The Carbon Cycle and the Nitrogen Cycle

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Unit 1.5 The Carbon & Nitrogen Cycles

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Key Questions

  • 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

  • Human activities are substantially modifying the global carbon and nitrogen cycles. The global carbon cycle is being modified principally by the burning of fossil fuels, and also by deforestation; these activities are increasing the carbon dioxide concentration of the atmosphere and changing global climate.

    The nitrogen cycle is being modified principally by the production of nitrogen fertilizer, and also by the planting of legumes and the combustion of fossil fuels; these activities are more than doubling the rate of fixation of nitrogen and contributing to the unbalanced productivity and acidification of ecosystems.

    With the aim of quantifying these disruptions, the principal flows among reservoirs in pre-industrial times and today are estimated in the framework of simplified models. The methane sub-cycle of the carbon cycle and the nitrous oxide sub-cycle of the nitrogen cycle are also discussed from this viewpoint.

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