By Dr. Wendy Taheri
Today we are going to examine the role arbuscular mycorrhizal (AM) fungi play in providing phosphorus and other benefits to plants and the impact of chemical fertilizers on plant-fungi symbiosis.
You often hear about the P-cycle (phosphorus) or the N-cycle (nitrogen) and have probably seen all sorts of graphics demonstrating how these nutrients move in the environment. I am going to focus on phosphorus (P) because it is generally the most limiting nutrient for plants. The reality however is that these cycles are interlinked and altering one cycle will impact other things. All plants need three basic macro-nutrients: N, P and K (nitrogen, phosphorus and potassium).
When you buy fertilizer those are the elements you look for. However when you saturate your agriculture system with these nutrients, carbon becomes the limiting factor, not to plants, but to the microbes in the soil that can serve as storage devices for nutrients, keeping them from leaching out of the system. Increases in P also lead to increases in utilization of other nutrients that are necessary for plant growth.
Because P is a highly reactive element it can occur as a constituent of many different kinds of molecules. This makes it very difficult to accurately measure, because testing methods tend to focus on a specific source of P, bound in a specific substrate. The two most commonly used methods for testing P in soil are the Bray and Olsen methods. The Bray method measures acid soluble forms of P bound mostly to aluminum and iron in the soil.
The Olsen test is used for more alkaline soils that typically contain a lot of calcium-bound phosphates. This method frees P from calcium carbonate and iron oxides and then measures the amount of P in solution.
Both the Bray and Olsen P tests target inorganic P (Pi). This is because plants can uptake Pi directly from the soil when it is dissolved in water; hence the effectiveness of phosphate fertilizers. Get enough P into solution at the right time and your plants get bigger. Pi, however, has a fleeting life in soil. It is quickly bound to other things, runs off, or leaches away. It leaves the system and exits the natural cycles that keep the phosphorus where you want it, which is within your field. If you run these tests on soil from natural ecosystems you will discover that there is very little inorganic phosphorus available. Yet these systems produce more plant biomass than our carefully cultivated and heavily fed cropping systems. How is this possible?
P is hoarded in natural ecosystems by living organisms. It’s a limiting nutrient so nearly every little bit is scavenged and incorporated into living cells. This system is highly efficient and leaves very little Pi lying around waiting to wash away and get lost from the system. This is the organic side of the equation, and our current testing methods tend to ignore organic phosphorus (Po). Why? Because plants can’t uptake Po directly. Organisms in the soil however, can make Po available to plants.
As those billions of microbes in a handful of soil go through their life cycles living and dying, they are providing a constant source of nutrients. Some bacteria mineralize Po, turning it into Pi, which makes it directly available to plants. Arbuscular mycorrhizal fungi are capable of passing both organic and inorganic forms of phosphate to plants. One of the huge differences between our cropland soils and natural ecosystems is the abundance of arbuscular mycorrhizal fungi. When I extract spores from cropland soil, most of what I see is literally black, dead or unhealthy looking spores. The extractions are starkly different than those from natural ecosystems, which are full of plump, brightly colored, healthy looking spores.
We found that 50 ml of soil (slightly less than 1/4 cup) from a remnant of native prairie contained over 1,300 AM fungal spores, while the same amount of cropland soil, from the same area, averaged fewer than 200 AM fungal spores (Figure1). Furthermore, non-AM fungal spores found in the extractions were the dominant fungi in agricultural soils, while they were a very small percentage of the spores found in native prairie soil (Figure 2). In agricultural soils there were nearly three times more non-AM fungal spores than there were AM fungal spores, while in native prairie soil AM fungal spore production outnumbered non-AM fungal spores by nearly five to one. This is a very important difference because non-AM fungi may compete with AM fungi for resources, while providing little or no benefit to crops.
What are we doing that is killing them?
Probably, the largest contributor to this situation is the application of inorganic phosphate fertilizers. When we saturate the soil with plant-available phosphate, plants reject their symbiotic partners. Under natural conditions, plants pay these fungi for phosphorus. They exchange sugars, which they can efficiently acquire through photosynthesis, to the AM fungi for harder to get phosphorus. Both organisms are specialized in this regard. When we provide a ton of phosphate, the plant rejects colonization. AM fungi are obligate symbionts. This means they cannot survive and reproduce without a host.
