Toxic Fertility.

• Author: Nierenberg, Danielle
• Position: Nitrogen

Over the past half century, the amount of biologically active nitrogen circulating through the world's living things has probably doubled. In unnatural excess, an essential nutrient is becoming a kind of ecological poison.

LAST DECEMBER, talks on the climate treaty reached an impasse. The treaty process is supposed to result in a blueprint for reducing carbon emissions. But when delegates met in the Hague, in the Netherlands, for their sixth official conference of the parties, the agenda focused not so much on cutting fossil fuel use as on the issue of "carbon sinks." Sinks are areas, primarily young forests, that are absorbing more carbon than they are releasing. Since they draw carbon from the atmosphere, sinks offer an attractive accounting option for the United States and some other nations that have high carbon emissions. These nations want to claim a "carbon credit" against their emissions, on the strength of their sinks. How big a credit--if any--should be allowed? In one way or another, that question underlay much of the discussion, and the delegates weren't able to agree on an answer. They failed, in other words, to agree on a way to balance the global carbon budget.

Apart from the immediate reasons for concern over this failure, there is the matter of another unbalanced natural budget. Nitrogen, like carbon, plays a key role in the vast biochemical cycles of life. And increasingly, the nitrogen cycle is being reshaped by human activity--a process that could eventually affect virtually every ecosystem on earth. Our economies are in urgent need of a "nitrogen audit."

Like carbon, nitrogen is a basic ingredient of living things. It's found, for example, in DNA, in proteins, and in chlorophyll, the pigment that drives photosynthesis. Nitrogen shares another key characteristic of carbon: it's very common. It comprises a whopping 78 percent of the atmosphere. But nearly all of this atmospheric nitrogen is elemental dinitrogen, or [N.sub.2]--it exists in the form of two nitrogen atoms linked together. Elemental nitrogen cannot be metabolized by most living things. Nitrogen becomes biologically active only when it is "fixed."-- that is, incorporated into certain other molecules, primarily ammonium ([NH.sub.4]) and nitrate ([NO.sub.3]). Fixed nitrogen flows throughout the food web: it is absorbed first by plants, then by plant-eating animals, then by their parasites and predators. Death at each stage of the way releases nitrogen compounds to begin the cycle anew. The fixation process is what makes the nitrogen cycle so different from the carbon cycle. Despite the abundance of e lemental nitrogen, fixed nitrogen is frequently what scientists call a "limiting nutrient." Under normal natural conditions, it is often in short supply, so the level of available nitrogen is a key regulator of ecological processes.

The gate-keepers to the biological part of the nitrogen cycle are certain micro-organisms capable of fixing elemental atmospheric nitrogen. Some of these organisms live in soil, often in close association with plants that belong to the bean family. This relationship benefits both parties: the plants get the nitrogen compounds; the microbes get carbohydrates, which the plants produce through photosynthesis. (Sometimes the plants themselves are said to be nitrogen fixing, but this is a kind of terminological shorthand.) Nitrogen fixing occurs in water as well. One of the biggest mysteries of the nitrogen cycle involves marine plankton. These microscopic plants are fixing enormous quantities of nitrogen, but their role in the global cycle has yet to be clearly defined. Finally, in addition to these living portals, there is an inanimate natural process that fixes large quantities of nitrogen: lightning fuses nitrogen and oxygen to create nitrate.

Recent human activity has greatly increased the rate at which nitrogen is being fixed. Since the 1950s, the amount of nitrogen circulating through living things is thought to have doubled. And increasingly, in forests and fields, in rivers and along the coasts, scientists are blaming excess fixed nitrogen for a range of ecological problems--some of them obvious, others very subtle. Any one of these problems can usually be linked to some local or regional cause; the nitrogen balance of a river, for example, might be upset by increased sewage outflow. But when you step back and look at the cycle from a global perspective, three general activities emerge as the primary reasons for the growing fixed nitrogen glut.

First, coal and oil combustion is releasing a huge, long-buried reservoir of fixed nitrogen by burning the residues of ancient plants, in the form of coal and oil. The fossil fuel economy is disrupting not just the carbon cycle but the nitrogen cycle as well. Second, the progressive destruction of forests and wetlands is releasing the nitrogen contained in these natural areas, just as it releases the carbon. Taken together, these two activities are releasing about 90 million tons of fixed nitrogen annually; that's about 43 percent of the human addition to the nitrogen cycle. (See table below.)

The remainder of the human addition--some 120 million tons--comes from agriculture. Nitrogen-fixing crops produce about a third of that amount; the rest comes from artificial fertilizer. Fixed nitrogen is the basic component of fertilizer. Through its dependence on artificial fertilizer, modern conventional agriculture has become, in a sense, a form of industrial nitrogen management. This is a relatively recent development in agricultural history. Low-cost techniques for synthesizing ammonia emerged shortly after the Second World War. Cheap ammonia led to mass production of artificial fertilizer and heralded what the ecologist and nitrogen expert David Tilman has called "the 35 most glorious years of agricultural production."

For farmers in the industrialized countries--and increasingly, in the developing countries as well--this limiting nutrient is now available in virtually limitless quantities. As is typical of cheap commodities, a great deal of it is wasted. Fertilizer is often very inefficiently applied; much of it never reaches the crop. It leaches out of the fields and into the streams, or it's converted into a nitrogenous gas like nitrous oxide and escapes into the atmosphere.

Nearly all crops grown in the industrialized countries are now nitrogen-saturated--that is, they're being exposed to more nitrogen than they can use. But fertilizer production...