Why is the nitrogen cycle important to life?
The nitrogen cycle is important because all living things require nitrogen.
Disrupting the nitrogen cycle can lead to a range of negative effects. For example, eutrophication is caused by an excess of nitrogen in aquatic systems. An increase in atmospheric nitrogen can contribute to acid rain.
Most ecosystems are well-adapted to low levels of nitrogen, because naturally most nitrogen is not biologically available. When humans drastically increase the amount of nitrogen available, though vehicle exhaust, fertilizers, and so forth, ecosystems are thrown off balance.
You can read more about the changing nitrogen cycle here.