Applied to soils as fertilizer and released as fuel burned in autos and industries, nitrogen readily spreads through the planet's natural systems, with devastating consequences.
For decades, farmers and researchers were convinced that agriculture wasted very little nitrogen. But closer study shows that modern food production loses more fertilizer than it uses.
On average, only about 3 ounces of every pound of nitrogen fertilizer actually leaves farms in the form of produce, according to an Ecological Society of America panel.
Of the rest, significant amounts can become airborne as ammonia, which travels as far as 30 miles on the wind and causes pollution when it hits surface waters.
More nitrogen is washed away by rains from the soil before plants absorb it. Some runs directly into waterways.
But a lot moves slowly underground, saturating the water table and seeping out to contaminate waterways for years, even after drastic cuts in fertilizer use.
Scientists say the land is "leaking" nitrogen. More leaks occur during droughts; they arrest crop growth and absorption of fertilizer -- which washes away with the next rainfall.
Even after harvests, nitrogen continues to leak from decaying stalks and leaves left in fields -- and from legumes such as soybeans, which aren't fertilized with nitrogen fertilizer because they capture it from the air.
The process might seem abstract, but the consequences aren't.
"How are you going to bring things back when you can't see the bottom in six inches of water?" Chesapeake Bay ecologist Walter Boynton asks, gazing into a creek near his home in Calvert County.
Boynton had just reviewed old aerial photos showing river bottoms in the area covered with sea grass meadows, vital fish and crab habitat, visible through several feet of clear water. Too much fertilizer has clouded the water with algae and killed 90 percent of the Chesapeake Bay's sea grasses.
World fertilizer production, at 88 million tons a year, is the main way that humans have glutted the Earth with new nitrogen.
Another estimated 22 million tons is added annually when we unearth and burn such fossil fuels as coal, oil and natural gas. And there is preliminary evidence that these airborne forms of nitrogen are especially potent in triggering excess algae growth when they fall directly on surface waters or wash into them.
Both major nitrogen sources, fertilizer and fossil fuels, are projected to grow rapidly. About half the commercial nitrogen fertilizer used in the history of agriculture on Earth has been applied since 1985. And automobiles, a major source of airborne nitrogen, increased tenfold as world population doubled since the 1950s.
Changes in agriculture make it even harder to plug the leaks in nature's nitrogen cycle.
Animals used to be raised on the same farm where their feed was grown; their manure went back onto the fields to nourish the coming crop.
But today, nitrogen applied to grow corn in Iowa is fed to chickens in Maryland or pigs in China. Farmers end up with mountains of manure too far away to be recycled efficiently back into growing Midwestern corn.
The southern United States, for example, grows nearly a third of the nation's livestock, but it produces just 6 percent of the grain fed to these animals, importing the rest. Nitrogen in runoff from their manure is linked to toxic algae blooms and oxygen losses in North Carolina's Pamlico Sound and elsewhere.
Such large-scale, concentrated production now dominates not only in meat and poultry growing, but also in farm-raised fish and shellfish -- increasingly a suspected cause of nitrogen pollution in coastal waters.
Nitrogen is not the only nutrient to cause water-quality problems. Humans have nearly tripled the amount of phosphorus fertilizers that run into rivers and the oceans in the past century.
Phosphorus, however, is usually more a problem in fresh water, more easily contained in farm fields and removed from sewage, and not produced by burning fossil fuels. So nitrogen is a more pernicious and widespread threat to coastal waters.
Experts continue to puzzle over a key mystery: Only about a fifth of all nitrogen leaking from human activities appears to end up in waterways. The rest is unaccounted for.
Scientists once assumed a lot of it was consumed by increased growth in forests. But studies at the Harvard Forest, a research station in western Massachusetts, failed to confirm that.
A significant amount of the missing nitrogen is likely being recycled harmlessly back into the atmosphere. Bacteria perform this beneficial deed wherever nitrogen-laden runoff flows through the sediments of wetlands. So the worldwide destruction of wetlands has been a major factor in the worsening impact of nitrogen.
And finally, large quantities of nitrogen seem to be accumulating in soils, posing a critical question: Is this nitrogen safely and permanently stored, or are soils becoming so saturated that millions of tons of nitrogen soon will begin leaking out to further taint our waters?