Role of Arbuscular Mycorrhizal Soil Fungus
James J. Hoorman Assistant Professor and Extension Educator Ohio State University Extension
Arbuscular mycorrhizae (AM) fungi are a type of fungi that infect the root of the plant and help plants to survive. There are an estimated 100 species of AM fungi (Brady and Weil, 2009). AM fungus typically have highly branched tree-like structures that form within the root cells and branch out into the soil to search for plant nutrients. Most grasses and agricultural annual crops have an association with AM fungus. AM fungi and rhizobia bacteria evolved together and may share a common ancestor that has a fungal origin because plants utilize the same protein to communicate with AM fungus and rhizobia (Dick, R. Personal Communication). See Figure 1 for mycorrhizal networks.
Arbuscular Mycorrhizae (AM) Fungus Functions
AM fungi have a symbiotic (beneficial) relationship with plants. AM fungi form a mycorrhizal network with plant roots, helping the plant roots be more efficient at gathering soil nutrients, especially N and P. Fungi produce enzymes such as proteases and phosphatases that mineralize and release N and P (Dick, W. Personal Communication). These enzymes are released into the environment and then the soluble nutrients are absorbed.
Most nitrogen from fungi is released as ammonium ions (NH4+). Nitrifying bacteria will convert ammonia to nitrate (NO3-) but since the soil conditions tend to be more acidic in fungal soils, with less nitrifying bacteria, more of the nitrogen stays as ammonium. Most grass and annual crops like corn and wheat prefer nitrates while most perennial crops like alfalfa and clovers prefer ammonia. The ammonium form of nitrogen is more energy efficient for the plant because in the plant cells, nitrate needs to be converted to ammonium to produce proteins (Lowenfels and Lewis, 2009).
AM fungi enhance soybean (legume) growth and production by increasing P absorption, nodulation and N fixation. Fungi enhance and cultivate good bacteria, especially Rhizobia bacteria for nitrogen fixation, which help legume plants grow (Brady and Weil, 2009). Legumes have a taproot and are less efficient than grasses with a fibrous root system at extracting P from the soil. AM fungi make phosphorus more plant available. The AM fungus however may reduce the number of Mn2+ reducing bacteria in the rhizosphere by a factor of five times, reducing the availability of Mn2+ for soybean absorption and seed production (Sylvia et al 2005).
AM fungi produce a sticky substance call glomalin. Glomalin is an amino polysaccharide composed of sugars from the plant root and protein from the AM fungi forming a glycoprotein. In a good soil, glomalin may represent 1–5% of the total carbon in the soil and glomalin is 30% carbon, 1–2% nitrogen, and up to 5% iron, which gives it a reddish soil color (Lavelle and Spain, 2005; Sylvia, 2005). Glomalin surrounds the microaggregate soil particles and glues them together to form macroaggregate soil particles. Polysaccharides like glomalin enhance soil tilth and soil structure (Dick, R. 2009, Dick W. 2009). Mycorrhizal fungi inoculums are on the market and may be added to plant seeds at planting time to increase specific fungal populations. See Figure 2 on Mycorrhizal Production of glomalin.
Fungi increase soil structure by increasing macroaggregates in the soil. Fungal hyphae increase water infiltration and water holding capacity by forming stable macroaggregates (>250 μm). The soil particles and soil debris are physically stuck together by glomalin and other plant exudates and microbial wastes (Ingham 2009). Sylvia et al 2005 states that "there are from 1 to 20 meters of AM hyphae in each gram of soil" and there could be as much as 5 miles of AM fungi hyphae in a pound of soil". See Figure 3 on Glomalin and soil structure.
Conventional tilled soils are dominated by bacteria, which do not produce glomalin. Tillage disrupts and breaks down the macroaggregates into microaggregates and results in denser, more compacted soils, lacking soil structure. Tilling the soil decreases soil organic matter. Tilled soils are not the ideal soil habitat to main beneficial fungal populations. Fungi need well-aerated soils with large amounts of residue and cannot tolerate saturated or anaerobic (lack of oxygen) soil conditions that occur under soils that are tilled, compacted or have poor soil structure. Tillage also injects excess oxygen into the soil, stimulating bacteria populations to expand and then consume active carbon (polysaccharides and glomalin) for food (Dick R, and Dick W, Personal Communication). Fungal populations tend to decrease with increasing depth in the soil.
