for plants to get phosphorus. Inoculation at high P levels had no effect on growth. The high P level allows the plant to have easy access to nutrients and help from the fungi is not needed. The data for Medicago truncatula (barrelclover) is similar.
This study also examined root growth. It was expected that the presence of AM would change the root structure to better make use of the symbiont, but such change was not detected. Root structures looked the same with and without the inoculant.
If you look at the growth without the inoculant, you see that adding extra phosphorus increased plant growth. You might incorrectly conclude that extra phosphorus stimulates plant growth and that transplant fertilizer works. The experiment was set up so that nitrogen levels were at adequate levels for good plant growth. Without the nitrogen present, adding extra phosphorus does not stimulate plant growth. However, It does increase root growth in situations where only phosphorus is deficient, like the above low P case.
Does Tilling Harm Fungi?
Gardeners believe that tilling harms soil microbes, but this is not entirely true. Bacteria are so small that they aren’t harmed at all. They are just moved around.
What about fungi? They exist as long filaments of mycelium. Surely they get chopped up? Tilling does disrupt mycorrhizal networks but the effects are not strong because the networks are able to re-form. Hyphal fragments can colonize roots and regrow. Dr. Maherali explained “a common method of propagation is to culture AM fungi using a host plant in a ‘trap culture’, and then to use the cut-up root fragments to inoculate other plants”. Root fragments containing pieces of fungi are called propagules and they are counted as living fungi on commercial product labels.
Tilling also breaks up the soil structure which can inhibit the rate at which a network is able to re-establish in the short term. The type and intensity of tilling will also have an effect. A single tilling event such as hoeing in the spring or even rototilling once will have limited long-term consequences.
Long Term Effect of Tilling
What about long term effect? Some AM fungi are more sensitive to repeated disturbance and their numbers are expected to decline with repeated tilling. Other AM fungi are more tolerant and their numbers are expected to increase at the same time. What makes them more or less sensitive? “Disturbance sensitive species are those that are slow to germinate and slow to colonize plants, which reduces their ability to complete a life cycle between disturbances”. Species that colonize roots faster, grow faster and produce spores more quickly are less affected by tilling.
Dr. Maherali says, “there is also some evidence suggesting that disturbance tolerant AM fungi are less mutualistic ( i.e. connect less to plants) than disturbance intolerant AM fungi, which could cause the fungal community in tilled soils to be less beneficial to plants. There aren’t many studies that have investigated these hypotheses, so we are not close to a consensus yet.”
It is clear that the populations will change over time with tilling.
No-till and Cover Crops
What is the effect of less tillage and cover crops on AM fungal populations? A meta-study looked at 54 field studies across five continents to find the answer.
- Less intensive tillage and winter cover cropping increased AM colonization of summer crops by ∼30%. The effect depends on the type of tilling and the type of cover crop with no-till and legumes being best.
- Low intensity tilling increased species diversity by 11% over conventional tillage.
Commercial Fungi Inoculants
There has been a real surge in commercial products and the manufacturers make all kinds of claims related to better plant growth. I recently did an investigation and contacted 5 brands. I asked them to provide evidence of the claims they made for their fungal products. Not one of them had supporting evidence to show their mycorrhizal products worked.
A recent study looked at 6 commercial AM fungal products to assess their effect on 9 grassland species. The products did not increase plant growth and a few innoculants decreased the growth of native species while at the same time increasing the growth of weed species. Two of the products contained excessive levels of phosphorus or nitrogen which probably accounts for the observed reduction in AM fungal root colonization.
Another study looked at 28 commercial AM inoculants in non-sterile soil and found no increase in AM fungal root colonization. Only one inoculant increased plant growth. Even more disturbing, when tested in sterile soil, 84% of the AM inoculants did not lead to AM fungal root colonization, indicating a lack of viable propagules in these products. Which means you are not even getting live fungi when you buy the product.
AM fungal inoculants are used in grape production but positive growth responses are seen mostly in the greenhouse. When inoculated plants are moved into the field there is no difference between those that were inoculated and those that were not.
Use in Agriculture
AM inoculants have been studied quite a bit in agricultural settings. A metanalysis of 168 global studies of both field and lab use for cereal crops is presented below. The horizontal line marked with a zero indicates no response. Dots above the zero are positive results and those below zero are negative responses. Lab results consistently show better results than field trials. Responses are crop specific with barely showing no positive effects. Overall, “AM fungi increased grain yields by 16% based on field studies”. Effects were lower in alkaline soil.
AM effects were lower for crops released after 1950 compared with those released before 1900. This same observation has been seen in other types of crops and may be due to the fact that breeding efforts are selecting for plants that rely more on fertilizer and less on mycorrhizal fungi.
The researchers of this study did point out a concern with using non-native inoculants. Non-native species are likely to compete with native species and may result in a reduction of native fungal populations.
What Came First, Fungi or Plants?
This is the old chicken and egg question, which came first on land? In those early days the ground was made up of rocks with no soil and plants can’t live on rocks. However, fungi can digest rocks and get nutrients out of them. It is believed that fungi were land-based first and slowly plants were able to leave their water environments because they formed symbiotic relationships with fungi. Plants were able to use the sun for energy and the fungi played the role of plant roots.
Over time soil was created and plants developed roots to be able to extract nutrients directly. In time, some plant species mutated and lost the ability to connect with fungi all together.
Do Gardening Techniques Improve Soil Fungi?
Should gardeners change the way they garden in an effort to improve fungi in the soil?
There is very little testing of fungal growth in garden situations but we can look at agricultural results. Unfortunately, even there scientists can’t agree on the value of modified practices. Some feel that the current evidence is positive enough to warrant changing agricultural practices to enhance fungal populations while others feel that there is “little evidence that farmers should consider abundance or diversity of arbuscular mycorrhizal fungi when managing crops”.
Although gardeners talk a lot about supporting soil life and compare different methods to see which improves soil health the most, the science is still being debated. What should you do? Follow sound gardening advice but don’t get sucked into using specialized techniques and concoctions. The expounded virtues of these are highly overstated with limited scientific backing.
Here are some other articles that will give you reliable information for building soil health.