Soil ecologist investigates the role of plant roots in regulating carbon cycling and reducing global warming

Soil ecologist Weixin Cheng is at the leading edge of scientific efforts to quantify the impacts of plant roots on the cycling of carbon between the atmosphere, where carbon dioxide contributes to global warming, and terrestrial ecosystems, where large amounts of carbon are stored in soil organic matter.

Cheng, an associate professor of environmental studies at the University of California, Santa Cruz, studies complex above- and below-ground systems to gather data on the movement of carbon in terrestrial ecosystems. He will present his findings this week at the Fall Meeting of the American Geophysical Union in San Francisco (session B51E, talk #2, on Friday, December 9, at 8:15 a.m.).

Plant roots contribute as much as 50 percent of the total carbon dioxide produced by terrestrial ecosystems, and they also play a major role in adding organic carbon to soil. But many studies of soil carbon have measured it in incubated soil samples that lack roots and vegetation.

"We've been oversimplifying the ecosystem," said Cheng, who has developed new techniques of measuring belowground carbon cycles.

In the greenhouse, Cheng is evaluating numerous plants, including soybeans, corn, wheat, Ponderosa pine, cottonwood, and amaranths.

By focusing on the rhizosphere, the soil that surrounds the roots of a plant, Cheng has documented changes in the rate of soil organic matter decomposition that vary from 70 percent suppression to 300 percent stimulation, a range that underscores Cheng's assertion that soil processes cannot be understood under plantless conditions.

Cheng isolates the impacts of different plant species by controlling atmospheric carbon dioxide and using soil samples with varied "signatures" based on carbon-13 isotope levels. The variation in soils allows Cheng to mix-and-match plant-soil combinations in the greenhouse and isolate preexisting soil carbon from carbon produced by plant roots. In addition, he uses a small, specialized videocamera to document belowground root changes over time.

"These methods generate a whole array of new questions," he said. "How do different plant species differ? Do different soils respond to different plants in different ways? It makes the research much more difficult."

The answers, however, can be invaluable to land managers developing strategies to store more carbon in the soil to counteract global warming.


Editor's Note: You may contact Cheng at (831) 459-5317 or

More information about Weixin Cheng's work is available on his web site at An overview of his rhizosphere work is available online at