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Soil fungi may not be effective carbon sink

Carbon balance Soil fungi may not be as effective at storing carbon and mitigating the effects of climate change as some scientists had hoped.

That's the conclusion drawn by US researchers who have studied the effect of rising carbon dioxide levels on the fungi that form mutually beneficial relationships with the roots of most plant species.

Many experts have assumed that arbuscular mycorrhizal fungi will play a major part in storing carbon in the soil as CO2 levels rise.

But the new study by Shuijin Hu and colleagues from North Carolina State University questions these assumptions.

Their study is published this week in the journal Science .

The researchers conducted four independent experiments in the lab and in the field. They found that enhancing the activity of these fungi by raising CO2 levels actually promoted the decomposition and release of organic carbon from the soil.

The carbon effects seen in this study are also likely to be affected by fertiliser use, says Hu.

"Carbon and nitrogen cycles interact, particularly in agro-ecosystems and forage lands where arbuscular mycorrhizal fungi dominate and human perturbations are routine. In these systems, nitrogen fertiliser inputs are high and some of them are emitted as greenhouse gases too."

"Effective management of nitrogen fertilisers may create a win-win situation," he says, "enhancing carbon sequestration, while reducing nitrogen-based greenhouse emissions."

To clearly understand the responses of terrestrial carbon cycling to climate change, interactions between carbon and nitrogen cycles must be taken into account, writes George A. Kowalchuk in a separate article in Science.

The decomposition seen in the study "presumably results from the activation of soil-borne microbes by the increased flux of [fungus]-derived substrates," says Kowalchuk, a microbial ecologist from the Netherlands Institute of Ecology.

"This allows the newly stimulated soil microbes to degrade other organic matter sources in the soil."

While the study may undermine some assumptions about plant-soil systems as potential carbon sinks, all is not lost, says Kowalchuk.

"Through manipulations of soil nitrogen, organic matter quality, and field management, the balance of soil decomposition patterns can be tipped in a more positive direction," he says.

Important questions

The study was well conducted and raises important questions, says biophysicist Professor Iain Young, of the University of New England, who was not involved in the research.

"How much carbon can the soil sequester? I've always felt very little," Young says.

But soil scientist Professor Alex McBratney from the University of Sydney remains optimistic.

Bratney would like to see longer studies to explore how much microbial biomass carbon is retained in the soil over a year or a decade.

"I still remain optimistic that we can increase the amount of carbon sequestered in soil if we can increase net primary productivity," he says.