Soil scientists from Cornell and Rice University have dug around and found that although adding carbon organic matter to agricultural fields is usually advantageous, it may muddle the beneficial underground communication between legume plants and microorganisms.

In a symbiotic relationship, microbes called rhizobia act like agricultural “butlers” to fetch nitrogen from the air for the legume plants. When carbon is added to the soil, it helps the soil retain nutrients, but it can repress plant-microbe communication by up to 70%, according to new research published Jan. 29 in Science Advances.

“The communication connection gets a lot of static, you might say,” said Johannes Lehmann, professor of soil and crop sciences. “With carbon amendments to the soil, the plants and the microbes cannot chemically communicate as well anymore. They can’t ‘hear’ each other.”

Lehmann is a senior author of “Soil Organic Matter Attenuates the Efficacy of Flavonoid-Based Plant-Microbe Communication.” Lead authors are former Cornell postdoctoral researcher Tara M. Webster, from Lehmann’s lab, and Ilenne Del Valle, a doctoral candidate at Rice University.

For more than a century, scientists have known about the symbiotic relationship between legumes and rhizobial microorganisms. To help the soil’s microorganisms and plants interact, flavonoids (plant and fungus metabolites) act as chemical “telephones,” but higher amounts of organic carbon – such as compost or wood chip mulch – in the soil hinder that communication.

Webster said that while adding carbon to soil does benefit legume plant growth, scientists must understand how it affects communication. The researchers were surprised to find that dissolved carbon moving through water in the soil disrupted signals.

“We weren’t expecting that,” she said. “But the takeaway here is, it’s good that we know. It helps to understand this interaction.”

Most knowledge of how microbes talk with plants comes from lab experiments in water, where it is easy to measure the chemicals the microbes and plants share, Webster said.

The legume plants use molecules of the flavonoids naringenin and luteolin to call for microbial nitrogen, but with dissolved carbon, these flavonoids were thwarted in their search for able microbes.

“We don’t always grow our plants in just water. We grow them in soil,” Webster said. “It’s a new angle to look at, but the carbon in the soil is having some interaction here. It’s not just plants and the microbes by themselves.”

Other contributors included Janice Thies, associate professor of soil microbiology; André Kessler, professor of ecology and evolutionary biology; and from Rice, Caroline Masiello, professor of biogeochemistry, and Joff Silberg, professor of synthetic biology.

Funding for the research was provided by the National Science Foundation and the Department of Energy.