Global Nitrogen: Cycling out of Control

Scott Fields

Environ Health Perspect. 2004;112(10) 

In This Article

Regaining Control

Reducing the amount of reactive nitrogen that is added to the environment is critical, Galloway says. Of the nitrogen that is created to sustain food production, only about 2-10% enters the human mouth, depending on the region. The rest, he says, is lost to the environment: "Unless an equivalent amount is denitrified back to molecular N2, then that means reactive nitrogen is accumulating in the environment, in the atmosphere, in the groundwater, in the soils, in the biota."

Some solutions are at best long-term, or simply unlikely. If many of the world's meat-eaters were to switch to a largely vegetarian diet, Townsend says, farmers could plant far less nitrogen-stoked grain, most of which goes to animal feed and sweeteners. But meat consumption in the United States and Asia is rising rather than falling. It has also been suggested that symbiotic bacteria could someday be genetically engineered to bestow grains directly with nitrogen-fixing capability.

A more practical, low-tech, low-cost solution is to improve the ways farmers rotate crops and fertilize their lands, says Stanford University Earth science professor Pamela Matson. In the American Midwest, for example, it's common for farmers to fertilize their fields in the fall. Winter snow and spring thaw wash away far more fertilizer than stays in the soil. Many farmers in all regions that have especially unpredictable weather intentionally overfertilize, she says, rather than run the risk of running short of nutrients in a year in which conditions would otherwise result in a bumper crop. The alternative, which Matson says some farmers manage well, is to add exactly the right amount of fertilizer exactly when it is needed.

In an effort to better understand the problems associated with changes in the nitrogen cycle and reduce their negative impacts, the Swedish-based International Geosphere-Biosphere Programme and the French-based Scientific Committee on Problems of the Environment have teamed up to support the International Nitrogen Initiative (INI). This international project is planned as a three-phase effort to assess the state of the knowledge of nitrogen flows and problems, develop region-specific strategies, and put those strategies into place, with regional centers to be established to carry out these goals. The INI will cosponsor the Third International Nitrogen Conference, scheduled for 12-16 October 2004 in Nanjing, China. There, scientists will focus on the problems specific to Asia and examine options for increasing food and energy production while reducing nitrogen pollution. During this meeting, the INI Scientific Advisory Committee will meet to plan one or more regional centers for Asia.

Experts warn that nitrogen-saturated soils may not be able to keep the excess from the environment. image credit: Jack Dykinga/USDA

Ultimately, however, the answer is to regulate reactive nitrogen the same as other pollutants, Likens says. In Europe, regulations have helped reduce nitrogen pollution, Galloway says. But the United States--not to mention developing nations--has a long way to go, not just in developing regulations, but in understanding the dynamics of the nitrogen cycle, Galloway says.

He cites the example of federal regulations to reduce nitrogen losses from hog farms. "A lagoon system was mandated to decrease reactive nitrogen-containing waste release into waters. The waste was stored in these big lagoons and then aerated--which released ammonia to the atmosphere--and the sludge was spread onto fields to grow cover crops," he explains. The system works insofar as it keeps the nitrogen out of the rivers fairly well. "But it just transfers [the nitrogen] to the atmosphere," Galloway says. "You need to have an integrated management policy."

We know the global nitrogen system is being disrupted, Galloway says. "What we don't know is the rate that nitrogen is accumulating. And because reactive nitrogen contributes to many environmental issues of the day, the more you have, the faster the rate of accumulation, and the more you're going to have an increase in the effects and distribution of the effects."

"Humans are changing the nitrogen cycle globally faster than any other major biogeochemical cycle--it's just going through the roof in a hurry," Townsend says. "The problems with that are remarkably diverse and widespread, and we really need to do something about it. But I think the good news is that there are a lot of ways to envision that we could do something about it without utterly turning socioeconomic systems on their ear."