Benefits of Soil Mycorrhizal Fungi
Fungi perform many functions in the soil including nutrient recycling; carbon decomposition and sequestration; water conservation; increased soil aggregate stability; produce plant hormones, antibodies, and vitamins; promote plant growth; and increase disease suppression (Ingham, 2009; Sylvia et al 2005). Mycorrhizal fungi increase the efficiency in plant and root extraction of nutrients leading to increased growth and production in nutrient poor soils and have the ability to make infertile soils more fertile (Brady and Weil, 2009, Sylvia et al., 2005).
In no-till agriculture, fungal populations dominate the soil food web (although they are less in number than the bacteria) and improve carbon sequestration. Fungi have 40–55% carbon use efficiency so they store and recycle more carbon (C) compared to bacteria. Bacteria are less efficient at retaining C and release more carbon dioxide into the air. Fungi have higher C content (10:1 C:N ratio) and less nitrogen (N=10%) in their cells than bacteria" (Islam, Personal Communication).
Due to their smaller size and much greater surface area, fungi can efficiently scavenge for nitrogen (N), phosphorus (P), calcium, magnesium, copper, iron, zinc and water (Lowenfels and Lewis, 2006; Brady and Weil, 2009) increasing plant root nutrient extraction efficiency. Fungi share nutrients with bacteria and excrete enzymes in the soil to release soluble nutrients. For example, fungi oxidize tightly bound P to make it more available for microbial uptake. Iron and manganese are also oxidized by fungi. Fungi, unlike most bacteria, can also release N from dead microbial biomass and from insects because they have the enzymes necessary to breakdown proteins, cell walls, and chitin in insects (Sylvia et al, 2005). Once the fungus and bacteria absorb the soluble nutrients, they become like living bags of fertilizer that becomes available when they die. When the fungi hyphae die, they leave microscopic tunnels 10 μm in size throughout the soil and these tunnels serve as a haven for bacteria from many bacteria predators and the tunnels allow for the movement of air and water through the soil profile (Lowenfels and Lewis, 2006).
During a drought, fungi grow when the bacteria do not. Fungi supply moisture to the plant roots by crossing cracking dry soils to obtain water not available to plant hair roots. Fungi also supply nitrogen to plants in dry soil, by accumulating soil nitrogen to break down hard to decompose residues low in nitrogen (Ingham, 2009). Mycorrhizal networks explore up to 20% of the soil volume due to their smaller size compared to only 1% of the soil volume for a typical plant root hair. These mycorrhizal networks even connect one plant to other plants, sharing and transferring nutrients among plants (Sylvia et al., 2005, Brady and Weil, 2009). See Figure 1 on Mycorrhizal Networks.
Fungal hyphae filaments translocate and store deficient nutrients to distant parts of the soil where nutrients may be lacking, allowing reproduction and growth to continue. The plant supplies simple sugars to the fungi while the fungi supply N, P, other nutrients and water to the plant. As much as 25 percent of the plant root carbohydrates are directly exuded into the soil to feed the microbes or 5 to 10% of a plants total photosynthetic production to mycorrhizal fungal networks (Kuzyakov, 2002; Brady and Weil, 1999).
There are several factors that decrease AM fungi in the soil. When excess nutrients like N and P are supplied by commercial fertilizer to plant roots, the AM fungi stop working. Tillage also decreases the effectiveness of the AM fungi by destroying the mycorrhizal network associated with plant roots (Dick, R, Personal Communication).
Mycorrhizal fungi may be harmed by many fungicides in the market place. So excess commercial fertilizer, tillage, soil compaction, pesticides (fungicides and fumigants that contain neonictinoides like Poncho, Cruiser, and Goucho or benzimidazoles like Benlate), short crop rotations, and long fallow periods tend to decrease fungal populations. Cereal crops and grass crops had three times higher density and length of fungi hyphae than land that was fallow (Lavelle and Spain, 2005; Lowenfels and Lewis, 2006; Sylvia et al, 2005).
Most soil fungi decompose recalcitrant organic residues high in cellulose and lignin. Fungi carbon use efficiency is about 40–55% so they store and recycle more C (10:1 C:N ratio) and less N (10%) in their cells than bacteria. Fungi are more specialized but need a constant food source and grow better under No-Till conditions. Arbuscular mycorrhizal (AM) fungi produce an amino polysaccharide called glomalin. Glomalin surrounds the soil particles and glues macroaggregate soil particles together and gives soil its structure. AM fungus store and recycle soil N and P and generally have a symbiotic relationship with most plants, greatly increasing the N and P extraction efficiency and improving soil structure and water retention.
This fact sheet was produced in conjunction with the Midwest Cover Crops Council (MCCC).
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