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Aimée Classen

Carbon Bandits – Aimée Classen

by Laura Buenning

Can turncoat fungi turn into carbon bandits?

Plants and mycorrhizal fungi share an ancient symbiotic bond. Soil fungi living in and among root structures receive carbon from the plant, which they use to grow and multiply. In return, they supply their hosts with soil nutrients and moisture.

“Because they have extremely fine mycelia, the fungi can go out and explore the soil and pick up things the plant needs to grow more easily than plant roots, which are bigger,” says JDRD team leader Aimée Classen. “There’s even some indication that mycorrhiza helps plants resist pathogens.”

Screen TrayRecent studies, however, show mycorrhizal fungi will sometimes cheat the plant, stealing its carbon without giving back adequate nutrients and they even turn into soil-carbon scavengers. Classen says carbon theft seems to increase when plants are stressed, perhaps because they have less fixed carbon to allocate to the fungi.

Current carbon models group mycorrhizae with plant root systems. In this view, “the plant is like a big straw. Carbon from the atmosphere goes into the plant and down into the soil, where it stays.

“There’s debate as to whether the mycorrhizae are decomposing soil carbon for themselves or degrading it just to get nutrients for the plants.” she says. “But, if mycorrhizae degrade soil carbon, assimilate some of it into their own mass, and then respire it back into the atmosphere, this challenges that assumption,” Classen says.

Jessica Bryant, the PhD graduate student on the project, thought it would be interesting to find out if the switch from symbiont to free-living—from carbon sink to carbon source—might significantly affect atmospheric carbon levels.

She first tackled the modeling aspects of her problem. Under the tutelage of ORNL ecosystem modeling expert Mac Post, Bryant built a model based on data published in academic journals, asking, ‘If mycorrhizae do in fact degrade carbon is that important feedback for CO2 models?’

Graduate StudentsThe affiliated LDRD project led by Melanie Mayes, of ORNL’s Environmental Sciences Division, fits tightly with Classen and Bryant’s interests. Mayes has turned her interest in the physical processes in soils—such as how microbes help fix carbon to soil particles—to improving the way carbon cycling mechanisms are represented in soil productivity models, and ultimately, to enhancing the land-surface component and global climate predictions of the Community Earth System Model.

“Bryant’s modeling work suggests soil carbon degradation by mycorrhizae could be significant in plants under stress—say, from global warming, climate change, insect outbreaks, or nasty invasive pathogens,” Classen says.

The modeling completed, Bryant will test this hypothesis, growing plants and mycorrhizae in soil containing a special signature carbon isotope and then measuring to see how much carbon the mycorrhizae scavenge from the soil before and after clipping the plant’s leaves to cause stress.

“Bryant’s work is pretty novel,” Classen says. “And, the training she received from ORNL sets her apart from her peers, who generally have either modeling or experimental experience, but not both.”

JDRD project:
Incorporating microbial dynamics that alter soil C fluxes into terrestrial C cycle models
Aimée Classen, UT Department of Ecology and Evolutionary Biology

LDRD project:
Incorporating molecular-scale mechanisms stabilizing soil organic C into terrestrial C cycle models
Melanie Mayes, ORNL Environmental Sciences Division