Nitrogen fixation is the process by which atmospheric nitrogen in the air is converted to nitrates and ammonia in the soil. Nitrogen fixation happens in two ways:

1. Via lightning - lightning converts atmospheric nitrogen directly into nitrates.
2. Via nitrogen-fixing bacteria - these convert atmospheric nitrogen into ammonia. Nitrogen-fixing bacteria are found in the soil, and also in the roots of some plants.

Nitrification is carried out by nitrifying bacteria in the soil, and consists of two steps:

1. Ammonia in the soil is first converted to nitrites.
2. Those nitrites are then converted to nitrates.

Assimilation is the process by which plants absorb nitrates and ammonium ions, and use them to create biological molecules such as proteins. These biological molecules form part of the plant's tissues.

This is how nitrogen enters the food chain, and when animals eat plants they assimilate these nitrogen containing compounds (e.g. proteins) into their own bodies.

Decomposition occurs when decomposers feed on the waste excretions of animals, and the rotting corpses of dead plants and animals. During this process they convert urea and proteins into ammonia.

This is important because the ammonia in the soil forms ammonium ions, which is one of few the nitrogen-containing compounds plants can absorb and assimilate. Note that the some of the ammonia produced by nitrogen fixation is also converted to ammonium ions.

(In exams, mark schemes often allow both 'ammonia' and 'ammonium ions' to be used interchangeably, so don't worry if you find it tricky to remember the difference)

In denitrification, denitrifying bacteria in the soil convert nitrates back into atmospheric nitrogen.

Denitrification is bad for plants because it means there are fewer nitrates in the soil, reducing the amount available for the plants to absorb and assimilate.

Don't worry if you find the nitrogen cycle confusing at this point. Read the following summary of the key learning points, and then try reading through the lesson again.

Nitrogen cycle summary

1. Both plants and animals need nitrogen in order to survive, but neither plants or animals can absorb the nitrogen in the air (atmospheric nitrogen). Animals get their nitrogen from digesting plants (or other animals), and plants must get their nitrogen from the soil, either as nitrates or ammonium ions (two chemicals that contain nitrogen).
2. The processes of nitrogen fixation and then nitrification cover how atmospheric nitrogen gets converted to nitrates. This happens either in a single step (via lightning), or via a multi-step process (nitrogen fixation and then nitrification) carried out by nitrogen-fixing bacteria, and then nitrifying bacteria.
3. Separately to the above, decomposers break down plant and animal waste to produce ammonia. This ammonia then reacts to form ammonium ions (which plants can absorb).
4. Plants absorb nitrates and ammonium ions from the soil, and use them to create essential biological molecules such as proteins and DNA. This process is called assimilation, and is how nitrogen becomes part of the plant's tissues, and enters the food chain.
5. At the same time as all of the above, some of the nitrates in the soil are being converted back into atmospheric nitrogen due to the process of denitrification (due to denitrifying bacteria).