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20.1: Biogeochemical Cycles

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    Key Points

    • Carbon, nitrogen, hydrogen, oxygen, phosphorus, and sulfur are conserved and recycled in the atmosphere, on land, in water, or beneath the earth’s surface.
    • Materials are recycled via erosion, weathering, water drainage, and the movement of tectonic plates.
    • Water is essential to all living processes, while carbon is found in all organic (carbon-based) macromolecules.
    • Nitrogen and phosphorus are major components of nucleic acids and play major roles in agriculture.
    • Sulfur plays a role in the three-dimensional folding of proteins and is released into the atmosphere by the burning of fossil fuels.

    Biogeochemical Cycles: Introduction

    Energy flows directionally through ecosystems, entering as sunlight (or inorganic molecules for chemoautotrophs) and leaving as heat during the many transfers between trophic levels. However, the matter that makes up living organisms is conserved and recycled. The six most common elements associated with organic (carbon-based) molecules (carbon, nitrogen, hydrogen, oxygen, phosphorus, and sulfur) take a variety of chemical forms and may exist for long periods in the atmosphere, on land, in water, or beneath the earth’s surface. The cycling of these elements is interconnected. Geologic processes, such as weathering, erosion, water drainage, and the movement of the continental plates, all play a role in this recycling of materials. Because geology and chemistry have major roles in the study of this process, the recycling of inorganic matter between living organisms and their environment is called a biogeochemical cycle.  It is important to remember that while matter and energy are processed in cycles, they aren't necessarily moving in a simple circle and don't really have a beginning or an end.  Today, anthropogenic (human) activities are altering all major ecosystems and the biogeochemical cycles they drive.

    The components of organic molecules are constantly being stored and recycled as part of their biogeochemical cycle. Water, which contains hydrogen and oxygen, is essential to all living processes. The hydrosphere is the area of the earth where water movement and storage occurs. Water can be liquid on the surface and beneath the surface or frozen (rivers, lakes, oceans, groundwater, polar ice caps, and glaciers) or exist as water vapor in the atmosphere. Carbon, found in all organic macromolecules, is an important constituent of fossil fuels. Nitrogen, a major component of our nucleic acids and proteins, is critical to human agriculture. Phosphorus, a major component of nucleic acid (along with nitrogen), is one of the main ingredients in artificial fertilizers used in agriculture and their associated environmental impacts on our surface water. Sulfur, critical to the 3–D folding of proteins (as in disulfide binding), is released into the atmosphere by the burning of fossil fuels, such as coal.

     

    A diagram shows labels for the atmosphere in the sky, the hydrosphere in a body of water, the biosphere by a tree, and the lithosphere as the ground.

    Figure \(\PageIndex{1}\): Earth has a hydrosphere, where water movement and storage occurs. The cycle is important for leaching certain components of organic matter into rivers, lakes, and oceans, and is a reservoir for carbon.

    The cycling of all of these elements is interconnected. For example, the movement of water is critical for the leaching of nitrogen and phosphate into rivers, lakes, and oceans. Furthermore, the ocean itself is a major reservoir for carbon. Thus, mineral nutrients are cycled, either rapidly or slowly, through the entire biosphere, from one living organism to another, and between the biotic and abiotic world.

    What pops into your mind when you hear the word reservoir? Probably a body of water, small or large, that is impounded behind a dam. In Earth systems science, the term reservoir is used for a distinctive kind of place where a certain kind of material is stored, or resides, for some period of time. This part of a cycle that holds an element or water for a short period of time is sometimes also called an exchange pool, or pool. For example, the atmosphere is an exchange pool for water. It usually holds water (in the form of water vapor) for just a few days. Some other examples of reservoirs or pools for water you will encounter in the course include glaciers; the soil layer; the aggregate of bodies of fresh water on the continents (rivers and lakes). Material moves into and out of reservoirs. The rate at which a given material moves between reservoirs is called a flux. If the flux of material into and out of a given reservoir is the same for some period of time, that reservoir is said to be in a steady state. Commonly, however, the flux in and the flux out are not equal.

    Contributors and Attributions

    Modified by Kyle Whittinghill (University of Pittsburgh) from the following sources

    20.1: Biogeochemical Cycles is shared under a CC BY-NC-SA license and was authored, remixed, and/or curated by LibreTexts.