July 14, 2011

Soil Microbes Aggravate Global Warming

By Craig Osenberg, University of Florida

Plants are good for the planet because, among other things, they help offset the effects of global climate change. It turns out, however, that other members of the terrestrial ecosystem, soil microbes, may exacerbate the problem, offsetting some of the beneficial effects of plants.

In a new study published in the July 14 edition of the journal Nature, researchers conclude that more carbon dioxide in the atmosphere causes soil to release the potent greenhouse gases methane and nitrous oxide, weakening the cooling effect of plants and amplifying global warming.

"With all the focus on carbon dioxide as the driver of global warming, plants are obvious buffers because they use CO2 as they grow," said Craig Osenberg, a biology professor at the University of Florida and co-author of the study. "But our work shows that more CO2 in the atmosphere enhances soil release of methane and nitrous oxide gases that cause even more warming than CO2."

Osenberg, along with co-authors Kees Jan van Groenigen from Trinity College, Dublin, and Bruce Hungate from Northern Arizona University in Flagstaff, gathered all published research to date from 49 experiments mostly from North America, Europe and Asia, and conducted in forests, grasslands, wetlands, and agricultural fields, including rice paddies. The common theme in the experiments: They all measured how extra carbon dioxide in the atmosphere affects how soils take up or release the gases methane and nitrous oxide.

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, but two strong patterns emerged when we analyzed all the data: more CO2 boosted soil emissions of nitrous oxide in all the ecosystems, and in rice paddies and wetlands, extra CO2 caused soils to release more methane," Osenberg said. Wetlands and rice fields are two major sources of methane emissions to the atmosphere.

The culprits? Specialized microscopic organisms in soil that use the chemicals nitrate and carbon dioxide, much like humans use oxygen when they breathe. In return, the microbes 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: "The higher CO2 concentrations reduce plant water use, making soils wetter, in turn reducing the availability of oxygen in soil, favoring these microorganisms," Van Groenigen said.

The other reason these microorganisms become more active is that increasing carbon dioxide makes plants grow faster, and the extra plant material 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, locking 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.

"It's an ecological point and counterpoint: the more the plants soak up CO2, the more microbes release these more potent greenhouse gases", Hungate said. The microbial counterpoint is only partial, he said, "reducing the cooling effect of plants by about 20 percent."

But it's an ecological surprise, too, and one that climate models will need to reckon with as they further refine pictures of the climate of the future.

"By overlooking the key role of these two greenhouse gases, previous studies may have overestimated the potential of ecosystems to mitigate the greenhouse effect" van Groenigen said.


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