Deny them a host for very long and they soon begin to die. Eventually you have the situation we see in our croplands. But they are not all gone. So who are the survivors?
Those species that are the most highly infective have managed to hold on. With all that fertilizer in the soil, they don’t have to work very hard either. We are literally selecting for less beneficial microbes in our soil. But we don’t stop there. We also coat our seeds with both fungicides and other pesticides. In fact, it’s getting harder and harder to find uncoated seeds. Since most of the microorganisms in soil fall into the “good” category, this sort of indiscriminate poisoning is likely to kill more good things than bad things. If you are combating a specific problem in your field, you treat for it. Prophylactic treatments to protect seeds “just in case” are not only an invitation to resistance, but it kills things that you want living in your soil.
Let’s consider the shift in fungal populations from AM fungi to non-AM fungi. Try to imagine the root system of a plant as the streets in a gated community. The street is lined with houses. Each house represents a niche in an ecological community.
A niche is where something can live and flourish. A plant’s root system provides a home for a wide range of organisms. Now we apply a pesticide and suddenly a large percentage of those houses are empty. Nature abhors a void.
Something else is now going to move into those empty houses. Will you get a good neighbor or a bad neighbor in each of those houses? Well, that is something of a craps shoot — it just depends on what is nearby. Every time you go through a pesticide cycle, you roll the dice again.
Eventually, something you really don’t want is going to move in. This is yet another reason why pesticides should be used sparingly.
How do you beat the odds?
You choose management techniques that favor the most beneficial organisms.
Remember, every organism living in the soil is competing with every other organism for resources. If you throw the balance to favor the beneficial organisms their populations can increase. This means when chemical intervention is necessary the odds are better of getting a good neighbor after you wipe out half the neighborhood. It also means during periods when intervention is not required, what the neighborhood is full and there’s little room for something else to move in. You stack the deck in your favor. It is much easier to jump into an empty house than it is to try and evict the current resident — so fill those houses with good neighbors.
We’ve told you most of what’s in the soil falls into the good zone. However some things are better than others for your plants, and you can’t manage for everything when there are literally billions of organisms in the soil. You need to manage for the support of the most beneficial group of organisms for plants.
This group is undoubtedly the arbuscular mycorrhizal fungi. Don’t get me wrong, there are a host of bacteria and fungi out there that promote plant growth, but nothing out there has shown the diverse benefits and defense capabilities that arbuscular mycorrhizal fungi demonstrate overall. Furthermore, managing for them is unlikely to seriously threaten other beneficial organisms, unlike chemical applications.
So let’s talk about these mysterious organisms so few people have ever seen and take a realistic look at what they can do for plants.
Arbuscular Mycorrhizal Fungi
AM fungi are plant symbionts. Symbiotic organisms are generally interdependent upon one another. In the case of AM fungi and plants, it is an obligate relationship for the fungi. The degree of plant dependency upon these organisms varies widely and some plants are unable to live without them, while others are completely non-mycorrhizal and do not act as hosts for AM fungi. It is estimated that 80-90 percent of all flowering plants are mycorrhizal. Most of our crops fall into this group.
The mycorrhizal symbiosis is the oldest symbiosis known to science. Fossil evidence suggests plants first colonized land with the help of AM fungi. That means this intimate relationship has been evolving for over 400 million years. It is a biologically complex relationship that we don’t fully understand yet.
It begins when a seed germinates. The young plant produces hormones called strigolactones that attract AM fungi to it. The plant wants to be colonized. In fact, it prepares channels for the AM fungi that run between the cells in its roots. The fungi penetrate plant roots with a filament called a hyphae, and grow through these channels. Along the way it occasionally penetrates individual cells where it produces a structure called an arbuscule. The plant grows a special membrane to surround the arbuscule.
The interface between these structures is where nutrients and minerals are exchanged between plant and AM fungi. Once colonization has occurred, the fungus sends its hyphae into the soil. They are finer than the smallest roots and can penetrate pores in the soil that roots are too large to access, collecting water and nutrients which would otherwise be unavailable to the plant. Because the hyphae grow out from the plant’s roots, extending beyond them into the soil, they also extend the total volume of soil available to the plant for nutrient uptake.
