Meat has a greater impact on the environment than pretty much any other food we eat. As The Salt has reported, billions of cows, pigs, sheep and poultry we raise as livestock guzzle massive quantities of water and generate at least 10 percent of the total greenhouse gases attributed to human activity.

But scientists say we've been slow to acknowledge yet another side effect of our taste for meat: nitrogen pollution.

Nitrogen is an essential nutrient and a major component of the air we breathe every day. However, certain forms of nitrogen, when released into the environment, cause a host of problems from contaminating drinking water to destroying the ozone. And most of this problematic nitrogen comes from agriculture, especially from producing meat.

That's because farmers apply nitrogen fertilizer to crops to help them grow, but only about half of that nitrogen is taken up by plants. The rest can leak out into the environment. And because cows, pigs and poultry gobble up heaps of corn and soy feed, more nitrogen fertilizer is applied — and emitted — in the process of producing meat and dairy than other foods. In total, when the additional pollution from nitrogen-rich manure is accounted for, raising beef produces almost 16 times as much nitrogen pollution as growing the same amount of bean protein, scientists say.

But consumers remain largely unaware of this, says James Galloway, a leading nitrogen expert at the University of Virginia. "Everybody knows about carbon, but not so much about nitrogen, in part, because it's complicated," he says. So, with the help of Allison Leach, a former student who's now a Ph.D. student at the University of New Hampshire, Galloway decided to create , a calculator to help consumers understand their role in the nitrogen story.

Like other footprint calculators you might have seen for carbon and water, N-print works by asking you about your diet and energy use and then calculates how much nitrogen they produce. Food accounts for roughly two thirds of the average American's nitrogen emissions, and burning fossil fuels also releases nitrogen oxides, a major component in urban smog, which makes up the remaining third. The calculator — which was created in 2011 and is getting an update and redesign in March — compares you to other consumers around the world.

Although it's only an estimate, Leach and Galloway hope the calculator will help consumers see the benefits of changing their behavior. For instance, if you are like most Americans, you eat about 1.4 pounds of protein per week, two thirds of which come from meat and dairy. But the researchers show that you could cut your nitrogen footprint by more than 40 percent just by reducing your total protein intake to 0.8 pounds, the amount recommended by the U.S. Department of Agriculture and the National Academy of Sciences.

Nitrogen pollution isn't limited to meat production, of course. Fertilizer leakage also occurs in vegetable fields. However, the researchers find that consumers can most efficiently reduce their footprint by changing the kinds of protein they eat to include more plants and meats like fish and poultry, which are less nitrogen intensive. Cutting down on the estimated 30 percent of food that goes to waste could also have a big impact. Currently, nitrogen is lost to food waste at several stops along the supply chain, including in processing, at the retailer, and at home. And some is also lost through sewage.

Natural ecosystems have evolved to make due with very little nitrogen. Although nitrogen gas makes up nearly 80 percent of Earth's atmosphere, that form of nitrogen is useless to most organisms, except certain bacteria that can convert — or "fix" — nitrogen into usable compounds. For centuries, farmers only had two options for boosting the amount of nitrogen available to their crops: occasionally growing legumes like beans, which host nitrogen-fixing bacteria and enrich the soil, or adding manure and compost to their fields.

In the early 1900s, however, researchers invented a way to produce virtually unlimited supplies of nitrogen fertilizer, sowing the seeds of the Green Revolution. This synthetic fertilizer helped feed the world, but it also unleashed an environmental disaster, says Benjamin Bodirsky, a nitrogen expert at the Potsdam Institute for Climate Impact Research in Germany. Humans have doubled the amount of biologically available nitrogen on Earth, and much of it has spread from fields into nitrogen-starved ecosystems.

In aquatic environments, this excess nitrogen can fuel massive algal blooms that suck oxygen out of the water, killing fish and other critters. Nitrogen can also reshuffle entire ecosystems by directly harming plants and animals or by giving invasive competitors an edge. A study appearing this week in the journal BioScience argues nitrogen pollution is also a big threat to global biodiversity. The researchers found that almost 80 species listed under the Endangered Species Act are affected by nitrogen pollution.

It also harms human health. In drinking water, high concentrations of nitrates can cause a potentially fatal blood disorder known as "blue baby syndrome," among other health effects. In the air, nitrogen pollution causes respiratory issues and erodes the ozone layer, which protects us from dangerous UV radiation.

And according to a recent study in Nature Geoscience, the effects of nitrogen pollution are not equally distributed. By tracing nitrogen emissions through our increasingly interconnected global trade system, researchers found that net exporters — mostly developing countries — suffered from nitrogen pollution associated with goods and services imported by developed countries. In fact, consumption in just four countries — China, India, Brazil, and the United States — accounted for almost half of the world's nitrogen emissions.

But Galloway says consumers "can put purchasing pressure back up food chain to the retailer, the processor, the grower," to be more conscientious about nitrogen. To that end, Leach and Galloway are now developing an environmental footprint label, similar to a nutrition label, which includes the item's nitrogen footprint and could help consumers make more sustainable choices.

Many of the ways researchers propose to address nitrogen pollution — like eating less meat and reducing food waste — could also help shrink the carbon and water footprints of our diets.

"We also need better management by farmers," Bodirsky says. He says huge gains could be made by increasing how much nitrogen fertilizer plants take up and by limiting leakage, both of which will save farmers money in the long run. Measures could be as simple as monitoring soil nutrient levels and timing fertilizer applications so they don't coincide with rain, he says.

In the end, reducing nitrogen pollution requires a coordinated effort across the food chain, Galloway says. "At every step along the way, you increase the efficiencies as much as you can."

Julia Rosen is a freelance science journalist based in Portland, Ore.

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