Consider the simplified version of the nitrogen cycle shown in the diagram - Leaving Cert Chemistry - Question c A - 2019

One method of natural fixation of nitrogen is through lightning, or the action of nitrogen-fixing bacteria, particularly those found in legumes, such as those in legume plants.

Nitrogen compounds are absorbed by most plants through their roots, from the soil or in water, utilizing active transport mechanisms or by being in solution.

Nitrogen compounds are transferred from plants to animals when animals consume the plants, a process commonly referred to as eating.

Plants and animals use absorbed nitrogen compounds primarily for protein manufacture, growth, and the repair of cells and tissues.

These conversions are collectively termed the nitrogen cycle because nitrogen is returned to the atmosphere, ensuring a constant concentration of nitrogen. Furthermore, nitrogen is never completely consumed; instead, it continuously cycles, being converted from one form to another as part of various processes.