“The underlying mechanisms that lead to carbon sequestration in soil are unclear,” Shuijin Hu of North Carolina State University told environmentalresearchweb. “AMF under elevated carbon dioxide have been assumed to facilitate carbon stabilization through protecting organic carbon from decomposition. Our results provide, for the first time, the direct evidence that enhancement of AMF by elevated carbon dioxide in the atmosphere can stimulate decomposition of organic carbon in soil.”

Species of AMF live alongside the roots of around 80% of land plants, taking carbon from the plant – up to around one-fifth of the amount it produces – in return for providing mineral nutrients such as nitrogen. The fungi can also protect organic matter from microbial attack by promoting soil aggregation; this helps to maintain carbon storage.

Hu and colleagues from North Carolina State University, the US Department of Agriculture and the Chinese Academy of Sciences used carbon and nitrogen isotopes to investigate the effects of enhanced carbon dioxide on the wild oat species Avena fatua. They also conducted field trials of a wheat-soybean system.

“Our findings challenge the assumption that AMF protect against degradation of organic carbon in soil and raise questions about the current prediction of terrestrial ecosystem carbon balance under future climate-change scenarios,” writes the team in Science.

Increased carbon dioxide levels enhanced AMF growth but resulted in considerable soil carbon losses. The team believes that plants experiencing higher carbon dioxide concentrations may rely more heavily on ammonium ions in soil as a source of nitrogen, as their ability to absorb nitrates is inhibited. Under these conditions, plants tend to send more carbon to AMF, which are relatively good at providing ammonium ions. This increases carbon storage. But in turn, the AMF boosting may prime saprotrophs – decomposers – to work on other organic matter, ultimately resulting in a net loss of carbon from the soil.

“We found that, under elevated carbon dioxide levels, AMF supply more nitrogen to their host plants by acquiring ammonium directly from decomposing residues,” said Lei Cheng, who is now at Pennsylvania State University.

According to Hu, soil carbon sequestration is more complex than it appears because the carbon and nitrogen cycles closely interact. “Our results suggest that we need to better manage nitrogen inputs to enhance soil carbon sequestration,” he added.

Now Hu and colleagues plan to “further examine the mechanisms underlying AMF stimulation of organic carbon decomposition and assess the impact of nitrogen inputs on AMF-mediated decomposition of organic carbon”.