Cover crops and the need to leave residue in the field throughout the season may get even more important in farm management over the next several years as researchers are learning both provide nourishment for soil microbes — and those microbes are actually far more important to building soil organic matter than previously thought.

In fact, university laboratory research shows soil microbes can produce stable carbon compounds very similar to those recognized as soil organic matter when fed nothing but table sugar.

Cynthia Kallenbach, an assistant professor of Soil Biogeochemistry at Montreal’s McGill University in Canada, says her work in 2015 as a graduate student at the University of New Hampshire showed microbial pathways — not plants or plant material — are the prime originator of organic matter found in stable soil carbon pools.

Matt Wallenstein, head of Colorado State University Soil and Crop Sciences Department says Kallenbach’s work proved what many researchers have thought for the past several years, and represents a significant change in the way scientists are looking at farm field organic matter.

“Dr. Kallenbach’s work was the first validation that microbes could take a simple carbon compound and transform it into complex molecules,” he explains. “We used to think a chemical process formed humus from plant-based carbon found in residues, but we’re finding plant material is actually food for the microbes, and evidence such as the New Hampshire studies suggest the most persistent soil organic matter is mostly comprised of necromass — the residue of dead microbes.”

Similarly, researchers in Sweden have found most of the carbon sequestered in northern boreal forests — an area of significant naturally stable carbon compounds in the soil of undisturbed forest floors — comes about because of fungi that live on and in tree roots, rather than from dead pine needles, moss and leaf matter.

Traditionally, the researchers reasoned carbon carried to the needles and leaves was redeposited on the surface when leaves and needles dropped to the forest floor where, over time it would decompose and leach into the soil. Under that scenario the scientists expected to find the newest carbon deposits near the soil surface. Instead, they found the newer deposits were more likely to be found deeper, near the roots, where much of the tree’s carbon was deposited in root exudates which were rapidly sequestered by mycorrhizal fungi that consumes the sugar and expels residues — carbon – into the soil.

Cover Crop Connection

Mycorrhizal fungi have been shown to provide the networks for nutrient and water conduction in networks formed by the fungi in fields and forest floors left undisturbed by the plow. The networks have been proven to provide naturally-occurring nitrogen from legumes in remote parts of farm fields to growing cash crops in other areas, along with moisture and other nutrients.

Cover crops, which can provide a nearly year-round set of growing roots — and exudates — to farm fields, help maintain the fungal networks and a rich diversity of soil microbial populations, all of which can add up to more efficient use of nutrients and ultimately, fewer inputs needed by growers.

How Do We Manage Microbes?

Wallenstein says the average farm field contains a wide variety of microbial DNA and rather than asking, “What bugs can we add to our fields,” growers should be asking “What can we do to enhance the work our microbes could be doing for us?”

The Colorado researcher and educator, who holds patents for isolated, use-specific microbes which enhance phosphorus availability in the soil, says so-called “designer microbes” are on the horizon for a number of agricultural applications, but explains growing a diversity of crops will maintain a healthy population of widely-adapted microbes eager to enhance agricultural crop production.

In many cases, the crops or cover crops grown determine what microbes are the most active in a field.

“We know no-till management and cover crops both enhance the ability of the soil to retain moisture and are widely adopted for benefitting soil health,” he explains. “Such practices also get more plant-based carbon in the ground and that is likely to help increase the activity and populations of beneficial microbes.”

Kallenbach, who took her laboratory UNH graduate studies to test fields in Michigan in 2015-16, says when compared with microbial activity in conventionally-farmed fields, the microbes in organic fields showed significantly higher carbon-use efficiency and growth rates.

“We found the organic treatment accumulated stable soil carbon even though we added no fertilizer and practiced greater tillage for weed control,” she explains. “This shows farming practices that enhance more efficient microbial populations may be an effective strategy for building soil organic matter,” she explains.

Currently, a number of researchers in the U.S. and Canada are studying various interactions of grower inputs and how those can change microbial communities in the root zone.

January 2019 Issue Contents