Fungi Not Helpful For Sequestering Atmospheric Carbon In Soil

Published on August 31st, 2012 | by

August 31st, 2012 by


Using the fungi found in plants to store carbon in the soil has previously been suggested as a partial solution to global warming, but new research shows that this doing wouldn’t help to reduce atmospheric carbon levels. The potential increased storage in the soil is offset by the increasing decomposition rates in the soil caused by the fungi.

“The researchers found that increased carbon dioxide stimulates the growth of arbuscular mycorrhizal fungi (AMF) — a type of fungus that is often found in the roots of most land plants — which then leads to higher decomposition rates of organic materials, said Lei Cheng, post doctorate fellow in plant science, Penn State. This decomposition releases more carbon dioxide back into the air, which means that terrestrial ecosystems may have limited capacity to halt climate change by cleaning up excessive greenhouse gases, according to the researchers.”

“Prior to our study, there have been few studies on whether elevated levels of carbon dioxide would stimulate organic carbon decomposition through AMF,” said Cheng.

“To study the effect of higher levels of carbon dioxide on AMF-mediated decomposition, the researchers conducted four experiments, two in greenhouses and two in fields to mimic Earth’s expected North American atmospheric levels of carbon dioxide. They studied plots of a wild oat species, which is native to Eurasia and now common in North American grasslands, and wheat.”

“In the experiments, one plot was treated with AMF, the other did not have the fungus. Both plots were exposed to higher than currently existing carbon dioxide levels. After a ten-week gestation period, the sample of plants with AMF had 9 percent less carbon in the soil than the plot that was not treated with AMF, indicating that the carbon was released back into the atmosphere.”

“Basically, we showed that elevated carbon dioxide increases carbon allocation to AMF to increase plant nitrogen uptake, and higher AMF facilitate organic residue decomposition which releases carbon dioxide into the air,” said Cheng.

“Elevated levels of carbon dioxide did significantly increase the size of the AMF colonies and carbon allocation underground, according to the researchers, who released their findings in the Aug. 30 issue of Science. However, the storage of carbon is offset by the role of AMF in facilitating decomposition.”

“We used to think that this excess carbon would be sequestered in the soil,” said Cheng. “So, that could help mitigate climate change, but it doesn’t appear to be so.”

“They also studied the effect on a wheat and soybean field. In this experiment, Cheng said elevated levels of carbon dioxide increased both the size of AMF colonies and decomposition.”

“AMF colonies, which are found in the roots of 80 percent of land plant species, play a critical role in Earth’s carbon cycle. The fungus receives and stores carbon — a byproduct of the plant’s photosynthesis — from its host plant in its long vein-like structures. A plant stores about 20 percent of its carbon in AMF, according to Cheng.
AMF also help the plant capture nutrients, such as phosphorus and nitrogen.”

“We found that, under elevated carbon dioxide levels, AMF supply more nitrogen to their host plants by acquiring ammonium directly from decomposing residues,” Cheng said. “So the good news is that AMF’s role in the plant’s nitrogen uptake may open up the possibility of keeping carbon in the soil.”

“When there are higher carbon dioxide levels, the plant’s ability to take in nitrates is inhibited and it then adds more carbon to fungi like AMF to acquire ammonium, said Cheng. The management of soil nitrogen transformations may provide a promising strategy of restoring levels of carbon sequestration under higher carbon dioxide conditions.”

“Cheng worked with Fitzgerald L. Booker, plant physiologist and professor of crop science, and Kent O. Burkey, plant physiologist and professor of crop science and botany, both of North Carolina State University and the U.S. Department of Agriculture-Agricultural Research Service; Thomas W. Rufty, Bayer Distinguished Professor, department of crop science, Shuijin Hu, associate professor of plant pathology, H. David Shew, professor of plant pathology, and Cong Tu, research specialist, all of North Carolina State University, and Lishi Zhou, department of plant pathology, North Carolina State University and State Key Laboratory of Vegetation and Environmental Change, Institute of Botany, Chinese Academy of Science.”

Source: Penn State

Image Credits: Yoshihiro Kobae

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  • Walt Brooks

    In undisturbed soils the carbon content of the soil is much higher than is typically found modern farm fields. In forests that are undisturbed there is an ever increasing storage of carbon not only from growing trees but in the trunks of fallen trees. In addition, the undisturbed soils have large amounts of roots and fungal threads that extend huge distances. I find it hard to believe that this report is correct in its conclusions. However, one of the major contributions that no one wants to notice is the methane and carbon dioxide that 7 billion human beings contribute directly even without our burning of carbon for fuel. I tend to believe that the real cause of global warming is the completely out of control growth of the human population with all its needs and complex financial systems.

  • Simon Eissen

    “The potential increased storage in the soil is offset by the increasing decomposition rates in the soil caused by the fungi.”

    My response is “Duh!”

    Any attempt to decrease atmospheric carbon by enhancing or accelerating the growth of bio-mass and increasing the concurrent absorption of CO2 by bio systems can only be a temporary short term measure since all living things eventually die and decompose, returning the carbon to the atmosphere as methane which degrades (oxidizes) to produce CO, CO2 and water vapor.

    We can increase the amount of the planets surface area that is forest but once we return all the worlds forests to the level of, say, a hundred years ago, then what? Fill the oceans with green algae?

    Increasing the planet’s total bio-mass will absorb atmospheric carbon as long as the bio-mass keeps increasing in volume.

    But when the growth of bio-mass begins to approach it’s limit it will level off regardless of what stimulus we can provide.

    And if the amount of bio-mass is not sustained at that level, if it begins to decline, it would become a significant contributor to global atmospheric carbon, if not the greatest contributor, perhaps dooming the planet to a runaway ‘greenhouse’ effect.