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When Captain N.C. Middlebrooks claimed the Brook Islands for the United States in 1859, he had no idea they would later be known as Midway Atoll, site of a World War II turning point more than eighty years later. The island cluster was coveted for a humbler reason: guano.

Home to colonies of seabirds for millennia, Midway had rich deposits of excrement rich in nitrates and phosphates — perfect for use in fertilizer and, ironically, gunpowder. The atoll was one of more than 100 claims under the United States Guano Islands Act of 1856, which allowed citizens to exploit feces in the name of America.

Today it’s hard to imagine the importance of such resources because we no longer need to collect bird droppings to meet our nitrogen needs. Thanks to early 20th century work by Fritz Haber and Carl Bosch, we have industrialized the conversion of atmospheric N2 into ammonia, which is then turned into fertilizers, explosives, and plastics, among other things. We no longer rely on invisible microbes to fix inert N2 gas into biologically reactive forms, which we then scavenge from bat caves and seamounts. We have harnessed the power of fossil fuels to fix more than 400 billion tons of nitrogen every year, twice the amount that natural processes (microbes, lightning strikes, and volcanic eruptions) capture.

This technology has proven to be a great boon to agriculture. We can supply our crops with a steady stream of a critical nutrients, increasing food production and ensuring consistent harvests. Abundant fertilizers supported the Green Revolution — a transition to higher-yielding (but also higher-maintenance) crop varieties. Today, some two billion people are fed by the extra food the fertilizers allow us to grow, and the Haber-Bosch process supports 40 percent of the world’s protein production.

Too bad there are flies in the ointment.

Tripling the magnitude of Earth’s natural nitrogen fixation process comes with dramatic consequences. All the extra nitrogen we pull out of the atmosphere must somehow find its way back, and in the process it upsets the balance of countless biological communities.

Excess fertilizer (here in the United States, we end up ingesting only one in every ten nitrogen atoms applied to a field) spreads beyond the bounds of agricultural fields. Nitrogen is not an equal opportunity fertilizer: Certain plants respond faster to extra nutrients, outcompeting their neighbors and reducing the biological diversity of the natural community. In fact, nutrient pollution is one of five critical threats to biodiversity named by the United Nations.

When fixed nitrogen seeps into groundwater or is leached away by runoff, it can pollute large water bodies, producing vast blooms of algae stimulated by the nutrient surplus. As bacteria devour sinking, dying algae, they also use up the water column’s oxygen supply. Without oxygen, fish and crustaceans cannot survive, and the food web collapses. In the Gulf of Mexico, agricultural runoff draining through the Mississippi River has produced a “dead zone” the size of Connecticut, destroying fisheries and livelihoods.

In addition, high reactive nitrogen levels are associated with elevated rates of human illness: Pathogens like West Nile Virus and malaria thrive. Weedy allergens like ragweed grow faster and produce more pollen. 

And still, the nitrogen must return to the atmosphere. Disturbingly, it does not always return in the inert N2 form from which it started. Three-to-five percent of chemically fixed nitrogen is released as nitrous oxide (N2O), which, molecule-for-molecule, has 300 times the global warming power of carbon dioxide. (In addition, fixing nitrogen requires a huge up-front energy investment: We emit roughly 200 megatons of CO2 each year burning natural gas to fuel the Haber-Bosch process.) Recent research casts a harsh light on biofuels, whose fertilizer demands (and attendant N2O emissions) may counteract — and even exceed — any carbon savings.

Perhaps most troubling of all, the effects of nitrogen pollution are not always felt close to the source. Issues of environmental justice arise: How can Midwest farmers be held accountable for Gulf of Mexico shrimp boat catches? Should food importers pay for the ecological damage caused by intensive agriculture in the producing countries? If so, how does one put a price on the commodities of healthy ecosystems and biodiversity?

Meanwhile, the more pressing task is a transition to a new relationship with nitrogen. Renouncing it is impossible: Billions of lives are on the line. In fact, while an excess of fertilizer drives food surpluses and obesity in our own country, 250 million Africans are malnourished because they lack fertilizer.

However, as fossil fuel supplies dwindle, fertilizer production will become more costly, and new levels of agricultural efficiency will be demanded of us.

Some strategies already exist. From the deceptively simple method of planting a winter cover crop (to prevent soil nutrient loss) to state-of-the-art localized fertilizer applicators (combined with soil quality monitoring), we can reduce our nitrogen demand. By placing buffer wetlands between field and stream, we can contain the spread of any remaining excess.

Ultimately, some radical transformations of our industrial agriculture system will be needed. We must close disconnected loops to improve sustainability: feed the cattle the corn, then use their waste to fertilize the field. That means the end of factory farming and an emphasis on small-scale, self-contained operations. But we could all stand to eat less meat, anyway. As we reevaluate our high-impact American lifestyles, it’s important to distinguish between quantity and quality, to challenge excess and reward efficiency.

Like most aspects of our lives, humanity’s relationship with nitrogen is nuanced: vital yet somewhat unsavory; powerful and therefore frightening. In a way, it’s like our relationship with Midway — the painful victory won there, and an older history mostly forgotten. We face uncomfortable truths and disconcerting choices, but we must move forward proactively, or lose the bigger battle: to live sustainably, in perpetuity, on Planet Earth.

Holly Moeller is a graduate student in the MIT/WHOI Joint Program in Biological Oceanography. She welcomes reader feedback at hollyvm@mit.edu. “Seeing Green” runs on alternate Tuesdays.