The 4 Steps of NITROGEN CYCLE: The Key to the Food Chain

Nitrogen Cycle

Last Updated: Dec 30, 2018

What is the Nitrogen Cycle?

As part of the nitrogen cycle, nitrogen transforms into several different forms.

For example:

  • Nitrogen fixing converts N2 from the air into nitrates.
  • In reverse, denitrification converts nitrates back to N2 as a gas.
  • Finally, nitrification converts ammonia into nitrates.

In the grand scheme of things, these processes are essential in the food chain.

The nitrogen cycle consists of 4 major steps: nitrogen fixing, decomposition, nitrification and denitrification.

1 Nitrogen Fixing

Nutrient Cycle

In terms of atmosphere composition, nitrogen makes up about 78% of the air. Plants and animals need nitrogen to make proteins but they cannot take it in from the air. Because nitrogen is unreactive as a gas, it has to be transformed into a new molecule. When bacteria in the soil takes nitrogen from the air, it becomes nitrates and can move through the food chain.

For example, legume like clovers, peas and beans, their roots can take N2 from the air and transform into another form called nitrates. In addition, lightning transforms N2 into NO2, which goes into the soil to form nitrates. Also, synthetic fertilizer from farming can bring nitrate to the soil.

Overall, nitrogen fixation takes unreactive nitrogen from the air and fixes it into a usable form. Nitrogen is essential for building amino acids which are building blocks for DNA and RNA. Nitrogen is used to make amino acids for growth.

2 Decomposition and Ammonification

Nitrogen Cycle

After nitrogen fixation, roots of plants absorb the nitrate. In the plant, they are in the form of protein and nucleic acids. In turn, animals eat these plants who break them down.

When animals produce waste or die, this waste decays and bacteria consumes this dead organic matter. As a result, the nitrogen in this waste is in the form of ammonium (NH4+).

This is a key process in the nutrient cycle that constantly exchanges inorganic and organic matter back and forth in an environment.

3 Nitrification

Soil Nutrients

However, it’s difficult for plants to use ammonium. So this waste (ammonium) is again broken down by bacteria through a process called ‘nitrification’.

Nitrification takes the ammonium and transforms it into nitrates in the soil. And again, plants can absorb nitrate and moves through the food chain.

If you deplete the soil from nitrogen, this deprives the plants from growing. Because bacteria convert dead plant material to nitrates, plants absorb it as food.

4 Denitrifying bacteria


But we need nitrogen back in the atmosphere, so we use another type of bacteria. There is a whole different process that does this.

Through the denitrification process, nitrates are converted nitrogen gas again. So it leaves the soil to go back into the atmosphere.

Overall, denitrification turns nitrates (NO3) in the soil to nitrogen (N2) which is returned to the air.

Nitrogen Cycle Highlights

This whole cycle how nitrogen transforms into different forms continues. Bacteria is essential in this process, as well as each process.

The nitrogen cycle continues and is key in the food chain.

  • Nitrification converts ammonia into nitrates.
  • Denitrification converts nitrates to N2.
  • Nitrogen fixing converts N2 into nitrates.


  1. I’m not a scientist, but this is a wonderfully easy to follow explanation, which now enables me to pass this on to a student I’m working with.
    Thank you- much appreciated!

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