Soils may be less of a carbon sink than previously thought, with new research indicating that the earth is likely to release more carbon as the planet heats up.
The role of soil in the carbon cycle is crucial because the near-surface earth contains as much as 2000 billion tonnes of carbon – or roughly four times as much as the plants sprouting from it. By comparison, the atmosphere contains about 800 billion tonnes of carbon dioxide.
A study of 22 soil types, mostly from the northern hemisphere, has found that with warming temperatures, microbes in the soil give off more carbon dioxide, a process that would amplify climate change.
"All the models at the moment have underestimated the loss of carbon under the elevated temperatures with global warming," said Brajesh Singh from the University of Western Sydney, one of the authors of the report, published in Nature on Thursday.
The strongest response was found in soils with high carbon-to-nitrogen ratios and in soils from cold climatic regions of the north – areas where temperature increases are among the fastest anywhere on the planet.
"It will have a significant impact on our carbon budget because that's where most of our carbon soil is locked in," Professor Singh said, referring to the boreal and arctic soils.
Some leading environmentalists have promoted carbon farming as a major opportunity to sequester emissions, both to reduce atmospheric concentrations of carbon dioxide but also to promote greater agricultural productivity.
Environment Minister Greg Hunt has championed carbon farming as key to the government's direct action approach to reducing carbon emissions, and has promoted a 25-year option for carbon sequestration projects.
Andy Pitman, director of the University of NSW's ARC Centre of Excellence for Climate System Science, said in the long term any hope that soils can serve as an additional sink for carbon "is totally wishful thinking" if temperatures continue to heat up.
"The implications for Australia are if you did manage to get carbon into the soil, as the soil warms, you will lose that carbon," Professor Pitman said.
Interestingly, plants tend to increase respiration of carbon dioxide as temperatures rise. However, after a certain period, many of them acclimatise to the warmer conditions and – providing nutrients and water are available in sufficient quantities – reduce their respiration amounts to their original levels.
Microbes, on the other hand, do not appear to acclimatise in the same way. Microbe populations tend to be higher in carbon-rich soils, with the number of species in one gram of soil between 10,000 and 1 million, according to Professor Singh.
"Such is the diversity [of microbes], rather than adjust physically, the microbial groups that are adjusted to that higher temperature just become dominant," Professor Singh said.
The effects on the ecological balance of the newly dominant groups in the soils are little understood, he said.
The tests were conducted on soils from the Peruvian Amazon to the Mediterranean and the Arctic, and took in a range of vegetation types, from grasslands and broadleaf forests to arable land.
The studies stretched over two 90-day periods to see how soils responded to increases of three degrees – the sort of temperature rise that may be expected by 2100 if not sooner, Professor Singh said.
Soil microbes' respiration releases amount to about 60 billion tonnes per year as carbon dioxide – or similar to the level released by terrestrial plants.
The experiments indicate the respiration rates from soils increase exponentially with temperature rises, the researchers said, implying that the feedback loop will accelerate further if the mercury continues to climb.