What caused the gradual increase of nitrogen in the atmosphere?

The correct reason for the gradual increase of nitrogen in the atmosphere is natural nitrogen fixation. This process involves the conversion of atmospheric nitrogen gas (N₂) into compounds that can be utilized by living organisms. This is primarily carried out by certain bacteria that possess the enzyme nitrogenase, allowing them to convert nitrogen gas into ammonia and other nitrogenous compounds, which are essential for plant growth.

Over geological time, these nitrogen compounds are further transformed and eventually return nitrogen to the atmosphere through various biological and chemical processes, contributing to the overall nitrogen level in the atmosphere.

While other processes like photosynthesis contribute to the uptake of carbon dioxide and the release of oxygen, they do not result in an increase of nitrogen in the atmosphere. Volcanic activity can release gases including nitrogen, but it does not significantly contribute to the long-term increase of nitrogen. Oxidation of atmospheric compounds typically involves reactive gases and does not play a significant role in adding nitrogen to the atmosphere.