As a scientist, it's the questions that keep me up at night.

When chemical nitrogen fertilizer is applied to crops, what happens to the nitrogen that isn't absorbed? When nitrogen is emitted to the atmosphere from fossil fuel combustion, where does it go, and what does it do?

Tackling these questions has been a major focus of my research, and that of a dedicated cadre of scientists around the globe, since the 1980s.

Several years ago, the U.S. Environmental Protection Agency Science Advisory Board recognized this major environmental challenge and gathered some of the nation's top scientists to examine the issue. I've been on the committee for five years and its chairman for three. This week, the Science Advisory Board is providing the EPA administrator with a seminal report. It's the first time nitrogen's impacts on the United States have been gathered in one place, and the findings are staggering.

Nitrogen, as any farmer or gardener knows, is essential to plant life. But there's a limited amount available naturally, so 100 years ago, chemists developed a way to turn inert nitrogen gas in the atmosphere into reactive, biologically available nitrogen in the form of ammonia. The widespread use of this chemical nitrogen fertilizer to boost crop production has resulted in food to feed the world. It has also resulted in too much nitrogen getting into waterways and our air, essentially adding unwanted, unneeded fertilizer to our natural systems — with disastrous results. The combustion of fossil fuels adds even more reactive nitrogen to our environment. A single atom of reactive nitrogen can contribute to aquatic dead zones, smog, acid rain, climate change and ozone depletion as it moves through the air, water and soil.

Our report, "Reactive Nitrogen in the United States: an Analysis of Inputs, Flows, Consequences, and Management Options," carefully details the many ways reactive nitrogen is wreaking havoc across the United States: smog, acid rain, algal blooms, human health problems, fish kills, polluted drinking water and the coastal "dead zones" we've spent hundreds of millions of dollars battling in theChesapeake Bay.

And while the report notes that the United States is one of the world's largest contributors to human-caused nitrogen pollution, it also shows that major decreases in nitrogen pollution can be achieved using available technology.

We've done a great job over the past decade decreasing nitrogen pollution from power plants and cars, and our report recommends ways to extend those reductions. Yet pollution from chemical fertilizers and livestock manure continues to increase.

Our report outlines how farmers can help reduce agriculture's nitrogen footprint — both to save money and to protect the environment. Our recommendations include using existing technology to grow crops and raise animals more efficiently, and creating or restoring wetlands to capture nitrogen losses.

Another solution is to plant less corn, a notoriously "leaky" crop. Less than 35 percent of the nitrogen fertilizer applied on corn fields in the United States is taken up by plants; the remainder gets into rivers and stream or reacts with the air to form nitrous oxide, a potent greenhouse gas. Reforming federal policies that incentivize the planting of corn for ethanol production would diversify our crop acreage and keep more nitrogen where it belongs: in the soil and plants. Taken together, these measures would decrease by 25 percent the amount of nitrogen to the U.S. environment. This is a great first step; more will be needed.

Farmers need support. Our report offers valuable recommendations at a time when lawmakers are considering changes to U.S. farm policy through the next Farm Bill. Whatever happens with cuts to the overall Farm Bill budget, we need to make sure we continue to fund valuable conservation programs that help farmers reduce nitrogen pollution. It's crucial that we begin to do a better job of balancing our use of nitrogen to maximize crop yields, so we can minimize its impacts on our environment and health. Knowing we're getting closer to achieving that balance would allow me to get a good night's rest, so I can move on to my next scientific question.

Jim Galloway is the associate dean for the sciences in the College and Graduate School of Arts and Sciences at the University of Virginia, and the Sidman P. Poole Professor in the Department of Environmental Sciences. His email is jng@virginia.edu.