While most people today are aware of the importance of reducing their carbon footprint, far fewer understand the harmful effects of nitrogen pollution. Yet many scientists believe that man’s disruption of the natural nitrogen cycle is a potential global tipping point and a serious threat to biodiversity. Nitrogen has played a beneficial and key role in helping to feed the world, but excess nitrogen is having a destructive effect on our environment and human health.
Nitrogen is one of five elements (the others are carbon, hydrogen, oxygen, and phosphorus) that make life possible. It is essential for the creation of DNA, amino acids and proteins. 79% of the earth’s atmosphere is made up of nitrogen, but living things can’t use it in this form called dinitrogen (N2). Most organisms can only use reactive nitrogen, which occurs when N2 atoms are combined with other atoms in a process called fixation. In the atmosphere, N2 can be fixed by lightning, which converts N2 into nitrate that is carried by rain to earth. On land and in the ocean, microbes fix 90% of N2 to produce ammonium that they use to grow; ammonium also enters the food chain and the environment. When organisms die, bacteria recycle the nitrogen in them, and it eventually returns to the atmosphere to complete the nitrogen cycle.
To feed our growing population, humans have disrupted the delicately balanced natural nitrogen cycle. The turning point came in 1909, when Fritz Haber and Carl Bosch figured out how to combine hydrogen with N2 to create ammonia, which was used to produce fertilizer. The use of synthetic fertilizer has vastly increased agricultural yields around the world. Today, the International Nitrogen Initiative estimates that 40% of the global population is dependent on crops fertilized with reactive nitrogen.
A study by University of Virginia environmental scientist James Galloway and colleagues reported that from 1970 to 2008, world population increased by 78% and reactive nitrogen creation grew 120%. Humans have introduced additional reactive nitrogen into the environment by expanding the production of soybeans, peanuts and alfalfa, (leguminous) crops which host nitrogen-fixing bacteria that convert N2 into reactive nitrogen. We use ammonia to manufacture nylon, plastics, resins, animal and fish feed supplements, and explosives. Fossil fuel burning industries and vehicles produce nitrogen emissions, and nitrogen is a component of the electronics, steel, drug, missile and refrigerant industries.
A single nitrogen molecule can cascade through the environment affecting air and water quality, human health and global warming in numerous ways.
Runoff from agriculture—from fertilized crops fed to animals, from manure, and from biofuel and leguminous crops—enters rivers and streams and can contaminate groundwater. When nitrogen-loaded runoff makes its way to the ocean, it can result in eutrophication, where algae bloom, then die, depleting the oxygen and suffocating plants and animals. Runoff from urban areas, sewage treatment plants, and industrial wastewater also contribute to eutrophication.
Nitrogen is also a component of acid rain, which can acidify soils, lakes and streams. While some trees may utilize the extra nitrogen to grow, others experience foliage damage and have reduced tolerance for stress. The acidification of lakes and streams can kill plants and animals, and pollute our drinking water.
Our air quality is affected by nitrogen emissions from vehicles, fossil fuel burning industries, and the ammonia from agriculture, which cause ground-level ozone. High concentrations of ozone affect human respiratory and cardiovascular health and disrupt photosynthesis in plants.
Climate change is both influenced by and exacerbated by nitrogen. For example, nitrogen may stimulate plant growth, resulting in more carbon dioxide uptake in some forests.
But when nitrous oxide from coal-fired power plants, and vehicle and fertilizer emissions rise into the atmosphere, it is 300 times more potent as a greenhouse gas than carbon dioxide. It also depletes the stratospheric ozone layer that protects us from UV rays.
The ever-growing nitrogen problem is complex, so solutions must also be multi-faceted. Scientists have stressed the need to reduce fossil fuel emissions, improve wastewater treatment, restore natural nitrogen sinks in wetlands, and both reduce the use and increase the efficiency of nitrogen fertilizers. Galloway’s study also underscores the importance of better management of animal waste from the concentrated animal feeding operations that produce most of our meat today.
But what can we individuals do to reduce nitrogen pollution? First, determine your nitrogen footprint with the nitrogen footprint calculator created by Galloway, Allison Leach, a staff research assistant at the University of Virginia, and colleagues.
If your nitrogen footprint needs cutting back, the most effective action we can take is to change the way we eat. Ten times more nitrogen is used to produce food than humans consume as protein, and not all the nitrogen in the food we eat is even used by our bodies—the excess enters the environment through human waste. And while most people require only 2 grams of nitrogen a day, the average American consumes 13 grams daily. The Recommended Daily Intake of protein is approximately 50 grams or less than 2 ounces, but the average American consumes 8 ounces of meat daily. So we need to eat less meat, and less nitrogen-intensive meat—for example, opt for chicken or fish instead of beef. We can also shop for more sustainably farmed products and reduce our food waste. And, because nitrogen pollution comes from fossil fuel burning power plants, we should reduce our energy consumption by using more efficient appliances and sustainable transportation, reducing our electricity use, and recycling.