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Hunting clues. Researchers spread a tracer to track minerals in the topsoil.

Sucking Nutrients From Thin Air

Like all plants, trees require tiny amounts of metal nutrients, which scientists have long believed came from rock decomposing deep in the soil. Now it turns out that nearly all the essential metals some kinds of trees depend on come from the atmosphere by means of rain. This pool of available nutrients is so small that it makes acid rain an even greater threat to forests than previously suspected.

When acid rain soaks topsoil, it leaches out metals such as potassium, calcium, and magnesium. But because rocks are rich in these elements, most researchers assumed that the nutrients would be abundant in soils. To pin down the amounts of vital metals from different sources, a team led by geochemist Martin Kennedy of the University of California, Riverside, went to pristine, mature forests in southern Chile. They tested soil, plants, and streams for the element strontium, because its movements mimic those of other metals. Because rainwater and rock give rise to variants, or isotopes, of strontium, the researchers could figure out where the trees' nutrients originated.

Kennedy's team found that southern beech, the dominant tree in the area, obtained about 90% of its strontium--and thus other similar elements--from rain, not rocks. Even more surprising, when the researchers treated the soil with a chemical that mimics natural nutrients, they discovered most of the tracer leached from the topsoil within 3 years. That means the reservoir of metal nutrients is far smaller than previous studies had suggested, Kennedy says: "We never realized trees lived this close to the edge."

The research in Chile and Kennedy's similar studies in Hawaii suggest that mature species on aging soils rely heavily on recycling and conservation of nutrients borne by rain. That's bad news for the maturing forests in the northeastern United States, where acid rain is leaching minerals out of the topsoil at alarming rates, Kennedy says.

The research supports the idea that the atmosphere might be the most important source of metal nutrients for some forest trees, says biochemist William Schlesinger of Duke University in Durham, North Carolina. He agrees with the worry about depleted nutrients: "Since the pool in these layers is small, forests are vulnerable," he says.

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