UC Davis team says plants can't absorb all the carbon we produce.

While plants do absorb carbon, we should not count on them to store enough of it in the soil to protect us from the steep rise in atmospheric CO2 levels implicated in global warming, say a team of researchers from the University of California Davis.

“That natural cleaning up [of excess CO2] is just not happening as much as was previously thought,” said Johan Six, an agroecologist and a co-author of a paper that analyzes a generation of studies on the effect of elevated atmospheric carbon dioxide on plants and soil. “We can’t rely just on nature cleaning it up. We’re going to have to do other things to mitigate the increase in atmospheric CO2.”

The recent findings will be published online in the April 18th edition of the Proceedings of the National Academy of Sciences. They challenge the projections of the Intergovernmental Panel on Climate Change (IPCC), which suggest that elevated levels of CO2 cause proportionally large carbon storage—with plants absorbing the gas through photosynthesis at an enhanced speed and sequestering it in the soil. According to the IPCC model, the feedback loop could potentially slow down global warming, as excess carbon dioxide could be safely and cheaply stored under the Earth’s surface.

“The idea was that there would be a natural way of cleaning up that extra CO2,” said Six. “Plants would grow better and therefore, take up more of that CO2 and transfer it into the soil. Then, the increase of CO2 in the atmosphere would not be as great as predicted just from how much is being produced because there’s a sink for that carbon in terrestrial systems.”

The UC Davis team, led by doctoral candidate Kees-Jan van Groenigen, ran an initial study on a site in Switzerland, where they discovered no increase in carbon sequestered in the soil, even when the carbon content of the air was artificially enhanced. Looking at several other papers, they spied a trend: Contrary to the IPCC’s prediction, when atmospheric CO2 levels rose, the levels of carbon sequestered in the soil did not.

The central role of nitrogen in photosynthesis accounts for the generally observed non-response to elevated CO2 in natural systems, Six explained. Some plants, especially legumes, pump nitrogen from the air into the soil—a process called nitrogen fixation—and the nitrogen helps to sequester carbon underground. But without the extra nutrients needed for fixation, including potassium, phosphorus and molybdenum, the influx of nitrogen cannot keep pace with the increase in carbon—so little extra carbon is absorbed in the soil.

“[People thought] that if these [nitrogen-fixing] plants are present in ecosystems, they would provide the nitrogen that’s necessary to keep up with the increased influx of carbon,” Six explained. “But what we’re finding is that they only do this if one also adds other nutrients, because the other nutrients are actually limiting the nitrogen fixation by these plants. We basically see a cascade of different nutrients playing a role in how much carbon can be sequestered.”

Originally published April 12, 2006

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