The most abundant naturally occurring gas in the atmosphere is the Nitrogen (N2); however, plants and animals are unable to use this directly because of their inability to break down its triple bond. In order for them to utilize this gas, it must be transformed into another form or molecules. There are ways where to convert nitrogen to make it useful for the plants and animals.
The first one is lightning. At the time when lightning strikes, nitrogen gas is transformed into nitrates by which plants can use.
The second one is through the process of nitrogen fixation. In this process, special nitrogen-fixing bacteria helps in converting this gas into useful forms.
Lastly, when there is a decaying organism. Nitrogen is released in the form of ammonium.