The Carbon Cycle and the Nitrogen Cycle

Key Questions

  • Answer:

    The nitrogen cycle describes how nitrogen moves through the biosphere and atmosphere. It is important because living things require nitrogen.


    Nitrogen cycles through the biosphere and the atmosphere through what is known as the nitrogen cycle. The major reservoir of nitrogen is the atmosphere, which is primarily made up of nitrogen. Atmospheric nitrogen cannot be used by most organisms and must be converted into a usable form. This occurs in nitrogen fixation.

    The major changes nitrogen goes through are nitrogen fixation, nitrification, anammox, denitrification, and ammonification.

    In nitrogen fixation, certain prokaryotes convert nitrogen gas to a form that can be used by other organisms (ammonia or NH3). This process can also occur due to human activities.

    In nitrification, NH3 is converted to nitrite by microbes known as ammonia-oxidizers. This nitrite is then converted to nitrate by nitrite-oxidizing bacteria. This process occurs in aerobic conditions (conditions that require oxygen).

    In anammox, nitrification occurs in anoxic conditions (conditions depleted of oxygen). Anammox bacteria oxidize ammonia so that is converted to nitrogen gas (N2).

    In denitrification, nitrate is converted to nitrogen gas by prokayrotes typically in soils, sediments, anoxic areas of lakes and oceans. This is an anaerobic process (process that does not require oxygen) and is how nitrogen is returned to the atmosphere.

    In ammonification, an organism dies and decomposers return inorganic nitrogen back into the environment in the form of ammonia.

    Nitrogen is important for all living things. It is a component in DNA, proteins, and chlorophyll in plants. Disrupting the nitrogen cycle can lead to imbalances in ecosystems. For example, soils with too much nitrogen can become acidic. Increased nitrogen in aquatic systems can lead to eutrophication.

  • Answer:

    The carbon cycle is one nutrient cycle on earth, and it is important because carbon forms the framework of all organic molecules.


    On the most basic level, the carbon cycle explains how carbon is recycled on Earth, describing how the biosphere, hydrosphere, atmosphere and sediments exchange carbon. Below is an illustration showing how green plants absorb CO2 (think of it as their food), animals then eat the food and the carbon, and animals then release it back into the atmosphere. This is one of the more basic examples of a carbon cycle.

    Organisms that photosynthesize (plants and phytoplankton) convert carbon to organic forms that are then consumed by animals and fungi. This process takes atmospheric CO2 and makes it available for others to consume.

    Producers, consumers, and decomposing organism also give off CO2 through cellular respiration.

    Below is a more complete picture of the carbon cycle, including the burning of fossil fuels, plant respiration, and the eruption of volcanoes, which all add CO2 to the atmosphere.

    We generally think of four sources of carbon, also called carbon reservoirs: the atmosphere, the terrestrial biosphere (forests, non-living organic materials, freshwater systems, and etc), the oceans, and the sediments (fossil fuels). Carbon is exchanged between all four.

    Check out this resource from NASA to learn more and read about North America's first State of the Carbon Cycle Report .

    Sources: Campbell and Reece's Biology and Earth Observatory at