by Laura Buenning
Take six protons, six neutrons, and six electrons and you have the means to form diamonds, one of the hardest substances on Earth, or graphite, one of the softest.
Extremely pliant, carbon’s configuration allows it to form several elemental structures (allotropes) and numerous compounds with ease. It is the core atom of all living organisms. Furthermore, life on Earth is only possible because carbon dioxide in our atmosphere protects us from heat and moisture loss.
Still, it’s a delicate balance because excess carbon dioxide can trap heat near the Earth’s surface and is thus linked to global warming. While, usually attributed to burning fossil fuels, “below-ground processes are also critical,” says JDRD team leader Aimée Classen.
“Soil properties and biological communities contribute to carbon loss and gain. For example, mycorrhizal 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.”
Recent studies, however, suggest that sometimes the fungi will cheat, stealing carbon from the soil without giving nutrients to the plant.
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,” Classen says.
But, mycorrhizae’s regulation of carbon dynamics might change from one soil type to another, Classen says. “If they degrade soil carbon, assimilate some of it into their own mass, and respire it back into the atmosphere, this challenges current carbon model assumption.”
In year-one, Ph.D. student Jessica Bryant and ecosystem modeler Wilfred M. Post set out to discover if this switch from symbiont to free-living—from carbon sink to carbon source—could significantly affect atmospheric carbon levels. Their computer simulations indicate the micorrhizae do decompose soil carbon for themselves and release it into the atmosphere when their host plants are under stress and have only a limited amount of carbon to give away. Year two follows with a greenhouse study that places plants under stress and measures the amount of carbon the mycorrhizae degrade from the soil.
The affiliated LDRD project led by ORNL’s Melanie Mayes fits tightly with the JDRD project. 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.
Ultimately both projects will use their results to enhance the land-surface component and global climate predictions of the Community Earth System Model.
Incorporating microbial dynamics that alter soil C fluxes into terrestrial C cycle models
Aimée Classen, UT Ecology and Evolutionary Biology Department
Incorporating molecular-scale mechanisms stabilizing soil organic C into terrestrial C cycle model
Melanie Mayes, ORNL Environmental Sciences Division