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Humans have doubled Earth's usable nitrogen, scientists say

A glut of nitrogen is wreaking biologic havoc worldwide ­ threatening human health, killing plants and damaging fisheries, according to a new report by a panel of ecologists led by Stanford biological sciences Professor Peter Vitousek.

The problem is an upset of one of Earth's fundamental cycles. In the last decade, humans have doubled the rate that nitrogen gas from the air is "fixed" changed into chemical forms that most plants and animals can use.

"Fixed nitrogen is essential for all life, but the added nitrogen is literally too much of a good thing," said Vitousek, the Clifford G. Morrison Professor of Population and Resource Studies.

Instead of a boon, usable nitrogen is becoming a curse ­ leaching soils of other nutrients, throwing smog into the air and clogging the coastal ocean with nitrogen-loving algae.

In some locations, such as in some northern European forests, the increase in nitrogen is up 10-fold. "The amount of nitrogen splashing around in northern Europe is really extraordinary," Vitousek said. "The rain there supplies all the land with as much fixed nitrogen each year as many farmers apply to their crops."

Solutions to the problem will not come easily, especially since the chief culprit is the nitrogen fertilizer used to boost crop yields for an ever-hungrier human population.

Vitousek and his colleagues report their conclusions in a scientific review requested by the Ecological Society of America. The society is publishing a summary this month.

Scientists have known for some time that nitrogen levels were creeping up, but the panel's findings about the magnitude of the increase and the global range of its effects have caused a stir. In her presidential address to the American Association for the Advancement of Science in February, Oregon State marine ecologist Jane Lubchenco cited the Vitousek study as an example of the rapidly shifting state of the global environment.

"Environmental changes occurring now are so different in scale, in rate and in kind from those of the past that the past offers little insight into likely responses," she said.

What scientists know so far

This month's summary of Vitousek's report, "Human Alteration of the Global Nitrogen Cycle: Causes and Consequences," is the first in a series called "Issues in Ecology" planned by the Ecological Society of America to inform the public and policy-makers about the prevailing state of scientific knowledge about major ecological issues.

The full scientific paper has been accepted for publication in the society's journal, Ecological Applications, and will appear there in August. In addition to Vitousek, the scientists on the panel were John Aber of the University of New Hampshire; Robert Howarth of Cornell University; Gene Likens of the Institute of Ecosystem Studies in Millbrook, N.Y.; Pamela Matson of the University of California-Berkeley; David Schindler of the University of Alberta; William Schlesinger of Duke University; and David Tilman of the University of Minnesota.

They reviewed the scientific evidence to determine what could be said for certain, and what could be said with confidence, about the tipped nitrogen balance.

In doubling the rate at which nitrogen is fixed, humans have spurred a substantial increase in the production of nitrogen-containing gases, including greenhouse gases that warm the globe and reactive gases that pollute the atmosphere with smog, the panel reported. Added nitrogen and the acidification of soils, streams and lakes that it causes are causing biodiversity to plummet in some areas, and likely causing long-term declines in coastal marine fisheries.

Nitrogen in pure form has always been abundant on the planet. Overall levels of the element ­ which cannot be synthesized ­ don't change from year to year. But even though 78 percent of Earth's atmosphere is composed of pure nitrogen gas, most organisms can't get to it. To be useful to plants and animals, nitrogen in the air must first be tugged from the air and combined with hydrogen, oxygen or carbon ­ a process called "fixation," which yields usable molecules such as ammonium and nitrate. In nature, only two things can do that job: specialized bacteria and lightning.

Now human activity has inflated the nitrogen budget ­ thus millions of extra tons of nitrogen have been sprayed onto the soil and into the air via fertilizer, exhaust pipes and smokestacks.

Worldwide agricultural usage of industrial nitrogen fertilizers is largely to blame for the rapid and growing increase, the scientific panel concluded. To boost crop production, farmers fertilize their fields. "That sounds like a good thing, and it is," Vitousek said. "If that's all the nitrogen did, there wouldn't be a problem," he said. "But it moves around the biosphere."

Reactive nitrogen compounds such as nitrate and nitric oxide leak from the soil into groundwater and drift up into the atmosphere, Vitousek said. The decline in air quality due to smog contributes to an increase in the incidence of respiratory problems ­ such as exercise-induced asthma ­ in susceptible people, especially children, he said.

When fossil fuels like coal and oil are burned, they thrust nitric oxide into the atmosphere, causing not only smog but also acid rain, which pollutes streams and rivers. The panel concluded that about 80 percent of nitric oxide emissions worldwide are caused by humans

Another gas, nitrous oxide, travels up to the stratosphere and eats away at the earth's protective ozone layer. By soaking up heat on its way out of the atmosphere, nitrous oxide warms the globe just like carbon dioxide and other greenhouse gases.

Gooey-green scum

Some of the nitrogen surplus is showing up in streams, rivers and oceans. "Especially in estuaries where rivers meet the sea, it encourages algal blooms that turn water that once teemed with a melange of species to nothing but gooey-green scum," Vitousek said.

The species that cause the scum, known as "nuisance algae," gobble up the nitrogen and other nutrients and leave little or none for other species. After pilfering all the nitrogen, the nuisance algae then use up the oxygen in the water when they die and decompose. Indeed, says Vitousek, some estuaries ­ often called the "nursery beds of the sea" ­ no longer give birth to the numbers of fish and shellfish they did just a few years ago.

Added nitrogen is also implicated in the recent upsurge in dangerous "red tides," toxic algae that contaminate shellfish and can kill people and marine mammals, Vitousek said.

On land, certain weed-like grasses, like the nuisance algae at sea, make use of the nitrogen glut and squeeze out neighboring species such as heath. "There is clear evidence that the heathlands of northwestern Europe are disappearing," said Vitousek.

Putting the nitrogen budget back in order is easier said than done, since the causes are hard to live without: industrial fertilizers for crops, fossil fuel combustion, and large-scale farming of crops like soybeans and other legumes that can abet soil bacteria in fixing even more nitrogen.

However, the panel did suggest some steps that can be taken.

Restoration of wetlands, which normally act as natural "nitrogen traps," could reduce the flow of nitrogen into streams and estuaries, the panel reported. Better fossil fuel combustion technologies must be devised and implemented ­ especially in developing countries, which according to one study are expected to more than double the amount of their nitric oxide emissions in the atmosphere over the next 25 years.

But inefficient fertilizer usage continues to be the biggest source of excess nitrogen, Vitousek said. Currently less than half of the nitrogen in fertilizers scattered onto croplands actually fertilizes the crops; "the rest is a waste and an environmental problem," he said.

Vitousek called for international efforts to share nitrogen-efficient agricultural techniques that are also highly productive. Fertilizer usage in developed countries has become more efficient, but much fertilization remains inefficient in developing countries. The rate of use of nitrogen fertilizer in those countries is rising dramatically.

"Often more efficient techniques exist, such as applying fertilizer in small applications, but they aren't widely used," he said. "These aren't new ideas, they just require some focused effort."


By Alison Davis

Alison Davis is a science writing intern with Stanford News Service.