The Carbon Disappearance Mystery

Vast areas of U.S. rangeland may absorb some of the CO₂ that is released every year into the atmosphere.

More than 7 billion metric tons of carbon enter the atmosphere in the form of CO₂ each year. But when scientists measure the increase in CO₂ concentrations in the air, they can only account for about half of the carbon. Where are the "missing" 3 billion metric tons?

That's about the amount of coal burned for electricity during a 3- to 4-year period in the United States.

"The answer matters because if the CO₂ concentration affects climate, we can't predict what will happen in the future until we understand the global carbon cycle," says Mayeux. "If the Earth's vegetation and soils are absorbing the CO₂ we're releasing, that could forestall the rate of CO₂ buildup in the atmosphere."

Some of the missing carbon might be stored in Nevada's high deserts, Oklahoma's prairies, or in grasslands near you.

"Plants take in CO₂ and convert the carbon to leaves, stems, roots, and fruit," says Mayeux. "Since rangelands cover half the Earth's land area and contain one-third of the plant life, they're a logical place to look for the missing carbon."

ARS scientists at 11 locations across western rangelands are doing just that. They're using sophisticated meteorological instruments called Bowen ratio/energy balance units to understand how CO₂ moves between the air and vegetation on U.S. rangelands. The units run continuously on plots of at least 15 acres each.

Participating ARS locations include Tucson, Arizona; Fort Collins, Colorado; Dubois, Idaho; Miles City, Montana; Las Cruces, New Mexico; Mandan, North Dakota; Woodward, Oklahoma; Burns, Oregon; Temple, Texas; Logan, Utah; and Cheyenne, Wyoming.

Bill Dugas, agricultural meteorologist at the Texas Agricultural Experiment Station in Temple, is compiling the data under a cooperative agreement with ARS. Tagir Gilmonov, a visiting Russian ecologist, is currently working at Logan to help some of the network participants develop predictive models based on the CO₂ fluxes and weather data.

Technician Edward Buenger prepares soil samples for analyses that will tell scientists how much carbon plants have pulled from atmospheric CO₂ and stored in soil organic matter.

"If rangelands store excess carbon, we will find that the amount of carbon in the plants and soil organic matter increases over time," says Phillip L. Sims, a rangeland scientist at Woodward. So far, ARS researchers have learned that the amount of CO₂ absorbed by the vegetation fluctuates significantly from location to location and even over short periods at each site.

"Within 3 years, we'll know what the fluxes are on undisturbed grasslands," says Sims. Many of the locations are also conducting smaller scale experiments that compare how various management strategies affect the land's ability to store carbon.

ARS researchers in Burns, for example, designed portable, 1-meter-square plastic chambers that allow them to measure CO₂ exchange around single plants, rather than over large areas of rangeland." This tool lets us conduct small-scale, replicated experiments," says ARS rangeland scientist Raymond F. Angell. He's evaluating the impact of fire on CO₂ absorption by rangelands.

"Prescribed burning is an effective way to increase the grass component of rangelands that have become dominated by shrubs and trees because of long-term fire suppression," Angell says. The controversy arises because burning releases CO₂ into the atmosphere. "But we believe that the increased growth right after the burn may take up more CO₂ than is released," he says.

Angell and colleagues are now measuring baseline conditions on the study sites. Then they'll burn some of the plots and use the chambers to measure changes in CO₂ uptake as the plants grow back.

Other locations are using the same techniques to study the effects of grazing and other land uses.

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Change on the Range

Range scientist Herman Mayeux checks a light-sensing bar that indicates solar radiation levels. CO₂ concentrations inside the tunnels range from today's 350 parts per million to the 200 ppm present during the last ice age.

Not only may global climate change affect tomorrow's world--it may already be shaping our natural environment.

ARS scientists have discovered that rangeland plants, like crop plants, can grow more and use less water when atmospheric CO₂concentrations rise.

"Shrubs have invaded and are in some cases replacing native grasslands worldwide," says ARS plant ecologist H. Wayne Polley of Temple, Texas. "Rising CO₂ levels over the past 200 years may be partially responsible," he says.

That's because some plants seem to benefit more than others from the extra CO₂. The shrub mesquite, Prosopis sp., is one of the winners.

"Woody plant populations tend to increase as precipitation increases. Improving plants' water use efficiency could be having the same effect as having more rain," Polley says.

In much of Texas, mesquite has replaced the native prairie grasses. Such a shift in the vegetation can have widespread impacts: less forage available for livestock grazing, a shift in wildlife species that inhabit the area, changes in soil nutrient cycling, and increased erosion because shallow-rooted grasses no longer hold soil in place.

Polley and colleagues are now looking at mesquite genetics, to see if some of the plants are better able than others to use the increased CO₂.

"If we find such genetic variability, then natural selection may be helping mesquite become more abundant," he says.

Plant physiologists Jack Morgan (left) and Dan LeCain have designed and installed six open-top chambers at the ARS Central Plains Experimental Range in eastern Colorado. Three of these greenhouse-like chambers are receiving injections of CO₂ to simulate anticipated global concentrations, and three operate under current atmospheric levels.

The grass species may also be changing.

Right now, warm-season grasses like blue grama, Bouteloua gracilis, dominate the shortgrass prairie in Colorado. Warm-season grasses are most productive during the summer months, while cool-season grasses like western wheatgrass, Pascopyrum smithii, grow in spring and fall.

In growth chamber studies, ARS plant physiologist Jack A. Morgan found that photosynthesis in cool-season grasses increases as atmospheric CO₂ rises.

"From research on other plants, we expected the cool-season grasses to respond more than the warm-season grasses," Morgan says. "Eventually that could give cool-season plants a competitive advantage and shift the ecosystem's species composition."

But he also found that the warm-season grasses respond more than previously believed to additional CO₂. Like mesquite, both types of grasses use less water and grow more.

Two complications in the future scenario are potential temperature increases and reduced forage quality.

"If temperatures go up without a corresponding increase in precipitation," says Morgan, "the soil may dry out enough each growing season that the plants can't take full advantage of the increased CO₂."

Morgan's and Polley's teams also found that while the plants grow larger, the concentration of nitrogen in the plant tissues goes down. That's important because protein, a key nutritional component of forage grasses, depends on the nitrogen. "The end result is more forage, but of reduced quality," says Morgan.

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