<human>:  Why is it called the Nitrogen cycle?
<bot>: Nitrogen is the most abundant element in the Earth’s crust. Nitrogen is used by plants and animals to build proteins and other important molecules. Nitrogen is also an important component of atmospheric oxygen. Nitrogen is found in two forms: as nitrogen gas (N2) and as a chemical compound called nitrogenous compounds. Nitrogen is also the main component of the atmosphere, making up 78% of the atmosphere by volume.