Coastal dead zones, global warming, excess algae blooms, acid rain, ocean acidification, smog, impaired drinking water quality, an expanding ozone hole and biodiversity loss. Seemingly diverse problems, but a common thread connects them: human disruption of how a single chemical element, nitrogen, interacts with the environment.

Nitrogen is absolutely crucial to life — an indispensable ingredient of DNA, proteins and essentially all living tissue — yet it also can choke the life out of aquatic ecosystems, destroy trees and sicken people when it shows up in excess at the wrong place, at the wrong time, in the wrong form. And over the past century, people have released so much of this type of nitrogen — known as reactive nitrogen — that scientists say we’ve passed the limit of what the planet can safely handle.

The result of releasing so much nitrogen to the environment — through excessive and inefficient fertilizer use, agriculture-related nitrogen emissions and nutrient-laden wastewater, along with fossil fuel and biomass burning — is this slew of adverse environmental impacts. These impacts are occurring worldwide and are exacerbated by warming temperatures. Though the nitrogen problem gets far less press, we’ve now upset the naturally occurring balance of nitrogen even more than that of carbon.

While many things contribute to the problem — including energy use, urban runoff and sewage — agriculture is the largest source of environmentally damaging nitrogen. According to scientists studying this problem, approximately 80 percent of the nitrogen currently used in agriculture (primarily synthetic and other fertilizers, like manure) is lost to the environment at some point in the food supply chain. These losses occur on farms and in food production, sales, distribution, preparation and consumption. Or, as University of Virginia professor of environmental sciences Jim Galloway puts it, losses occur “all along the way from the field and bare soil to the sewage plant.”

A big part of the problem, according to Jan Willem Erisman, University of Amsterdam professor of integrated nitrogen studies and CEO of the Louis Bolk Institute in the Netherlands, is that “people don’t connect it to food and food production.” In fact, in the U.S., European Union, Japan and likely China and elsewhere, food accounts for more than 75 percent of the average person’s nitrogen footprint (individual contribution to nitrogen pollution), according to University of New Hampshire natural resources and environmental studies Ph.D. candidate Allison Leach, who is among the scientists working on the Nitrogen Footprint, a project designed to raise awareness of the issue. And it turns out that meat and other top-of-the-food-chain animal products — those that consume the most resources before they themselves become food — are among the biggest culprits in contributing to excess nitrogen in a form that can be damaging to the environment.

Such nitrogen pollution is responsible for the harmful algae blooms plaguing Chesapeake Bay, the Great Lakes and the Gulf of Mexico; for last year’s drinking water crisis in Toledo; and for “blue baby syndrome,” a potentially fatal oxygen depletion disorder that harms infants around the world — among other effects. Part of the difficulty in making people aware of these connections is that they often show up as “disruptions in distant places,” Erisman explains. What happens on a farm field can show up as algae many miles downstream and offshore, for example, or show up in groundwater that supplies well water to residents without direct connections to the source of contamination. Connecting water pollution or air pollution with food choices is even more of a stretch.

But before unraveling why a cheeseburger will expand your nitrogen footprint more than rice and beans, it helps to understand how we got to where we are now — with excess nitrogen creating dangerous river-choking algae blooms, fish-killing dead zones, unsafe drinking water and unhealthy levels of smog in cities worldwide — and how nitrogen works in the environment.

First, it’s important to know that the type of nitrogen that makes up a large part of the Earth’s atmosphere is not reactive but inert. For that nitrogen to be used by plants and other organisms, it must be converted into what’s called a “fixed,” or reactive, form. The main way this happens in nature is through microbes that live in soil and plant roots and convert inert nitrogen to ammonia, a reactive form that can be used in — and is essential to — plant growth. While plants need this reactive nitrogen to thrive, if excessive amounts enter the environment, they contribute to a suite of adverse effects. And ammonia is not the only form of reactive nitrogen; others — nitrous oxide,nitrogen oxides, nitrate and nitrite — can also become serious air and water contaminants and prompt respiratory, cardiovascular and other diseases. Key to understanding this problem — and its solutions — is that this overload didn’t happen on its own.

Without human intervention of some sort, the world’s naturally occurring supply of reactive nitrogen essential to plant growth is relatively limited. So by the beginning of the 20th century it became apparent that there wasn’t going to be enough of this form of nitrogen available to produce the volume of food needed to adequately feed a growing population. This problem was solved by the invention of synthetic fertilizers that supply plants with nitrogen in a “fixed” form they can use. Agricultural productivity soared. But the use of these fertilizers has not been very efficient, resulting in the release of large amounts of reactive nitrogen into the environment. According to Galloway, Leach and colleagues, so much anthropogenic reactive nitrogen has been produced that by 2010, human activity was creating at least five times as much as were natural systems.