Why did the percentage of nitrogen slowly increased when the atmosphere was formed?

The percentage of nitrogen in the atmosphere increased over time primarily due to volcanic outgassing and the release of nitrogen compounds from Earth's interior. As the planet cooled, these gases, including nitrogen, accumulated in the atmosphere. Additionally, nitrogen is relatively inert and does not easily react with other elements, allowing it to build up as other gases, like carbon dioxide and water vapor, were removed or transformed through processes like photosynthesis. Over geological timescales, this led to nitrogen becoming the dominant gas in the modern atmosphere.