Nitrogen is essential for synthesizing proteins and nucleic acids. Although the atmosphere is its largest reservoir, nitrogen gas ($N_2$) is non-reactive and cannot be used directly by plants and animals. It must be converted into soluble compounds through the nitrogen cycle, which moves nitrogen between air, soil, water, and organisms.

The cycle begins with nitrogen fixation, where bacteria like Rhizobium and Azotobacter convert atmospheric $N_2$ into ammonia ($NH_3$). Lightning also fixes some nitrogen oxides. Next is nitrification, where Nitrosomonas converts ammonia into nitrite, and Nitrobacter converts nitrite into nitrate.

During assimilation, plants absorb these soluble nitrogen compounds from the soil, and animals obtain nitrogen by eating plants or other animals. When organisms die or produce waste, decomposers like bacteria and fungi break down organic matter, returning ammonia to the soil through ammonification.

The cycle concludes with denitrification, where denitrifying bacteria like Pseudomonas convert nitrates back into atmospheric nitrogen gas, maintaining the ecosystem's nitrogen balance.

If nitrogen were not cycled, this essential nutrient would not be recycled or remain available to support life. Plants and animals could not synthesize proteins and nucleic acids, and dead matter would not be broken down to return nitrogen to the soil. Consequently, ecosystems would degrade, and life on Earth would be severely disrupted.