The research team used a statistical technique called meta-analysis, or quantitative data synthesis, a powerful tool for finding general patterns in a sea of conflicting results. "Until now, there was no consensus on this topic, because results varied from one study to the next," explained Professor Craig Osenberg of the University of Florida and co-author of the study. "However, two strong patterns emerged when we analysed all the data: firstly more CO2 boosted soil emissions of nitrous oxide in all the ecosystems, and secondly, in rice paddies and wetlands, extra CO2 caused soils to release more methane." Wetlands and rice fields are two major sources of methane emissions to the atmosphere.
The culprits are specialised microscopic organisms in soil, that respire the chemicals nitrate and carbon dioxide, like humans respire oxygen. The microbes also produce methane, a greenhouse gas 25 times more powerful than carbon dioxide, and nitrous oxide, 300 times more potent than carbon dioxide. Their oxygen-free habit is one of the reasons these microorganisms flourish when atmospheric carbon dioxide concentrations increase. Van Groenigen explained: "The higher CO2 concentrations reduce plant water use, making soils wetter, in turn reducing the availability of oxygen in soil, favoring these microorganisms."
The other reason these microorganisms 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 ecosystems could slow climate change. With more CO2, plants grow more, soaking up carbon dioxide through photosynthesis, and, the hope is that they also lock away carbon in wood and soil. But this new work shows that at least some of that extra carbon also provides fuel to microorganisms whose byproducts, nitrous oxide and methane, end up in the atmosphere and counteract the cooling effects of more plant growth.
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