Here's an environmental riddle. Why is a cornfield like an automobile? Answer: They both pollute the air with nitrogen oxides, which then help form photochemical smog.
Cars pollute by burning fuel. cornfields pollute by releasing nitrogen oxides from the fertilizer that helps the corn grow.
This illustrates a basic challenge in the battle to have clean air. It's not enough to go after the obvious polluters like cars. You also have to take account of such subtle and diffuse polluters as cultivated farmland.
Air-quality scientists have suspected this for some time. Studies in recent years have documented nitrogen releases from soils, which result from the action of bacteria. Now atmospheric chemist Kevin Civerolo at the University of Maryland in College Park and several colleagues have shown how significant such nitrogen releases can be when farming practices unwittingly intensify that kind of pollution.
Natural soils emit relatively little nitrogen oxide. In contrast, farmland laced with nitrogenous fertilizer can be a rich source of pollution. This pollution tends to be strongest on the hottest summer days when the risk of forming smog also is greatest. But not all farmland is equally culpable. Mr. Civerolo and his Maryland colleagues Russell Dickerson and Bruce Doddridge, together with Richard Valigura of the National Oceanic and Atmospheric Administration's Air Resources Laboratory in Silver Spring, Md., took a detailed look at what was happening at the seaside experimental site of the Wye Research and Education Center in Queenstown, Md.
Located about 100 miles east of Baltimore, the center runs comparison studies of cultivated cornfields and cornfields under "no till" management. That is, they compare fields that are plowed, harrowed, and weeded with fields whose soil is unbroken and weeds chemically controlled.
As Civerolo reported during the recent meeting of the American Geophysical Union in Baltimore, they found that cultivated fields released some six times more nitrogen oxides than did their "no till" neighbors, which also were fertilized. The conclusion, Civerolo says, is that switching to "no till" farming could help hold down air pollution even from fertilized farmland.
A simple rough estimate makes the point. Assume cars typically emit 0.3 grams of nitrogen per mile, as Environmental Protection Agency studies suggest. Also assume they are driven 10,000 miles a year. A six-fold reduction in oxide emissions from soils would then be equivalent to removing about 200,000 cars from Maryland roads during the summer, Civerolo says.
Considered on a global or national basis, such soil emissions are a big deal. They contribute some 20 percent of the global nitrogen oxide budget, which represents about 50 billion tons of nitrogen per year. In the United States, soil emissions account for about 6 percent of man-made nitrogen-oxide releases. They can run as high as 14 percent of that total in summer.
Civerolo and his colleagues found that fertilized soils contribute about 1 to 2 percent of the pollution in Maryland. It can run as high as 10 percent to 15 percent in rural areas.
They also found that, while cultivated cornfields make up only 4 percent of the local land area, they contribute about 40 percent of the soil nitrogen-oxide pollution.
Whether this is sufficient reason for favoring "no till" farming depends on a complexity of factors. These include any negative aspects of "no till" techniques with their dependence on chemical weed control. But it certainly would help clean up the air.
That conclusion didn't jump out at Civerolo right away. The instruments were set up at the boundary between cultivated and "no till" fields. He thought something was wrong with the experiment when the numbers began coming in with such large differences between the two fields. But as often happens in scientific research, what seem to be errors can open the door to discovery. In this case, it's a discovery that could benefit us all.