At first, it seems odd to think of nitrogen as pollution. After all, 78 percent of the air we breathe is nitrogen. But that nitrogen is inert. What worries Weiss and others is so-called "reactive" nitrogen.

The compounds that belong to this category -- nitrogen dioxide, ammonia, nitrate, nitric acid -- are biologically and chemically labile. They bounce from the land to the atmosphere to the ocean and back to the atmosphere again, triggering a complex sequence of chemical reactions. "The nitrogen cascade," University of Virginia environmental scientist James Galloway terms it. Among the cascade's byproducts is low-level ozone, which can be toxic to humans as well as trees.

Both plants and animals require moderate amounts of reactive nitrogen in order to synthesize protein. A judicious addition of nitrogen greatly increases crop yields. But an overload can strip soils of calcium and magnesium, leading to lower fertility as well as acidification. Excess nitrogen can also lead to eutrophication -- literally, an enrichment of nutrients -- which disrupts both terrestrial and aquatic ecosystems by spurring the growth of some species at the expense of others.

Nature doles out reactive nitrogen parsimoniously. Lightning, for example, produces enough heat to drive the production of nitric oxide. In addition, many microbes release reactive nitrogen as they decompose organic matter. Microbes that colonize the roots of legumes such as soybeans have the additional knack of taking nitrogen from the air and converting it into a form usable by plants.

Around 1970, human emissions of reactive nitrogen surpassed terrestrial emissions from natural sources, then kept on going. At present, human-related sources release around 190 million metric tons of reactive nitrogen per year, compared to an estimated 90 to 120 million metric tons for natural sources. By 2050, our annual contribution could top 270 million metric tons.

"Of all the chemical cycles essential to life on earth, the one we've changed the most is the nitrogen cycle," says Cornell University biogeochemist Jed Sparks.

Reactive nitrogen makes its way into the biosphere in myriad ways. Some of it flows directly into rivers and streams from agricultural runoff and urban sewage spills. Increasingly, though, it wafts into the atmosphere from exhaust pipes, power plants and factories, as well as from fields doused with ammonia-based fertilizer and manure piles associated with cattle feedlots and dairy farms.

As a result, biologically significant quantities of reactive nitrogen are now reaching the highest places. In the Colorado Rockies, reactive nitrogen has upped the metabolic activity of certain soil microbes and overturned once-stable communities of algae in high-altitude lakes. The plants that compose the alpine tundra are also responding. Some species -- native bunchgrasses, alpine bluebells -- clearly like the extra jolt. Others, however, appear to be losing ground, among them a slow-growing bog sedge.

At lower elevations in the West, introduced grasses stoked by nitrogen are overwhelming many ecosystems, including the coastal scrub of Southern California. Nitrogen is also implicated in the grass invasion occurring in the Mojave Desert, downwind of the smog-filled Los Angeles basin. The grasses expand their range during the winter rainy season, and then, in summer, once the rains end, they dry up. And then, quite often, they burn.