Benefits of AM Fungi
First and foremost among the benefits of AM fungi is the uptake of phosphorus. Literally every species of AM fungi known provide this crucial service to plants. Phosphorus and sugar are the currency of this symbiosis. AM fungi are so specialized they can only feed on sugars obtained through their arbuscules.
If you try to grow them in a Petri dish with sugar in the agar, they germinate, go looking for a host and die when they don’t find one. They don’t start taking up sugar through their hyphae and growing independent of a host plant. This gives the plant a measure of control. It can dissolve arbuscules that are not providing phosphorus and deny the fungi carbohydrates.
However if the plant is not providing sugar, the fungi can stop providing phosphorus, or meter it out based upon how well the plant feeds it. Since both partners have control over metering an essential nutrient to the other partner, it becomes hard to cheat. This also demonstrates how saturating the system with phosphate fertilizer interferes with the relationship.
The plant suddenly no longer needs the AM fungi. And if that were the sum total of what AM fungi did, replacing them with chemical phosphate might be a good idea. However, AM fungi are complex organisms that do much more than just mine and meter phosphate.
Unfortunately for the fungi, the mechanism that regulates how plants respond to them depends on phosphate concentrations in the soil. Heavy applications of chemical phosphate destroy the symbiosis, and a host of other benefits are lost with it. These benefits include: drought tolerance, salt tolerance, improved plant nutrition, defense against pathogens including nematodes, improved soil quality and aggregation, tolerance to herbivory, increased fecundity (more seeds, fruits and flowers), reduced compaction stress, and reduced transplant shock. That is the short list, emphasizing only the benefits of particular interest to farmers and horticulturists.
Defending Your Home
If the plants are not strong and healthy they can’t afford the cost of supporting a population of AM fungi. The fungi want a healthy host — if their host dies they may also die. That plant is their home and they actively defend it against invaders. They don’t want to live in a derelict home, and they don’t want bad neighbors. Imagine the population of AM fungi populating your plant’s roots as a complex biological monitoring system. That fungus is making sure the plant has everything it needs.
It senses water stress, it provides more water. It senses a nutrient deficiency and it provides that nutrient. It senses nematodes approaching and it signals the plant to produce defensive chemicals. There is absolutely no product on the market available anywhere that can monitor every plant in your field and attempt to compensate for whatever stress factors the environment throws its way. Yet this is what AM fungi are doing for your plants every day, and we are killing them all off with our current and most common agricultural practices, the most basic of which is how we fertilize our fields. We have pushed so much Pi through our fields over the years that they are now rich in unavailable phosphorus. Remember much of the phosphate fertilizer we apply winds up bound to soil particles. Only something between 5-20 percent of the fertilizer you spent those hard earned dollars on, ever reaches your plants. However that untapped reserve of bound Pi and Po tied up in the microbial community and decomposing litter could be made available to plants through AM fungi.
It has been demonstrated on average that AM fungi can replace up to 25 percent of the phosphorus we currently utilize without a decline in yield. However this generalization is not crop specific. Many experiments utilizing AM fungi depend on only a few easy to grow species that do well under greenhouse conditions.
Furthermore, these studies are often conducted in low P soils. If you suddenly stopped applying phosphate fertilizers to your fields, with a resident population of AM fungi that are now adapted to being highly infective and “lazy” (because they haven’t had to work hard with you providing the phosphorus) you would probably have trouble maintaining your yields. Natural ecosystems don’t rely upon 75 percent of the recommended amount of P. Theoretically, we should be able to replace most if not all of our P requirements with appropriate management techniques and restoration of AM fungal communities.
Transition studies in high P soils are needed to determine how long it will take for AM fungal populations to rebound, and to measure how effective those populations are at taking up phosphorus, and maintaining or increasing yields.
Editor’s Note: This article appeared in the September 2012 issue of Acres U.S.A. magazine.
Dr. Wendy Taheri is a mycorrhizal ecologist currently employed as a research microbiologist for the USDA Agricultural Research Service at the North Central Agricultural Research Laboratory in South Dakota. If you have specific questions about arbuscular mycorrhizal fungi, or about these articles, contact her at firstname.lastname@example.org.
Learn more about mycorrhizal fungi and its impact on soil health with this article, from the May 2018 issue of Acres U.S.A. magazine.