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December 5, 2005

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

By Jennifer McNulty

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.

Photo: Weixin Cheng

Soil ecologist Weixin Cheng
Photo: Jennifer McNulty

Cheng, an associate professor of environmental studies at UCSC, studies complex above- and below-ground systems to gather data on the movement of carbon in terrestrial ecosystems.

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 below-ground 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 below-ground 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.


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