Nitrogen makes up almost 80 percent of the air in Earth's atmosphere. But, since the 1960s, it has had a growing — and increasingly unsustainable — presence in the planet's waterways, as well.
The 1960s was when the use of chemical fertilizers began to take off. Over the years, those fertilizers washed into rivers, bays and, eventually, oceans, becoming a major contributor to coastal pollution and "dead zones."
One of the world's largest dead zones today stretches from the mouth of the Mississippi River west along Louisiana's Gulf Coast. In the Chesapeake Bay, an annual summer dead zone results in multiple fish kills a year. Meanwhile, airborne nitrogen pollution from vehicles and power plants is affecting inland, high-altitude lakes, according to a study in last week's issue of the journal Science.
The runoff from farms and from urban and suburban communities carries excess nutrients that eventually settle into coastal ecosystems. These nutrients provide food for huge algae blooms that both form a layer preventing the oxygen in the atmosphere from reaching deeper water and, when the algae die, use up the water's oxygen as they decompose, thus creating the numerous dead zones that now dot the world's seas.
In the Chesapeake Bay, the federal government is starting to take action.
Following an executive order by President Obama on May 12 directing the federal government to become more involved in the cleanup of the Chesapeake, the Environmental Protection Agency issued a draft plan on Monday for the cleanup.
The plan allows states to continue programs that have worked, but also holds them accountable for reducing nutrient pollution and sets up possible sanctions if they fail. In light of the slow pace of cleanup efforts thus far, it would usher in what EPA senior Chesapeake adviser J. Charles Fox has called "a new era of federal leadership" in those efforts.
Congress is also currently considering bills to reauthorize the Chesapeake Bay and Gulf of Mexico Programs of the Clean Water Act.
Cities are part of the problem, but modern agriculture practices are the largest contributor of nitrogen, phosphorous and sediment to the Chesapeake Bay, Peter Hughes of the agricultural consulting firm Red Barn Consulting told the Senate Committee on Environment and Public Works on Monday.
Ann Swanson of the Chesapeake Bay Commission echoed that conclusion, noting that nearly one-quarter of the bay watershed's land is devoted to agricultural production. Agricultural runoff hasn't been given the attention or blame it deserves, she said.
"While the states have made significant progress with point sources, we have not been successful with reducing the more diffuse non-point sources of nutrient pollution entering the bay," Swanson said. "When one considers the vast and diverse nature of these pollution sources across the bay's 64,000-square-mile watershed, it is not hard to see why we have fallen short in this area."
Much like climate change, the effects of a nutrient-overload of the planet's waters have only begun to be felt recently and come at the hands of human activity.
Likewise, the effort to contain the amount of nutrients in water has strong echoes of that to contain the amount of carbon in the air. "We must reduce the average total loads of nitrogen and phosphorus pollution to no more than 175 million pounds and 12.8 million pounds per year, respectively," says the Chesapeake Bay Foundation's 2008 State of the Bay report.
Nitrogen in fertilizers is captured from the vast amount in the air around us, and it can be returned to the air by bacteria in soil and water that convert it into gas — a process called denitrification. But U.S. waterways are becoming nitrogen-saturated, scientists say.
Denitrification also has a drawback: It produce nitrous oxide, a powerful greenhouse gas that the Intergovernmental Panel on Climate Change estimates has 298 times the global warming power of carbon dioxide over a 100-year period. It is the third largest contributor to climate change, behind carbon dioxide and methane.
Nitrous oxide can be produced naturally as part of the nitrogen cycle, but human activities have led to a sharp increase in its emission. They have pumped far more nitrogen into waterways than the life there knows what to do with. In the case of fertilizer, nitrogen for soils — and thus for food — which is naturally converted into biologically absorbable forms by bacteria in plants like legumes, is instead being mass produced in factories and added to farmland.
A September study in the journal Nature called the disrupted nitrogen cycle one of three human-caused phenomena that threaten the habitability of the Earth.
"The rate at which N2 is removed from the atmosphere and converted to reactive nitrogen for human use," say the study's authors, is one of the "rates of change that cannot continue without significantly eroding the resilience of major components of Earth-system functioning."
(Illustration: Nitrogen cycle, NASA/GSFC)