Scientists have recently found humanity's nitrogen footprint on watersheds once thought to be isolated and pristine, indicating our impact on the world is more widespread than previously imagined.
These findings show that natural nitrogen equilibriums have shifted drastically and are now driven largely by human factors. This world awash in nitrogen can have devastating effects on health, ecology and the climate.
Nitrogen is an important element to all life as we know it. However, its various forms are not equal in how they affect the environment. Nitrous oxide, for example, is an especially potent greenhouse gas. According to U.S. EPA, it is 300 times more powerful than carbon dioxide at trapping heat, and it can stay in the atmosphere 300 years.
Meanwhile, nitrogen gas, which makes up close to 80 percent of the air we breathe, is relatively inert. Other forms, like nitrates, form aerosols in the atmosphere that can seed clouds, which generally have a net cooling effect on the climate.
Nitrates, nitrites and nitrous oxide can arise naturally, or they can come from human activities. Many of these compounds are byproducts of combustion, whether in cars and trucks or factories and power plants. The largest source, however, is agriculture. As the world's population surges toward 7 billion people, many see nitrogen fertilizers as the only way to increase crop yields enough to feed a hungry planet.
But what's really surprising is the scale of these impacts. According to a report published last month in the journal Science, researchers found that human activities produce more reactive nitrogen than all of nature combined.
"Almost certainly, more new fixed nitrogen and utilizable nitrogen is produced by the sum total of human activity than by the sum total of natural processes," said Alexander Wolfe, a professor of Earth sciences at the University of Alberta, who co-authored the paper. "It turns out the nitrogen cycle is more pervasively controlled by humanity" than previously thought.
A big component is liquid fertilizer
Fixed nitrogen refers to nitrogen that is taken from the air and turned into more reactive forms like ammonia, either by bacteria or by man-made techniques. Humanity's impacts, however, are so vast that nitrogen compounds from combustion and agriculture have found their way into lake beds isolated from farms, roads and suburban neighborhoods.
"We see this definite human fingerprint, and now there's nowhere on the planet that's completely untouched by humans," said Gordon Holtgrieve, a postdoctoral researcher in the School of Aquatic and Fishery Sciences at the University of Washington.
While studying salmon in lakes, Holtgrieve, also a co-author, found that the nitrogen in the lake beds was lighter than he expected. Nitrogen comes in several isotopes, differing in the number of neutrons in their nuclei. Nitrogen produced through burning gasoline or making fertilizer tends to use lighter isotopes, he explained. Examining 25 lakes and ice cores in the United States, Canada and north of the Arctic Circle in Greenland, Holtgrieve found these distinct anthropogenic signatures dating back to 1895.
Much of this nitrogen can be attributed to the Haber-Bosch process, which is used around the world to help make 500 million tons of fertilizer annually by pulling nitrogen gas out of the air and converting it to ammonia and other forms. This allows farmers to raise crops more effectively, increasing yields.
However, about half of all nitrogen for agriculture ends up running off the soil and into the air or water. Runoff depletes oxygen in oceans and rivers, creating dead zones. It can also lead to large blooms of algae that choke out other species while contaminating drinking water. In addition, excess fertilizer spraying increases reactive nitrogen in the atmosphere.
Eric Davidson, executive director and senior scientist at the Woods Hole Research Center, co-authored a study in the journal Issues in Ecology last month that evaluated impacts from excess nitrogen in the United States. Nitrogen compounds don't just affect nature; according to Davidson, they can harm human health. Nitrates in water can lead to birth defects and are linked to cancer.
Climate change could make control harder
Nitrogen oxides from vehicles can create ozone and small particles in the air, making it more difficult to breathe, Davidson said.
As global weather patterns shift, these issues could get worse. "With climate change, it will be more difficult to manage nutrients well," said Davidson. "Extreme weather may drive up fertilizer use by making it more difficult to raise crops."
However, it's not clear whether nitrogen compounds will have a net warming or cooling effect on the global climate on their own. They generally trap heat in the upper atmosphere, but they also create clouds and haze, which reflect solar radiation back into space. Excess nitrogen falling from the sky also helps the growth of forests, which collect and sequester greenhouse gases. These factors make it difficult to calculate nitrogen's effects in climate models.
Nonetheless, the United States has made some progress in addressing nitrogen emissions. The Clean Air Act, passed in 1963, forced automobile manufacturers to make cleaner-burning engines. "That's the bright spot in the picture. There's been tremendous improvement in the technology for catalytic converters. Nitrogen emissions have decreased in this country since the passage of the Clean Air Act and its improvements," said Davidson.
As for agriculture, policymakers need to balance the short-term demands of keeping people fed against the long-term issues of ecological damage. "We know how to reduce losses of nitrogen from agricultural systems. The question is how to do it in an economical way," said Davidson. He suggested that some old-fashioned techniques like crop rotation and crop diversification can help replenish soils and insulate farmers from droughts and bad weather.
"A better-managed crop is also more resilient to climate change," he explained.