告诉你你吃的食物会造成何种如染

Looking to lead a more eco-friendly lifestyle? You could start by cutting back on meat. A new nitrogen footprint calculator shows you the impact your diet has on the environment.  You may have heard of the concept of a carbon footprint. That's how much of the greenhouse gas carbon dioxide your lifestyle generates. Now researchers have developed a way to measure how our lifestyle - and in particular our diets - impacts another major source of climate-changing pollution: nitrogen.

Everything needs nitrogen, from plants to plankton to people. It's a key element in the proteins that make up our bodies.

Good news, bad news

In the early twentieth century, scientists figured out a way to take nitrogen out of the air and turn it into a form that plants could use. The invention of synthetic nitrogen fertilizer dramatically improved food production.

"That's the good news," says environmental sciences professor Jim Galloway at the University of Virginia. "We're able to feed the world's population because of this wonderful invention."

The bad news, Galloway says, is that many parts of the world use far too much nitrogen fertilizer. Burning fossil fuels also creates nitrogen pollution. Galloway says the excess nitrogen "contributes to smog, acid rain, loss of biodiversity, dead zones along the coast, global warming, stratospheric ozone depletion. The list is quite long."

Visualizing impacts

Galloway and his colleagues developed the nitrogen footprint calculator to show how people's behaviors contribute to that long list of environmental impacts.

The calculator starts with a graph showing the amount of nitrogen pollution produced by the average person. One of the first things you notice is that food is far and away the biggest contributor: 72 percent of the 42 kilograms the average American produces comes from food consumption. Housing, transportation and goods and services make up the rest.

To see exactly how your diet affects your nitrogen footprint, the calculator lets you adjust how many times a week you eat 16 different kinds of foods.

"As you change your consumption, you can see how the graph changes," says co-creator Allison Leach at the University of Virginia. "So as you scale down one bar, you can see your nitrogen footprint shrinking."

Too much protein

Americans eat far more protein on average than the U.S. Department of Agriculture recommends, Leach says. "Reducing your protein to the recommended levels is going to have a huge impact on reducing your nitrogen footprint. [That] reduces it by almost half."

Reducing the amount of animal protein is one of the quickest ways to shrink that footprint. Galloway says that's because feeding plants to animals is an inefficient use of nitrogen.

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

Beef is less efficient than chicken or pork, while plant sources like legumes are the most efficient, Leach says.

Nitrogen footprints around the world

She and her colleagues also created nitrogen footprint calculators for Germany and the Netherlands. She says their footprints are smaller largely because they tend to eat less meat, not because of major differences in their farming practices.

"Industrialized agriculture production is pretty consistent among developed countries. But it's going to be very different in developing countries."

The next step, Leach says, is a nitrogen footprint calculator for India, a developing country where farming practices are very different from in the industrialized world.

Footprints growing

But even in the developing world, nitrogen footprints are changing. The world's population is not only growing, it's also growing richer. One consequence is that people are eating more animal protein. Galloway says they have looked at what the world might look like in 2050 - and it raises concerns.

"If the entire world had the per capita resource-use habits that people in North America have, then we would be putting three to four to five times more reactive nitrogen into the environment than we are now," he says. "And that's frankly untenable. Our ecosystems can't handle that."

Galloway says that’s why it’s so important that we all do a better job managing the amount of nitrogen we put on our fields, in our mouths, and into our environment.