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Massive erosion is an obvious disaster in any agricultural system, but more subtle changes to the soil can be just as devastating. WSU ecologists Dave Evans (’90 Ph.D. Botany) and Rick Gill are finding that high levels of carbon dioxide (CO₂), the major cause of our current climate change, are altering plants and soils in ways that could profoundly affect the health of ecosystems and our ability to feed ourselves.

Unlike previous “natural” warming trends, present-day warming is not the result of sunspot activity or cyclical fluctuations in temperature. This episode is being caused by a huge increase in the amount of CO₂ and certain other gases in the earth’s atmosphere. Like panes of a greenhouse, these gases keep heat in; hence the name “greenhouse gases.”

Among the greenhouse gases, carbon dioxide merits special attention because, quite apart from its role in boosting temperature, it has profound effects on living things. Plants take in atmospheric CO₂ through their leaves. If they have enough water and nutrients such as nitrogen, they use the CO₂ to make sugars and other molecules. Plants then leak some of those sugars from their roots, which benefits the soil bacteria and fungi that, in turn, provide the plants with nitrogen that they process from sources in the soil. The reciprocal interactions among plants, fungi, and bacteria are the basis of all plant growth, all forests, all grasslands, all agriculture.

^{Courtesy Rick Gill}

Rick Gill, along with colleagues from Duke University and the USDA’s Agricultural Research Service, has been studying how a grassland ecosystem copes with different levels of CO₂. On their research site in north Texas, a 110-yard-long tube of greenhouse fabric encloses a narrow strip of native prairie in a sort of atmospheric time tunnel. Constrictions in the tube divide it into 5.5-yard-long segments, and pumps create a stepwise gradient of CO₂ in the segments from one end of the tunnel to the other. At one end, representing the future, CO₂ is at 550 parts per million (ppm), the level it's predicted to reach by 2100. In the middle of the tunnel, CO₂ is at today’s level of 375 ppm. At the other end, CO₂ is at 220 ppm, the level that prevailed in earth’s atmosphere for at least ten thousand years, until the Industrial Revolution and our large-scale burning of fossil fuels began nudging it upwards in the mid-1800s.

The researchers first asked whether plants would grow bigger and faster in the presence of a higher level of CO₂. That possibility is the basis of one of the major strategies proposed for fighting climate change: using plants to take more CO₂ out of the air and sequester it as part of the structure of the plant. Gill and his colleagues found that higher CO₂ levels did lead to more plant growth—for a few seasons. Then growth stalled.

Gill says there’s a limit to how much carbon plants can take in and how much carbon-rich plant material, such as fallen leaves, soil microbes can recycle before they run into other constraints. Both plants and microbes need nitrogen and other nutrients as well as carbon. Some nitrogen sources are small molecules that microbes can process easily. Others are so large and complex, and require so much energy to break down, that most microbes don’t bother. They simply slow their growth rather than attempt to mine the nitrogen out of those sources.

Under elevated CO₂, says Gill, “plants are much more efficient, they photosynthesize more, they grow more. But then that litter falls to the ground. Microbes have to break it down, and before long they run out of nitrogen. So there’s a natural negative feedback in the system that slows down carbon sequestration.”

The feedback is called “progressive nitrogen limitation.” It’s been observed in forests as well as grasslands, and it poses a big problem for carbon sequestration schemes.

“It doesn’t let the system respond to the rising CO₂ in the way that we’re forecasting it will,” says Gill. “We spend all of our time focusing on carbon, carbon, carbon, with the hope being that, as we pump more and more CO₂ into the atmosphere, native ecosystems will suck some of that out.”

Gill says the notion of carbon sequestration is so widespread, it’s become embedded in many of the models used to predict the extent of climate change.

“They assume that we’re only going to be at 550 parts per million 90 years from now, because a third of the CO₂ we produce will end up in trees or in soils,” says Gill. “What we’re showing is, don’t count on it. Because the assumption that’s built into that is that you’ve got abundant water and abundant nutrients. And we’re saying, that doesn’t happen.”

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Continued

"We spend all of our time focusing on carbon, carbon, carbon, with the hope being that, as we pump more and more CO₂ into the atmosphere, native ecosystems will suck some of that out. . . . What we're showing is, don't count on it."