When Nitrate Reigns: Air Pollution Can Damage Forests More Than Trees Reveal

By Raloff, Janet | Science News, February 11, 1995 | Go to article overview

When Nitrate Reigns: Air Pollution Can Damage Forests More Than Trees Reveal


Raloff, Janet, Science News


As all good foresters know, trees crave nitrogen. No matter how much you give them, forests just grow and ask for more.

In fact, 6 years ago, Ernst-Detlef Schulze, a plant ecologist at Bayreuth University in Germany, made a big splash in the world forestry community by tying the woodland's appetite for nitrogen to northern Europe's acid-rain-induced timber die-offs.

Schulze argued compellingly that an acid rain of nitrates and other readily digestible forms of nitrogen stimulated trees to shunt unhealthy amounts of energy into growth. These trees, already suffering from pollution-induced chemical imbalances, in time became too weak to handle even the normal vicissitudes of daily life -- insects, fungal blights, and extremes of weather (SN: 7/22/89, p.56).

Imagine Schulze's shock, then, when he learned last year that much of the nitrate raining down on the upper forested slopes in Germany's Fichtelgebirge -- a range of mountains within 25 miles of the Czech border -- is not taken up by trees or nitrogen-hungry microbes in the soil.

If atmospheric nitrate can pass through these forests "without being touched by biology," he says, "something is going wrong. Nitrate is passing through a soil with all sorts of organisms, and none of them is interested in [this nutrient! any longer."

At a minimum, Schulze's new finding confounds scientists' understanding of how nitrogen cycles through woodlands. But even more provocatively, it suggests that apparently healthy trees can in fact be on the verge of collapse.

Many foresters read these data, collected by Schulze and his colleagues from heavily polluted stands of Norway spruce, as signaling that these woodlands can become saturated with nitrogen in a self-accelerating process that poses a threat to forest vitality worldwide. Some also suspect the timber will adapt to nitrogen saturation -- by growing more slowly.

This would force the wood-products industry to wait longer for such trees to reach harvestable size. More troubling to the environmental community, these forests would become increasingly ineffective at sopping up the carbon dioxide releases that threaten to trigger a global warming.

One neutron spells the difference between two naturally occurring forms (or isotopes) of nitrogen: nitrogen-14 and nitrogen-15. Similarly, a pair of neutrons distinguishes oxygen from 18 its lighter sibling, oxygen-16. For reasons not now fully understood, the nitrate ([NO.sub.3]) in combustion-generated air pollution has more nitrogen-15 and oxygen-18 than does the nitrate produced by soil bacteria.

Several research groups have observed nitrate in water draining out of forest soils recently. In theory, there should be little, if any, present. To investigate this curiosity, Schulze and his coworkers employed a novel technique: They determined the ratios of those heavy and light isotopes of nitrogen and oxygen in spring-fed streams.

Because heavy nitrate comes only from air pollution and light nitrate only from soil microbes, Schulze explains, "we could now trace that nitrate [leaving forests] to distinct sources," either soil bacteria or air pollution.

The Germans found that 16 to 20 percent of the nitrate leaving two healthy-looking Fichtelgebirge stands bore air pollution's signature, they report in the Dec. 22, 1994 NATURE. In three "slightly declining" stands of spruce, 23 to 30 percent of the nitrate in runoff came from air pollution. But the isotope tracers showed that fully 60 to 100 percent of the nitrate leaving two dying stands of trees came directly from the atmosphere.

"To have this experiment turn out so cleanly says that the phenomenon the authors describe is correct . . . [that] nitrate [here] tends to pass through soils rapidly," says John Aber at the University of New Hampshire's Complex Systems Research Center in Durham.

However, he adds, this "is exactly what microbiologists would predict. …

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