The nitrogen cycle is the process by which nitrogen is converted and recycled through the Earth’s land, oceans, and atmosphere. Nitrogen is an essential component of many biological molecules, such as DNA and proteins, and is a critical nutrient for the growth of plants and other organisms.
The nitrogen cycle starts with nitrogen fixation, which is the process by which atmospheric nitrogen (N2) is converted into ammonia (NH3) or other forms of nitrogen that can be used by plants. Nitrogen fixation is carried out by certain types of bacteria, either through symbiotic relationships with plants or through free-living forms in the soil.
Once nitrogen has been fixed, it can be taken up by plants and incorporated into organic compounds. Consumers then eat the plants, transferring the nitrogen from the plants into their own bodies. When these organisms die or excrete waste, the nitrogen is returned to the environment through decomposition or excretion.
Nitrification is the process by which ammonia is converted into nitrate (NO3-) by bacteria in the soil. This process makes nitrogen available to plants for uptake and use.
Denitrification is the process by which nitrate is converted back into atmospheric nitrogen by bacteria. This process occurs in environments where oxygen is limited, such as wetlands and the deep ocean.
Human activities, such as the use of synthetic fertilizers and the burning of fossil fuels, have significantly altered the nitrogen cycle. The excess nitrogen from fertilizer runoff and emissions can lead to eutrophication of waterways and the formation of harmful algal blooms. Nitrogen oxides released from the burning of fossil fuels contribute to air pollution and the formation of acid rain.
Efforts to reduce nitrogen pollution and increase nitrogen use efficiency, such as the use of organic fertilizers and conservation agriculture, are critical to mitigating the impacts of human activities on the nitrogen cycle and the environment.