The nitrogen cycle refers to the movement of nitrogen within and between the atmosphere, biosphere, hydrosphere and geosphere. The nitrogen cycle matters because nitrogen is an essential nutrient for sustaining life on Earth. Nitrogen is a core component of amino acids, which are the building blocks of proteins, and of nucleic acids, which are the building blocks of genetic material (RNA and DNA). When other resources such as light and water are abundant, ecosystem productivity and biomass is often limited by the amount of available nitrogen. This is the primary reason why nitrogen is an essential part of fertilizers used to enhance soil quality for agricultural activities.
What is the nitrogen cycle?
Nitrogen cycles through both the abiotic and biotic parts of the Earth system. The largest reservoir of nitrogen is found in the atmosphere, mostly as nitrogen gas (N2). Nitrogen gas makes up 78% of the air we breathe. Most nitrogen enters ecosystems via certain kinds of bacteria in soil and plant roots that convert nitrogen gas into ammonia (NH3). This process is called nitrogen fixation. A very small amount of nitrogen is fixed via lightning interacting with the air. Once nitrogen is fixed, other types of bacteria convert ammonia to nitrate (NO3‑) and nitrite (NO2–), which can then be used by other bacteria and plants. Consumers (herbivores and predators) get nitrogen compounds from the plants and animals they eat. Nitrogen returns to the soil when organisms release waste or die and are decomposed by bacteria and fungi. Nitrogen is released back to the atmosphere by bacteria get their energy by breaking down nitrate and nitrite into nitrogen gas (also called denitrification).
A simplified diagram showing terrestrial nitrogen cycling. Credit: Wikimedia
Nitrogen levels can vary significantly in aquatic and terrestrial habitats, and can be affected by various human activities and environmental phenomena, including:
- The production and use of fertilizers for agricultural activities that increase the amount of nutrients in soil or water, especially nitrogen (and phosphorus). These nutrients increase plant and algae growth. However, increased nutrient is not always a good thing. For example, in aquatic environments nutrient-rich runoff (erosion) can cause large numbers of algae to grow. When these algae die they are consumed by bacteria which can reduce oxygen levels in the water, killing fish and other species. This process is known as eutrophication.
- The abundance (biomass) and biodiversity of bacteria, plants, fungi species that can fix nitrogen. Certain species of agricultural crops, including legumes (plants in the bean family) such as soy, clover, and peas, also host symbiotic nitrogen-fixing bacteria in their roots. As humans increase nitrogen soil levels, this can also reduce populations of plant species that are adapted to low-nitrogen soils.
- The burning of fossil fuels releases nitrous oxide (N2O), a greenhouse gas, into the atmosphere. The burning of fossil fuels also releases nitrogen oxides (NOx), sulfur dioxide (SO2), and carbon dioxide (CO2) that react with water vapor, oxygen, and other chemicals to form acid rain. Acid rain can affect freshwater sources, where increased nutrients can result in harmful algal blooms that reduce water oxygen levels and harm fish populations and other wildlife. Additionally, acid rain increases chemical weathering of rocks, including manmade structures.
- Increased precipitation can increase erosion and thus increase the transport of nitrogen (and other chemical nutrients) into soils, freshwater environments, and coastal waters.
- Deforestation, habitat loss, and erosion can reduce the nutrient levels in soils. The process of producing fertilizers also introducespollutants into the environment which alters habitats.
- Changes in ocean circulation patterns can alter the concentration and distribution of nutrients that are transported offshore. Dissolved chemical nutrients, especially nitrogen (and phosphorus) are critical for marine organisms, including the growth of plankton and algae which form the base of most ocean food webs. When organisms die they sink to the bottom of the ocean where their nutrients are released as they decay. These nutrients can be returned to the surface by ascending currents through a process known as upwelling, which is caused by offshore winds. Regions with coastal upwelling have highly productive ecosystems because of the nutrient rich upwelled water.
- Changes in atmospheric circulation patterns can alter the concentration and distribution of dust (airborne particles) that contain nitrogen (and other nutrients) for life on land and in aquatic environments.
Earth system model about the nitrogen cycle
The Earth system model below includes some of the processes and phenomena related to the nitrogen cycle. These processes operate at various rates and on different spatial and temporal scales. For example, fixation of nitrogen by bacteria happens on small spatial scales, but human use of fertilizers can impact entire ecosystems. Can you think of additional cause and effect relationships between the parts of the nitrogen cycle and other processes in the Earth system?
Explore the Earth System
Click the linked bolded terms (e.g. agricultural activities, productivity and biomass, and nutrient level) on this page to learn more about these process and phenomena. Alternatively, explore the Understanding Global Change Infographic and find new topics that are of interest and/or locally relevant to you.
