The nitrogen cycle represents one of the most important nutrient cycles found in ecosystems. However, despite its abundance in the atmosphere, plants cannot use this gaseous form (N₂). Rather, they need nitrogen to be converted into a usable form, such as ammonium (NH₄⁺), nitrite (NO₂^-), or nitrate (NO₃^-). This process of converting nitrogen into a form usable by organisms is known as nitrogen fixation. Nitrogen can be "fixed" by two natural forces: lightning and bacteria. Electricity in the form of lightning can break the strong bond between the N₂ molecule, enabling free nitrogen atoms to bond with oxygen in the air to form nitrites and nitrates through the process of nitrification. These compounds dissolve in the atmospheric moisture to form nitrates that then fall as rain.

Bacteria in the soil and on the roots of plants fix more nitrogen than lightning. Bacteria can break the bonds in the nitrogen gas too, and combine the free nitrogen atoms with hydrogen to form ammonium (this process is known as ammonification), which is readily absorbed by plants. Some types of nitrogen fixing bacteria have formed symbiotic relationships with certain types of plants. Legumes, such as peas and beans, support colonies of bacteria called rhizobium, in special structures called nodules, on their roots. While the bacteria fix nitrogen for the plant, the plants provide the bacteria with energy-rich carbohydrates and a moist environment in which to grow and thrive. Bacteria also perform denitrification, the process of converting nitrites, nitrates, and ammonium back into N2 gas, which is then released into the atmosphere.

The following animation describes the movement of a nitrogen molecule through the process of fixation from the atmosphere, into the soil, through various organisms and back to the atmosphere in a continuous cycle.