There has been a general consensus that plant life such as fungi and other biomass would be a key asset in preventing and mitigating climate change effects by storing additional carbon in the soil. However, an international team of plant biologists from Penn State say that may not be the case. Their research has found that increased carbon dioxide stimulates the growth of arbuscular mycorrhizal fungi, a type of fungus that is often found in the roots of most land plants, which in turn leads to higher decomposition rates of organic materials.
Leading the team was Lei Cheng, a post doctorate fellow in plant science, who noted that the decomposition of the fungus released more carbon dioxide back into the air. The team concluded that terrestrial ecosystems, which were previously thought to be a bastion against climate change, may have limited capacity to absorb excessive greenhouse gases.
During the team’s studies, they looked at the effect of higher levels of carbon dioxide on AMF-mediated decomposition. Mimicking the 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 as well as wheat. In the experiments, one plot was treated with AMF and 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, thus 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. “We used to think that this excess carbon would be sequestered in the soil, so that could help mitigate climate change, but it doesn’t appear to be so.”
However there is a silver lining. 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, Cheng noted. The management of soil nitrogen transformations may provide a promising strategy of restoring levels of carbon sequestration under higher carbon dioxide conditions.
“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.”