Scientific American, June 1997



A little nitrogen is nice, but too much is toxic

If global warming seems ominous, consider this new
assessment of how humans have disrupted the natural
cycling of nitrogen. By using fertilizers, burning fossil
fuels and cultivating crops that convert nitrogen into
forms plants can use, humankind has over the past
century doubled the total amount of atmospheric
nitrogen that is converted, or fixed, every year on land.
The nitrogen glut is already causing "serious" loss of soil
nutrients, acidification of rivers and lakes, and rising
atmospheric concentrations of the greenhouse gas
nitrous oxide. Moreover, the oversupply probably
explains decreases in the number of species in some
habitats, as well as long-term declines in marine fish
catches and, in part, the algal blooms that are an
unwelcome spectacle in many coastal areas.

That alarming evaluation, to be formally published this
summer in "Ecological Applications," is the work of
eight senior ecologists chaired by Peter M. Vitousek of
Stanford University. Their study identifies as the chief
culprit the industrial fixation of nitrogen gas to make
fertilizer. "The immediacy and rapidity of the recent
increase of nitrogen fixation is difficult to overstate," the
researchers say. More than half the nitrogen fertilizer
ever made before 1990 was used during the single
decade of the 1980s, they note.

Industry now fixes 80 million metric tons of nitrogen
every year to make fertilizer. Leguminous crops, which
harbor nitrogen-fixing bacteria, and fossil fuels, which
liberate nitrogen compounds when burned, together
make another 60 million tons of nitrogen available to
living things. The natural global rate of nitrogen fixation
on land is between 90 and 140 million metric tons, and
the excess stimulates plant growth. Moreover, by
clearing forests and draining wetlands humans make the
situation worse, because those activities liberate
nitrogen that would otherwise be stored.

The Environmental Protection Agency, recognizing the
damage caused by nitrogen oxides from combustion,
has introduced regulations to limit by several million tons
emissions from power stations and other industrial
plants. And it is negotiating further limits on the already
tightly controlled amounts emitted by vehicles. But there
are no effective federal controls on the amount of
fertilizer a farmer can use. "It is my feeling that this is an
emerging issue," says Gary T. Gardner of the
Worldwatch Institute. Gardner asserts that demand for
industrially produced fertilizer could be reduced if
farmers instead put on their fields recovered municipal
food and yard waste, rich sources of nitrogen that
together make up a third of the waste volume.

Employing fertilizers more efficiently might be "our best
hope for doing something," Vitousek suggests. The
Sierra Club Legal Defense Fund is pressuring the EPA
to limit runoff into the Mississippi, which the litigation
group contends is responsible for a 7,000-square-mile
"dead zone" that appears every summer in the Gulf of
Mexico. Reductions are possible: some states, including
Arizona, have initiated successful incentive programs to
lower fertilizer runoff. And some U.S. farmers have
reduced fertilizer consumption voluntarily.

A spokesman for the Fertilizer Institute in Washington,
D.C., a manufacturers organization, says industry is
already developing ways of getting more growth from
less fertilizer. But assessments such as Vitousek's report
should, the institute argues, acknowledge the rapid
escalation in the human population's demand for food. It
points out that global nitrogen fertilizer use in 1995 was
down 3 percent from the peak year of 1988--although
it apparently is on the rise again. Those numbers may
need closer scrutiny as the global population zooms to
an estimated 10 billion during the next century.

--Tim Beardsley in Washington, D.C.