AMF fungi boost plant, soil health

Their name is not easy to pronounce and it may take some time to understand what they are, but South Dakota and North Dakota farmers are likely to be seeking more information about arbuscular mycorrhizal fungi, or AMF, in the future.

AMF fungi boost plant, soil health

Their name is not easy to pronounce and it may take some time to understand what they are, but South Dakota and North Dakota farmers are likely to be seeking more information about arbuscular mycorrhizal fungi, or AMF, in the future.

Research microbiologists at the USDA North Central Agricultural Research Laboratory in Brookings, S.D., are finding more evidence that these microscopic soil dwellers are key to both healthy soil and robust plants.

“Myco” means fungus and “rhizae” means root. Mycorrhizae are beneficial fungi growing in soil where plant roots are present. They survive by gathering nutrients and moisture, bringing them to plants , and then and taking sugars from plants. Mycorrhizae act as extensions to a plant’s root system, more than doubling the amount of soil area available to the plant’s root system. When viewed under a microscope, mycorrhizae resemble tiny trees.

Key Points

Scientists are discovering the role beneficial fungi play in plant health.

Fungi colonize roots and transfer water and nutrients to plants.

No-till, crop covers and other practices promote fungi networks.

“There are many types of mycorrhizae,” says Wendy Taheri, a USDA microbiologist. “AMF are most important to farmers because they’re associated with pretty much all flowering plants.”

Once plants germinate, AMF spores germinate, too, colonizing the plant root, and developing a network of hyphae. AMF take up nutrients and moisture and give them directly to the plant.

“Because they’re so tiny, AMF can get into soil pores that are too small for plant roots,” Taheri says.

“That increases the amount of nutrients and moisture available to the plant. They also occupy plant root cells, which means the cells can’t be occupied by something that causes trouble, like pests, because they’re already taken up.”

One of the greatest benefits of AMF is that they bring phosphorus to plants early in their growth stage. Phosphorus is used to build cell walls and DNA.

Taheri says heavy chemical fertilization causes plants to provide less carbon to AMF, reducing their presence in the soil.

“Chemical fertilization upsets the natural balance. In a natural setting, there’s never that much phosphorus available. In agricultural settings, heavy use of fertilizers causes plants to reject colonization and AMF begin dying out. Without them you’re trapped in a chemical cycle of fertilizer, pesticides and other high inputs," she says.

"It costs a lot of money to apply chemical fertilizer, and if you’re lucky about 20% of the P you put down might make it into your plants. As soon as the P goes into the soil, it becomes unavailable to plants because they’re designed to use microbes to uptake nutrients.”

AMF hyphae also act as an alarm for plants, signaling them if a pest, such as nematodes, disturbs the hyphae.

“But hyphae don’t just connect to one plant,” Taheri says. “They branch out into the soil and interconnect with many nearby plants. So you have large areas all connected to one another. If they send out a signal that something’s eating a plant, plants around activate their chemical defenses. That forces predators to travel further before they find a plant that’s not prepared for them.”

In monocultures of corn and soybeans, there is less AMF diversity.

“We’re selecting for the least beneficial AMF, the ones doing the least work,” Taheri says. “We’re doing their work for them. It’s been shown by another researcher that AMF in fields where we’ve been using modern farming practices for years don’t work very hard, compared to those in a more natural setting. We need to reverse that effect. Cover crops are one way to restore that diversity.”

Also, tillage disturbs AMF hyphae networks. “When we till soil, it causes erosion and breaks up the hyphae network,” Taheri says. “That’s a problem, because plants supply all the carbon to the AMF. If you destroy those hyphae networks, when spring comes and plants are colonized, the fungi tries to connect to whatever network was there last year, but it’s gone. So they begin growing a new network, increasing the carbon cost to your plant.

"No-till is a great way to benefit AMF. If you use tillage and kill some AMF species without using cover crops, you’re going to lose many AMF benefits. Over time the largest, most established networks of hyphae also take up the most nutrients and moisture.

“AMF have many positive effects,” Taheri says. “If AMF aren’t present, the numbers of beneficial soil bacteria goes down, too. We’ve found that the more AMF diversity in the field, the more benefits there are for plants. It’s a community, and some AMF are better at some things than others.”

Sorensen is from Yankton, S.D.

04141244A.tif

TINY TREE: This arbuscule (which literally means “tiny tree”) is magnified 200 times. It is the organ of nutrient exchange between plants and mychorrhizal fungi.

04141244B.tif

SOIL SPORE: These are AMF spores (at 90x magnification) taken from prairie soil.

This article published in the April, 2014 edition of DAKOTA FARMER.

All rights reserved. Copyright Farm Progress Cos. 2014.

Crop Management

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