What caused the amount of nitrogen to gradually increase in the atmosphere?

The increase of nitrogen in the atmosphere can primarily be attributed to the process of volcanic activity. Early in Earth's history, volcanic eruptions released a variety of gases, including nitrogen, into the atmosphere. At that time, Earth's atmosphere had very little nitrogen, but as volcanic activity continued over millions of years, it contributed significantly to the nitrogen levels.

Nitrogen is relatively inert and does not easily react with other elements under normal conditions, which helps it to accumulate in the atmosphere over time without being significantly depleted by biological processes. As a result of ongoing volcanic eruptions and the release of nitrogen gas, the concentration of nitrogen in the atmosphere gradually rose.

While the other options involve processes that may have influenced the Earth's atmosphere, they do not have the same direct effect on the nitrogen levels as volcanic activity does. For instance, solar radiation can break down certain compounds but is not responsible for adding significant nitrogen. Respiration by early organisms typically consumes oxygen and releases carbon dioxide rather than producing nitrogen. Rock weathering processes contribute various elements to the soil and atmosphere but are not a major source of atmospheric nitrogen themselves.