What Are the Steps of the Nitrogen Cycle?

Everybody needs nitrogen, but as far as non-negotiable, life-sustaining elements go, it's tricky. Living things require nitrogen for their cells to function and, furthermore, we are virtually steeping in the stuff since our atmosphere is made up of 78 percent nitrogen gas. However, there's a catch: It's a "water, water everywhere, but not a drop to drink" situation.

Although nitrogen's lurking basically everywhere, it's not terribly abundant in the Earth's crust, and it's incredibly difficult for living things to capture atmospheric nitrogen and use it for their purposes. It's like having a pocketful of Icelandic krónur in Minneapolis, where you can't spend it.

"Nitrogen is a major part of amino acids, which are the building blocks of proteins and nucleic acids such as DNA," says Jessie Motes, a Ph.D. candidate in the Odum School of Ecology at the University of Georgia, in an email. "In addition to needing nitrogen for proteins in plants, it is a main component of chlorophyll, which makes it crucial for photosynthesis."

The Nitrogen Cycle

Since nitrogen is a limited resource on this planet, a nitrogen atom doesn't spend much time doing nothing when it's in a form living things can use — scientists call this nitrogen "fixed." Fixed nitrogen is taken up by plants, which are eaten by animals, which eat other animals, which die and decompose and release nitrogen back into the ecosystem to be worked on by bacteria or plants. This is the cycle of a nitrogen atom on Earth, and its journey starts either very quietly or with a humongous bang.

Step 1: Nitrogen Fixation

Believe it or not, lightning and bacteria are primarily responsible for turning atmospheric nitrogen into nitrogen living things can use. Atmospheric nitrogen (N2) is very stable, so it takes an incredible amount of energy to convert it to a different form. If you've ever wondered why your outdoor plants seem happier after a rain than they do when you turn a sprinkler on them, there's a reason for that: Lightning electrifies atmospheric nitrogen (N2) and water (H2O) to reconfigure them into ammonia (NH3) and nitrates (NO3). This falls to the ground as rain, where plants slurp it up and use it for their biological processes.

On the other end of the spectrum, the most common way nitrogen is made available to organisms is when atmospheric nitrogen is fixed by bacteria, some of which live free in the soil and others of which enjoy a symbiotic relationship with certain plant species. Legumes like peas, clover and peanuts have little nodules on their roots that attract bacteria that convert stubborn atmospheric nitrogen into ammonia or ammonium, which can then be used to power the plant.

Step 2: Nitrification

Ammonia in the soil can be used directly by plants, but it's also the first step in the process of nitrification, through which specialized bacteria and archaea convert ammonia into nitrite (NO2), and then pass it off to an entirely different set of prokaryotes that further oxidize the nitrite into nitrate (NO3-). This process is slow, but it's the way that nitrogen is built as a nutrient in soil and aquatic and marine environments — terrestrial plants, for instance, can absorb ammonium and nitrate through their root hairs. The organisms that specialize in nitrification are also important in treating municipal wastewater.

Step 3: Ammonification

Everything living eventually dies, and the nitrogen a particular organism was using when it croaked is taken to hand by bacteria that turn the nitrogen-rich corpse into ammonium, which can be picked back up by plants and used again.

Step 4: Denitrification

It's possible to convert bioavailable nitrogen into atmospheric nitrogen again, and that process is called denitrification. Nitrification is performed by bacteria and archaea that can tolerate oxygen — not all prokaryotes can. In the case of denitrification, certain anaerobic bacteria that don't need oxygen convert nitrate to nitrogen gas, which floats up into the atmosphere and plays hard to get until some lightning or a crafty nitrogen fixing bacterium comes along and ropes it into the nitrogen cycle yet again.

Humans and the Nitrogen Cycle

"Like most natural processes, anthropogenic activities are disrupting the nitrogen cycle through nitrogen deposition," says Motes. "Too much nitrogen can lead to increased emissions of the greenhouse gas nitrous oxide, as well as eutrophication, which is nitrogen pollution of water sources."