The nitrogen cycle is out of balance.

As the prospect of climate change continues to bring attention to the global carbon cycle, scientists are beginning to voice concern about another key circuit: the global nitrogen cycle. Human activities are doubling the amount of nitrogen each year that is "fixed" (combined with hydrogen, oxygen, or carbon to form compounds that living things can use) on land. This large-scale intervention in the nitrogen cycle is already seriously altering ecosystems around the world, according to a panel of ecologists chaired by Stanford University's Peter Vitousek.

Nitrogen, the atmosphere's most abundant element, is an essential component of all life on earth. However, only a small amount of inert atmospheric nitrogen is converted into forms that plants can use, so it serves as an important limiting factor in many ecosystems. Soil microbes and lightning fix just 90 to 140 million tons of nitrogen per year. But humans are now adding at least 140 million tons annually by manufacturing industrial fertilizer, burning fossil fuels, and cultivating legumes, which support nitrogen-fixing bacteria. Industrial fertilizer - distinct from manure and other organic fertilizers that merely transfer already-fixed nitrogen from one place to another - contributes the most newly-fixed nitrogen, 80 million tons. And the industrial fertilizer boom is relatively recent: the amount used on crops during the 1980s alone exceeded all industrial fertilizer applied previously in human history.

This human-induced nitrogen infusion is creating various types of local, regional, and global pollution that tend to be studied separately. However, the Vitousek panel, whose findings appear in the August issue of Ecological Applications, has reviewed the whole range of problems. For instance, surplus nitrogen is contributing to increased global concentrations of nitrous oxide ([N.sub.2]O), a potent greenhouse gas in the lower atmosphere that lives long enough to travel to the upper atmosphere, where it accelerates depletion of the ozone layer. Also on the rise is nitric oxide (NO), which, as a shorter-lived gas, has local and regional effects: smog and acid rain. And as concentrations of airborne nitrogen compounds increase, more nitrogen is being deposited on land and water. Nitrogen spurs the growth of plants, but at some point, added nitrogen yields diminishing returns. Soils in many regions are now so saturated with nitrogen that nitrates are carrying away vital nutrients such as calcium and potassium into streams or groundwater. When this happens, plants adapted to excess nitrogen thrive at the expense of other species, speeding loss of certain plants and animals on land. Nitrogen that ends up in the water can also lead to decreased biological diversity, as seen by dwindling fish populations in acidified lakes throughout Scandinavia, Canada, and the northeastern United States. Overenrichment of estuaries and coastal waters with nitrogen is thought to be at least partly responsible for the surge in toxic algal blooms in the 1980s that killed huge numbers of fish and shellfish.

Given the importance of industrial fertilizer for food production, the panel acknowledges that curbing nitrogen pollution is a tremendous challenge that will require greater efficiency in farm management practices. Along with the threat of climate change, it may also lend urgency to the need to replace fossil fuels as a primary energy source. Copies of the report "Human Alteration of the Global Nitrogen Cycle" can be obtained from the Ecological Society of America, 2010 Massachusetts Ave., NW, Suite 400, Washington, DC 20036 [phone]. A summary is available on the Internet at http://www.sdsc.edu/~ESA under the heading "Issues in Ecology."