This is the VOA Special English Agriculture Report.

A carbon footprint is the amount of carbon dioxide pollution that we produce as a result of our activities. Some people try to reduce their carbon footprint because they are concerned about climate change. Now, researchers have a way for people to measure how much nitrogen pollution they produce -- their nitrogen footprint.

All plants and animals need nitrogen. Nitrogen is a major element in the proteins in our bodies. The atmosphere is mostly nitrogen.

In the early 20th century, scientists learned how to take nitrogen out of the air and make it into a form that plants could use. University of Virginia professor Jim Galloway calls synthetic nitrogen fertilizer a "wonderful invention" because it increased food production. But he says many parts of the world use too much nitrogen fertilizer, and that harms the environment.

JIM GALLOWAY: "It contributes to smog, acid rain, loss of biodiversity, dead zones along the coast, global warming, stratospheric ozone depletion. The list is quite long."

So Professor Galloway and other researchers have developed a nitrogen footprint calculator. This Web-based tool asks people about the foods they eat and questions like how much they fly and drive and how big a house they live in.

Researchers say the average American produces 42 kilograms of nitrogen pollution a year. Some of that comes from fossil fuels like oil and coal. But more than 70 percent involves food.

University of Virginia researcher Allison Leach says Americans eat almost twice as much protein as the government recommends. She says eating only the recommended levels could reduce nitrogen footprints by almost half.

Professor Galloway says animal protein plays a big part. He points to the example of cattle that are given feed grown with nitrogen fertilizer.

JIM GALLOWAY: "For large animals like beef, a very large fraction of the nitrogen that enters the cow's mouth is excreted out the back end."

Last year, a study supported by American beef producers found that their industry's environmental impact has decreased over the past 30 years. Jude Capper at Washington State University reported that by producing more beef from fewer animals, the industry cut its carbon footprint by 18 percent.

The nitrogen calculator is currently designed for people in the United States, Germany and the Netherlands. Another online calculator will be designed for India.

And that's the VOA Special English Agriculture Report, written by Jerilyn Watson and Steve Baragona. For a link to the nitrogen footprint calculator, go to voaspecialenglish.com. I'm Jim Tedder.

ozone depletion: 臭氧消耗；臭氧枯竭

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（來源：VOA 編輯：崔旭燕）