Of Dirt and Ducks

Twenty thousand years ago, the last great continental ice sheet plowed the northern Midwest, planing the land down to bedrock, grinding stone to dust, mixing sediment and then dropping it as the ice melted. For a few hundred years, the boreal forest replaced the glacier, but as the weather grew warmer and drier, spruce and fir gave way to oak and hickory, then to bluestem and indiangrass.

For thousands of years, generations of tallgrass prairie plants lived and died, penetrating eight to 10 feet below the surface with extensive root systems and building a thatch of rotting stems at the surface. Slowly and steadily, the organic material turned into humus, mixing with the minerals below to form as much as eight feet of topsoil.

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Soil is black for the same reason oil and coal are black -- carbon. Plants take in carbon dioxide and use the energy of the sun to forge the carbon into long-chain organic molecules. Plant physiologists call the process photosynthesis, but a new generation of climate specialists and entrepreneurs has another name for it: carbon sequestration, the long-term storage of carbon in its solid form. Carbon sequestration has become one of the hottest topics in discussions about climate change and wetland conservation.

Given the chance, the planet does a pretty good job of locking up carbon. The oil, coal and natural gas we burn by the billions of tons are, after all, the result of natural carbon sequestration that occurred as much as 360 million years ago. Natural vegetation continues the process today. A walnut tree on the northern plains might absorb more than a ton of carbon per year over the first 80 years of its life, much of which is right where we can see it, in that exquisite dark wood we use in our gun stocks.

The carbon uptake in prairies and wetlands is harder to recognize because so much of it happens underground. The taproot of a prairie rose might grow 20 feet straight down, and the main roots of compassplant and leadplant aren't far behind. Most prairie plants have huge systems of roots and underground stems -- more than three-quarters of the plants are below ground, which makes sense for vegetation that has evolved to handle the extremes of prairie weather and the not-so-tender attention of millions of grazing animals.

When one of these plants dies, much of the carbon in its roots is left below the surface, out of reach of many of the creatures, large and small, that might eat it and release it into the atmosphere. Little wonder prairie soils are exceptionally rich in carbon. An acre of tallgrass prairie might have more than 50 tons of organic carbon bound in the soil underneath. Marshes in the pothole country of the north-central U.S. bind about the same amount of carbon per acre.

When the prairie sod was first broken, the soil immediately started leaking carbon.

Because nobody was taking careful soil samples in the last half of the 19th century, it is hard to know exactly how much carbon was lost to the plow. One study in the prairie potholes estimates that 16 percent of the carbon in wetland basins is lost when they're plowed. Data from a long-term study plot at the University of Illinois suggests the loss there was well over 30 percent, and a similar study at the University of Missouri suggests it can be as much as 96 percent.

The conversion of native prairie to cropland, combined with the clearing of American forests and wetlands, released unimaginable amounts of carbon into the atmosphere.

Scientists at the Woods Hole Research Center have estimated the amount of carbon released as the United States was settled. According to their data, the loss reached a broad peak in 1881 and began to decline slowly afterward. In 1881 alone, more than 300 million tons of carbon was liberated from America's soil and timber.

Obviously, this couldn't go on forever. Eventually, no more pristine grasslands and forests remained to convert, so the amount of carbon stored in American cropland and pasture found a new, although much lower, equilibrium. Beginning in the 1940s, the net loss of forests that had gone on for more than 300 years actually began to reverse itself as farms in New England and many parts of the Appalachians were abandoned and reverted to timber. Shelterbelts on the Great Plains and shade trees in growing urban areas also took carbon dioxide out of the air and transformed it into cellulose.

By 1946, the capture of carbon by trees and other plants in America was enough to balance the release of carbon from the oil, gas and coal we were burning. According to the Woods Hole estimates, we actually locked up about 4 million tons of carbon in 1946.

Since then, changes in our land use have led to a slight increase in the net amount of carbon we are trapping in the United States. The stems, leaves and husks produced on our grain fields are more likely to stay on the field these days, rather than being used for livestock forage and bedding. Minimum-till techniques leave more crop residue on the field as well, reducing the need for fertilizer and herbicides while maintaining carbon in the soil. Long-term land retirement programs such as the Conservation Reserve Program trap carbon as well -- not as much as native prairie or marshes, but still a substantial amount.

The Woods Hole team estimates in 2005, America actually captured about 29 million tons of carbon, thanks to our improved stewardship of the land. Before we congratulate ourselves on this achievement, however, we ought to remember a couple of other statistics. If the Woods Hole numbers are right, America has released 17 billion tons of carbon since 1850. If we continue to trap carbon at the rate we did in 2005 -- and don't burn any more fossil fuel than we did in 2005 -- it will take us 600 years to get the genie back in the bottle.

And our historical example has convinced a lot of other societies to follow our lead. In 2005, the world as a whole released 1.3 billion tons of carbon into the air.

You might be wondering what all of it has to do with waterfowl. There is an important link. The prairie potholes offer a great way to trap carbon. In a recent technical paper,

Ned Euliss, a biologist with the Northern Prairie Wildlife Research Center in Jamestown, N.D., and several colleagues pointed out that wetlands absorb an exceptional amount of carbon. According to their analysis, a pristine prairie pothole in native prairie holds about 23 tons of organic carbon per acre and a restored wetland in the region holds about 19 tons of carbon per acre.

Wetland restoration in the region could sequester as much as 340 million tons of carbon on about 17 percent of the land area in the northern prairie. They conclude, "Wetland restoration has potential to offset 2.4 percent of the annual fossil carbon dioxide emission reported for North America in 1990."

Preserving native prairie on the uplands surrounding the potholes will also keep a

substantial amount of carbon in the ground, and CRP plantings on retired cropland will trap even more. It seems everything we do for breeding ducks helps shift the balance between gaseous carbon in the atmosphere and solid carbon in the ground.

The bottom line is good waterfowl management can be good climate change policy as well.