Above: Image © 3dalia, iStockphoto

Nitrogen gas (N2) makes up 78 per cent of Earth’s atmosphere. That’s a lot of nitrogen in the air! But what does it do? Understanding the nitrogen cycle can help answer this question.

The nitrogen cycle describes how nitrogen moves through the environment. It has three main steps: nitrogen fixation, nitrification and dentirfaciton. Sound complicated? Well, it’s worth taking the time to understand, because the nitrogen cycle has many important impacts on the planet. For example, it helps many living things get the nitrogen they need to survive.

Step 1: Nitrogen fixation

Simplified (unbalanced) chemical reaction for conversion of nitrogen gas to ammonia

N 2 O + H negative becomes N H 3 + H 2

N2 + H+ → NH3 + H2

Most living things need nitrogen in their diet. However, your body can’t use the nitrogen gas in the atmosphere. Other animals and plants can’t either. That’s why the the first step of the cycle is so important: it converts nitrogen gas into a form living things can use. Plants can use a form of nitrogen called ammonia (NH3), a nitrogen atom bonded to 3 hydrogen atoms.

The process of converting nitrogen gas (two nitrogen atoms bonded together) to ammonia is called nitrogen fixation. Bacteria in the soil and on the roots of certain leguminous plants roots can make this happen. Nitrogen fixation can also occur when lightning strikes the air.

When nitrogen is in the form of ammonia, plants can metabolize and use it. Animals, including humans, can then eat these plants to get the nitrogen they need.

Did you know? Soybeans are one of the leguminous plants that can perform nitrogen fixation, converting nitrogen gas to ammonia.

Step 2: Nitrification

Simplified chemical reaction for conversion of ammonia to nitrite

N H 3 + O 2 becomes N O 2 negative + H positive

NH3 + O2 → NO2 + H+

2 N O 2 negative + O 2 becomes 2 N O 3 negative

Chemical reaction for conversion of nitrite to nitrate

2 NO2 + O2 → 2 NO3

Not all of the ammonia produced during nitrogen fixation is used by plants. Bacteria in the soil can use some of it to create nitrite N O 2 negative(NO2-). Nitrite can then be converted into nitrate N O 3 negative(NO3-), which helps plants grow.

Nitrification is an example of an aerobic process. That means it needs oxygen gas (O2) to happen. To create both nitrate and nitrite, a nitrogen atom has to connect to oxygen atoms.

Step 3: Denitrification

Denitrification is the final step in the nitrogen cycle. It happens when certain types of bacteria take nitrate and convert it back to nitrogen gas, which is released into the air. Unlike nitrification, this process is anaerobic: it doesn’t require oxygen.

Human impact on the nitrogen cycle

Simplified chemical reaction for conversion of nitrate to nitrogen gas

NO3 → NO2 → NO + N2O → N2

Human activities can affect the nitrogen cycle in potentially harmful ways, such as by introducing more nitrogen in certain parts of the cycle.

For example, plant fertilizers contain nitrogen, in the form of ammonia and nitrate. When too much fertilizer is used, the excess nitrogen can run off into nearby bodies of water. This leads to eutrophication, a process in which excess nutrients in the water cause plants and algae to grow rapidly. This depletes the oxygen in the water, which can cause fish to die.

Human activities can affect the nitrogen cycle in potentially harmful ways, such as by introducing more nitrogen in certain parts of the cycle.

Excess nitrogen can also cause the release of nitrous oxide (N2O), a greenhouse gas. Nitrous oxide is produced during nitrification and denitrification, and more nitrogen in the soil means more nitrification and denitrification. In fact, nitrous oxide is one of the main greenhouse gases emitted by human activities!

A solution to both eutrophication and nitrous oxide emissions is for gardeners and farmers to apply only as much fertilizer as their plants need. But that’s easier said than done. It can be very difficult to determine this exact amount.

Did you know? Burning fossil fuels can release nitrogen oxides, which contribute to acid rain.

Plants, animals and bacteria to need nitrogen to help them grow. The nitrogen cycle is how nitrogen gas in the atmosphere gets converted to a form they can use. It affects life on the planet in many ways—including by making sure your body gets its daily dose of nitrogen!

Illustration of the nitrogen cycle, including fixation, nitrification and denitrification (Eme Chicano, Wikimedia Commons)

Learn More

Understanding nitrogen in soils (2014)
J.A. Lamb, F.G. Fernandez & D.E. Kaiser, University of Minnesota

The Nitrogen Cycle: Processes, Players, and Human Impact (2010)
A. Bernhard, Nature Education Knowledge 3

The Nitrogen Cycle
Partnership for Environmental Education and Rural Health

Kelly Resmer

Kelly is a chemistry undergraduate laboratory instructor in Halifax.  She loves working with students in the lab, watching chemistry happen! She has a PhD in chemistry and is very interested in studying and learning about bacteria, the good and the bad ones! 

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