Researchers from the universities of Arizona and Florida in the US say that their meta-analysis of 49 different studies suggests that the capacity of land ecosystems to slow climate change has been overestimated, possibly by as much as 20%.

They found that as CO2 concentrations increase, so do concentrations of the potent greenhouse gases methane and nitrous oxide, due to feedback mechanisms in the soil. While it is a well-known fact that these gases are produced by decomposition of organic material, Kees Jan van Groenigen and his colleagues found that the increase in methane and nitrous oxide was larger than anticipated.

"This feedback to our changing atmosphere means that nature is not as efficient in slowing global warming as we previously thought," Van Groenigen told environmentalresearchweb. "By overlooking the key role of these two greenhouse gases, previous studies may have overestimated the potential of ecosystems to mitigate the greenhouse effect. It is important that climate models take this feedback mechanism into account."

The mechanisms that lead to nitrous-oxide emission from soil are relatively complex, so van Groenigen and his colleagues were surprised to find a definite clear trend – an increase in CO2 boosted soil emissions of nitrous oxide in all the ecosystems. They also found that in rice paddies and wetlands, extra CO2 caused soils to release more methane.

The two greenhouse gases are emitted by microbes in the soil, which flourish when atmospheric CO2 concentrations increase.

"The higher concentrations of CO2 reduce plant-water use, making soils wetter and reducing the availability of oxygen in soil, which provides favourable conditions for these microorganisms," said van Groenigen.

The other reason the microbes become more active is that increasing CO2 makes plants grow faster, and the extra plant growth supplies soil microorganisms with extra energy, pumping up their metabolism. This extra plant growth is one of the main ways that ecosystems could slow climate change: with more CO2, plants grow more, soaking up CO2 through photosynthesis and, the hope is, locking away carbon in wood and soil.

But this new work shows that at least some of the extra CO2 also provides fuel to microorganisms whose by-products end up in the atmosphere and counteract the cooling effects of increased plant growth.