A team led by Dr Kees Jan van Groenigen of Trinity College Dublin analysed published results from 49 different experiments that measured how extra CO2 in the atmosphere affects how soils take up or release methane and nitrous oxide.

‘Until now, there was no consensus on this topic, because results varied from one study to the next,’ explained one of the study’s co-authors, Professor Craig Osenberg of the University of Florida. ‘However, two strong patterns emerged when we analysed all the data: first, more CO2 boosted soil emissions of nitrous oxide in all the ecosystems, and second, in rice paddies and wetlands, extra CO2 caused soils to release more methane.’

The effect is due to anaerobic soil microbes, which produce nitrous oxide and methane as waste products. ‘The higher CO2 concentrations reduce plant water use, making soils wetter, in turn reducing the availability of oxygen in soil, favouring these microorganisms,’ Van Groenigen explained.

Increasing CO2 also makes plants grow more rapidly, which, in turn, supplies the soil microorganisms with extra energy, increasing their metabolism. ‘It’s an ecological point and counterpoint: the more the plants soak up CO2, the more microbes release these more potent greenhouse gases,’ said another of the study’s authors, Professor Bruce Hungate of Northern Arizona University.

September 2011