Nitrogen is good for plants but harmful to water bodies because it causes algae blooms which choke off oxygen. This can affect fish and other living things. Beneficial bacteria in the soil and water convert much of the nitrogen into nitrite and nitrate, eventually evaporating into the atmosphere as harmless nitrogen gas. The air we breathe is made up of 79% nitrogen and 21% oxygen. In addition, plants naturally remove nitrogen from the water via uptake through their roots. Approximately half of the nitrogen in urban runoff can be removed using wetlands.