05/01/93
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STANFORD -- Scientists studying the effects of "greenhouse" gases in the earth's atmosphere have a major puzzle on their hands: there has been a huge increase of nitrous oxide in the atmosphere that simply can't be accounted for.
They think the cause may be the use of nitrogen-based fertilizers in the developing world, and have set up a series of experiments in Brazil and Hawaii to test that notion.
"It's one of the many things in the atmosphere that is changing under the influence of humans," said Peter Vitousek, professor of biological sciences at Stanford, who participates in the Hawaiian study. "We know exactly what causes [some of them], like the increase of carbon dioxide in the atmosphere, and some of them we're not sure about. This is one of the 'not sure'."
With Pamela Matson of the NASA-Ames Research Center, and colleagues at Cornell, the Woods Hole Research Institute, NASA-Goddard, the University of Hawaii and research institutions in Brazil, the scientists are trying to account for a massive increase in nitrous oxide being added to the atmosphere each year.
"Good measurements of the amount [of nitrous oxide] in the atmosphere have been made over about a 15-year period at a number of stations around the world," Vitousek said. "The gas has a lifetime in the atmosphere of 160 years, so you don't have to filter out much of the local contribution to it. If you get an increase in concentration, the stuff is increasing in the atmosphere."
Increasing it is, he points out, at a rate of about 3-1/2 million tons per year. The source is not easily explained.
Land clearing, particularly the slash-and-burn techniques used in places such as the Amazon rain forest, certainly has produced some nitrous oxide, Vitousek said, "but not enough to make the difference."
Pushing the group's numbers to an extreme, he estimated that land-clearings could account for no more than 20 percent of the increase. No one knows what is producing the other 80 percent.
"A lot of the logical things have been checked," he said. "We went back and asked, 'what's changed globally in the nitrogen cycle and how has human activity altered that?' "
A likely candidate is the worldwide use of nitrogen fertilizers.
When researchers first looked at agriculture as the source for the increase, they found that nitrogen-based fertilizers were used mostly in the developed world and that usage had leveled off as food supplies more than met the demand.
In the developing world, fertilizers were just coming into widespread use, especially in the tropics. Populations were growing enormously and fertilizers presented the only viable way of producing enough food for the population growth.
"If we have any hope of improving their standard of living and nutrition, the countries are going to have to double food production more quickly than they double the population," Vitousek said.
The result is intensive agriculture, factory farms, in which the use of nitrogen fertilizer has now equaled that of the industrial world.
-"We don't know anything about what its effects are," he said. "They could be different because some of our earlier studies showed that soil in tropical forests produces much more nitrous oxide than do most temperate forests, both in absolute terms . . . and total.-
"Most tropical forests have all the nitrogen they need and treat it more or less as an excess nutrient."
The next question was: What happens when nitrogen fertilizer is added to those soils? "Do they have proportionally the same emissions from fertilizer as do temperate soils, which we know fairly well, or might they be a lot greater proportionally?" he said.
The researchers set up experimental stations near intensively managed agricultural fields in the tropics to measure the effects of the fertilizers. The Cornell and Brazilian scientists are working near Brasilia; Matson and Vitousek set up shop near a sugar-cane plantation on Maui.
The researchers hope to expand the experiments to Indonesia.
The results of the study aren't in yet, Vitousek said. The researchers should be able to start submitting papers in about six months.
"The Hawaiian systems are really nice because, basically, they have the whole range of soil you find anywhere in the tropics, in a small area," he said.
"You have a very modern agricultural system on some of them, 300 pounds per acre of nitrogen fertilizer, which is a lot, and it is applied to a very wide variety of soils, with everything else the same, the same crop."
Would the use of nitrogen fertilizer be worse than the destruction of the rain forest?
"That's the question in terms of both local effects and global effects," Vitousek said. "There's no way you can feed urban populations that are growing at the rate they're growing with slash-and-burn. You have to do some concentrated intensive farming on good soil. . . . It may be that it's better overall in a global environmental sense to farm a few acres very intensively and not grow over a broad area for relatively small yields."
Unfortunately, he said, it is likely people will do both.
There is, however, the possibility of altering the way nitrogen is used in a fertilizer, or replacing it entirely.
At one plantation in Hawaii, nitrogen is applied to each field seven times through the irrigation water during the life of a crop, instead of having it dumped all at once at the beginning of the crop's growth. If 300 pounds of fertilizer per acre are applied when not much is happening with the crop, most of it is wasted and winds up in the atmosphere or ground water.
American and European scientists have found that if farmers can pay the cost of making the transition from nitrogen-based fertilizers to fertilizers with other nutrients, crop yields can be as big.
American and European farmers might be able to afford this transition - it takes about five years before it becomes cost-effective - but farmers in developing countries probably would not.
"It may be the sort of thing where the evolutionary pathways from extensive systems to intensive would involve putting on a lot of fertilizer in the cheapest, simplest way possible," Vitousek said. "But as you accumulate capital and a better-developed system, you can intensify in other ways as well that are more knowledge-based."
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