Soon after Dr. Vitousek's report, the journal Geophysical Research Letters branded as a "missing greenhouse gas" nitrogen trifluoride, which is used in production of semiconductors and in liquid-crystal displays found in many electronics. According to the report, it causes more global warming than coal-fired plants. Nitrogen trifluoride, which is not one of the six gases covered by the Kyoto Protocol, the celebrated international global warming accord, is about 17,000 times more potent than carbon dioxide. Its estimated worldwide release into the atmosphere this year is equivalent to the total global-warming emissions from Austria.

    "The nitrogen dilemma," Dr. Vitousek added, "is not just thinking that carbon is all that matters. But also thinking that global warming is the only environmental issue. The weakening of biodiversity, the pollution of rivers, these are local issues that need local attention. Smog. Acid rain. Coasts. Forests. It's all nitrogen."

    Dr. Vitousek's summer report followed a similar account in May in the journal Science by James N. Galloway, an environmental sciences professor at the University of Virginia and a former chairman of the International Nitrogen Initiative, a group of scientists pushing for smarter use of nitrogen.

    Dr. Galloway is developing a universal calculator for individual nitrogen footprints. "It's Goldilocks's problem," he said in an interview. "Reactive nitrogen isn't a waste product. We need it desperately. Just not too much and not too little. It's just more complicated than carbon." He continued, "But we're not going to get anywhere telling people this is simple or easy."

    Dr. Giblin of Woods Hole spent the summer at the field station here, midway between the Arctic Circle and the Arctic Ocean, researching the nitrogen content of lakebed sediment - not the inert nitrogen that makes up 80 percent of air, the reactive nitrogen that Dr. Galloway referred to. In forms like nitric acid, nitrous oxide, ammonia and nitrate it plays a variety of roles.

    Nitrogen is part of all living matter. When plants and animals die, their nitrogen is passed into soil and the nitrogen in the soil, in turn, nourishes plants on land and seeps into bodies of water. Dr. Giblin is pursuing her research because as the Arctic warms, the tundra's permafrost will thaw, and the soil will release carbon and nitrogen into the atmosphere.

    When an ecosystem has too much nitrogen, the first response is that life blossoms. More fish, more plants, more everything. But this quickly becomes a kind of nitrogen cancer. Waters cloud and are overrun with foul-smelling algae blooms that can cause toxic "dead zones." Scientists call this process eutrophication, but the laymen's translation is that the water gets mucked up beyond all recognition. A recent such plague bedeviled China when its Yellow Sea was smothered in algae at Qingdao, the planned site of Olympic sailing events this summer. More than mere inconvenience, such problems routinely threaten many coastal areas and riverside communities.

    Nancy Rabalais, executive director of the Louisiana Universities Marine Consortium, is known as Queen of the Dead Zone. She cruises around the Gulf of Mexico every summer in the research vessel Pelican to look for damage from nitrogen-rich river flows into the gulf. This year, she expects a dead zone that will beat the Massachusetts-size 8,500-square-mile bloom of 2002.

    One of the problems, Dr. Rabalais said, is that the Mississippi River involves so many communities that it requires stronger federal guidance, which she said was not a part of the Bush administration's policies. She is part of a national research committee financed by the Environmental Protection Agency and run by the National Academies of Science, but, she said, "it's so much talk and not enough action."

    She continued: "Because you're not just going up against the agribusiness lobby, but also the livelihood of farmers. It's not exactly popular in the Midwest."

    Fertilizer use is largely inefficient. With beef, only about 6 percent of nitrogen used in raising cows ends up in their meat; the rest leeches out into air or water supplies. With pork, it is 12 percent; chicken, 25 percent. Milk, eggs and grain have the highest efficiency, about 35 percent, or half of what, in the metric of report cards, is a C-minus.

    "Look," she said, "you just can't have all these states and all these communities knowingly overfertilizing their land because they want a bumper crop every year. That's just all kinds of bad. But Des Moines, for example, is willing to filter their drinking water to an extra degree just to be able to flood their water supply with more-than-normal levels of fertilizer."

    Reactive nitrogen competes with greenhouse gases that have greater public awareness. "But it's like looking at malaria and AIDS in Africa," Dr. Rabalais said. "They're both problems. And they both need vigilant attention."

    Environmentalists face the puzzle of how to deal with multiple problems at once. And some worry that after the hard-fought campaign spotlighting carbon, turning to focus on nitrogen could upset that momentum.

    The tension can plague even the most informed and articulate campaigners. "One of the many complexities that complicate the task I've undertaken is complexity," said Al Gore, the former vice president who won a Noble Peace Prize for his environmental work. Mr. Gore added, "Look, I can start a talk by saying, 'There are 14 global warming pollutants, and we have a different solution for addressing each of them.' And it's true. But you start to lose people."

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