Earlier research into slowing global warming from fossil-fuel pollution suggested that soil could be a good place to store excess carbon. But the latest research warns otherwise, says a report on the Science and Development Network website.
Increasing levels of carbon dioxide in the atmosphere may turn soil from a potential carbon sink into a carbon source, according to research published online in Proceedings of the National Academy of Sciences in the USA.
Climate change could stimulate microbial communities to release, not store, carbon dioxide.
Lead researcher Patrick Megonigal of the United States-based Smithsonian Environmental Research Centre said: ”We thought that higher plant growth at elevated carbon-dioxide levels would add more carbon to soils or at least leave it the same.”
His new study reports that despite increased plant growth, doubling carbon-dioxide levels in an ecosystem actually caused a reduction in carbon storage in the soil.
”We now need to consider a third possibility — that the carbon already in soils will end up back in the atmosphere as a greenhouse gas,” he said.
The researchers say elevated carbon-dioxide levels increased microbial activity in the soil, leading to decomposition of organic matter and the release of carbon dioxide.
The study compared samples in a Florida scrub-oak ecosystem over a six-year period. Researchers found that the carbon-dioxide loss from soils negated about 52% of the additional carbon that had been sequestered by the trees. Soils exposed to the elevated carbon dioxide were found to contain more fungi and higher levels of activity of a carbon-degrading enzyme.
”Because the loss of soil carbon was due to the actions of soil micro-organisms, our work highlights the importance of understanding these tiny creatures and their responses to rising carbon dioxide and global climate change,” Megonigal said.
He plans to investigate further the changes in microbial activity and the extent to which this happens in other forests. It might be that altering microbial activity in soils could still help stabilise atmospheric carbon-dioxide levels and help slow global warming.
Help from the forests
Old-growth forest might store far more carbon than previously thought, making their preservation a higher priority in carbon trading and other efforts to tackle global warming.
Classified as forests at least 100 years old, old-growth forests are widespread in tropical and subtropical developing countries. Until recently, they were not thought to absorb and store significant amounts of greenhouse gases from the atmosphere.
In a study published in the journal Science, however, scientists show that a 400-year-old forest in southern China is soaking up atmospheric carbon considerably faster than expected.
Knowing this, developing countries with abundant old-growth forest cover could ask rich countries for compensation through the global carbon trade, said team leader Guoyi Zhou, of the Dinghushan forest station in Guangdong province.
The finding could also help reduce deforestation in the developing world, added Zhou, whose research falls under the Chinese Academy of Sciences.
So far, protecting forests from deforestation is not included in the global carbon trade, a mechanism enabling industrialised countries to pay developing nations for reducing the latter’s carbon-dioxide emissions.
In the study, Zhou and colleagues discovered that the top 20cm of soil in old-growth forests can contain much more carbon than people had expected.
Until now, scientists believed that carbon taken up by old-growth forests from the atmosphere was balanced by the carbon they release, partly because of the large amounts of decomposing organic matter in their soils, ”unlocking” significant amounts of carbon.
The group of scientists measured carbon in the soil collected between 1979 and 2003. They found that organic carbon concentrations in the top 20cm of the soil increased in that period from about 1,4% to 2,35%.
”We began to collect data on soil organic carbon in the mid-1970s, which enabled us to make this comparison,” said Zhang Deqiang, one of the researchers and an associate professor at Dinghushan forest station.
The forces driving this rise are not yet clear, and the authors say their findings point to the need for further research on the complex responses of old-growth forest to global environmental change.
”We estimate that the increased acidity resulting from industrial pollution might have curbed the activities of micro-organisms, hence reducing the speed of soil degradation process. But this needs more systematic studies,” Zhang said. — SciDev.Net