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Agricultural Fertilizer May Increase Threats of Nitrogen Pollution - 2002-07-10

From mountain summits to ocean shores, plants and animals are suffering from exposure to the chemical nitrogen. Though this element is a key building block of life, it can also be a pollutant so serious that some biologists rank its effects on par with global warming.

At Welsch Farms in Jersey County, Illinois, workers are preparing the soil for planting. They are injecting into the ground anhydrous ammonia, a man-made fertilizer. This is modern agriculture, the foundation of civilization.

University of Minnesota ecologist David Tilman said, "When agriculture first began about 10,000 years ago there were four million people living on Earth. There are now six billion. And the only reason we can have so many people is because we have perfected agriculture."

Mr. Tilman said a critical element to perfecting agriculture was inventing synthetic fertilizer containing the element nitrogen, like the ammonia being applied here. Plants need nitrogen to make life's building blocks, including DNA and proteins. So billions of pounds of factory-made nutrients are applied to crops every year.

Of course nitrogen is all around us. The air we breathe is mostly nitrogen. But this gaseous form is kind of like crude oil to a car, it needs to be chemically modified, the same way oil has to be refined to be used. The natural refineries of atmospheric nitrogen are primarily microbes that convert it to a form plants can use. But today, the dominant source of this key nutrient on all Earth's land is not microbes, it is fertilizer factories.

These, along with nitrogen compounds created when fuel in cars and power plants is burned have doubled above natural levels the production of nitrogen nutrients.

Gerry Melillo is a researcher at the Marine Biological Laboratory in Woods Hole, Massachusetts. He said the problem is not just how much of this nutrient is made, but where it goes. Mr. Melillo said, "The problem is that there can be too much of a good thing. It can cascade through the environment in a variety of forms, moving to places that we as humans did not intend it to move."

More than half of the fertilizer applied by growers is never used by plants. It percolates into ground water. It evaporates into the air and flows from fields to streams and lakes. In rare instances water supplies become so contaminated with excess nitrogen they become unfit for human use. Nitrogen compounds in such water can replace oxygen in the blood of infants causing a life-threatening condition sometimes called "blue-baby syndrome."

Waterways polluted with excess nitrogen flow to the ocean. There, in shallow coastal waters, it spurs algae growth. When these algae die and decomposes they use up oxygen, suffocating fish.

Cornell biologist Robert Howarth said this is happening to a huge degree at the mouth of the Mississippi River. "The nitrogen pollution coming down the Mississippi River," he said, "has created an area of about 20,000 square kilometers - that's an area bigger than the state of New Jersey - where a lot of higher forms of life have just been wiped out by this nitrogen pollution."

The degree of damage there is unique because the mighty Mississippi drains so much farmland. But Mr. Howarth says two-thirds of U.S. coastal waters suffer from some nitrogen damage. Life on land is at risk as well. Liquid fertilizers evaporate from farmers' fields. These vapors, combined with smaller amounts of nitrogen compounds from tailpipes and smokestacks, drift in the wind and fall to land as acid rain and dust.

The effect of nitrogen pollution on trees is the subject of a study at the Harvard Forest outside Boston.

While walking briskly up a steep dirt track, forester John Aber said, "As we go up the path here there's two research plots on either side. The high nitrogen forest is over here." For the last 12 years, Mr. Aber, a professor at the University of New Hampshire, has been applying fertilizer to this plantation of mature red pine. The scientist turns off the path into a stand of tall trees.

"Some of the trees appear, actually, to be dead," he continued. "And the remaining trees that are alive the crowns are very sparse [with some foliage left at the tops and at the ends of the branches. But it's very open."

Daniel Grossman: "This is a pretty sick looking forest!"

John Aber: "It is indeed! It's looking a lot worse than it did than even a year or two ago."

John Aber said the extra nitrogen is probably binding up soil minerals, making them inaccessible to roots. The trees in this plot are getting big doses - 20 times the amount of nitrogen falling as air pollution.

But Mr. Aber has some evidence that even smaller amounts of nitrogen, comparable to that coming from acid rain and dust, could be causing subtle forest damage. "It's kind of a warning," he said, "that those potential responses are out there, if we didn't take care in controlling nitrogen pollution."

It's not just forests with these responses. Half a continent away, ecologist David Tilman is studying the effects of nitrogen on native grasses. He's been surprised by his results. He said, "Adding a small amount of nitrogen - just the amount that comes out of the atmosphere every year in Minnesota, year after year for twenty years - has caused us to lose a little over 30 percent of the plant prairie species that occurred at our site."

Like many plants, native grasses are adapted to live in low nutrient sod. They cannot take advantage of nitrogen-rich soil. Plants that can, like the European transplant quack grass, outgrow and replace the natives.

Mr. Tilman said it is the gradual erosion of the diversity of plant and animal life that may be the most serious impact of nitrogen pollution. Mr. Tilman adds that U.S. farms and factories must reduce nitrogen pollution if this country's diversity of life is to be preserved.

The Great Lakes Radio Consortium is a production of Michigan Radio. Support comes from the W.K. Kellogg Foundation and the Charles Stewart Mott Foundation.