Fossil fuels are deposits of essentially animals or plants that have died in the past and become buried and then converted through to highly concentrated forms of energy like oil or coal.
If you think about plants and animals, both of them contain quite a lot of nitrogen, so when they get converted through to energy forms such as fossil fuels, that nitrogen is still contained in that fossil fuel. When you burn the fossil fuel, either say coal or oil, you are essentially releasing that nitrogen back up into the atmosphere – not necessarily as dinitrogen gas but maybe other forms of nitrogen that are more available to plants and animals.
So if that material goes up into the atmosphere, it can be rained out again and deposited on the land as a form of biologically available nitrogen.
Fossil fuels release nitrous oxide, which is one of the greenhouse gases that is released into the atmosphere, so fossil fuel burning does contribute to greenhouse gases, not only through the production of CO2 but also nitrous oxide.
Professor Louis Schipper, University of Waikato