16.4A: Sources and Sinks of Essential Elements

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Biogeochemical cycles are pathways by which essential elements flow from the abiotic and biotic compartments of the Earth.

Learning Objectives

Identify sources and sinks of essential elements

Key Points

Biogeochemical cycles are pathways by which nutrients flow between the abiotic and abiotic compartments of the Earth. The abiotic portion of the Earth includes the lithosphere (the geological component of the Earth) and the hydrosphere (the Earth’s water).
Ecosystems rely on biogeochemical cycles. Many of the nutrients that living things depend on, such as carbon, nitrogen, and phosphorous are in constant circulation.
Essential elements are often stored in reservoirs, where they can be taken out of circulation for years. For example, coal is a reservoir for carbon.
Humans can affect biogeochemical cycles. Humans extract carbon and nitrogen from the geosphere and use them for energy and fertilizer. This has increased the amount of these elements in circulation, which has detrimental effects on ecosystems.

Key Terms

Reservoir: Reservoirs are places where essential elements are sequestered for long periods of time.
biogeochemical cycle: A pathway by which a chemical element or molecule moves through both biotic (biosphere) and abiotic (lithosphere, atmosphere, and hydropshere) compartments of the planet.

Most important substances on Earth, such as oxygen, nitrogen, and water undergo turnover or cycling through both the biotic (living) and abiotic (geological, atmospheric, and hydrologic) compartments of the Earth. Flows of nutrients from living to non-living components of the Earth are called biogeochemical cycles.

Nutrient Cycles and the Biosphere

Ecosystems hinge on biogeochemical cycles. The nitrogen cycle, the phosphorous cycle, the sulfur cycle, and the carbon cycle all involve assimilation of these nutrients into living things. These elements are transferred among living things through food webs, until organisms ultimately die and release them back into the geosphere.

Figure: **The Carbon Cycle**: The element carbon moves from the biosphere to the geosphere and the hydrosphere. This flow from abiotic to biotic compartments of the Earth is typical of biogeochemical cycles.

Reservoirs of Essential Elements

Chemicals are sometimes sequestered for long periods of time and taken out of circulation. Locations where elements are stored for long periods of time are called reservoirs. Coal is a reservoir for carbon, and coal deposits can house carbon for thousands of years. The atmosphere is considered a reservoir for nitrogen.

Humans and Biogeochemical Cycles

Although the Earth receives energy from the Sun, the chemical composition of the planet is more or less fixed. Matter is occasionally added by meteorites, but supplies of essential elements generally do not change. However, human activity can change the proportion of nutrients that are in reservoirs and in circulation. For example, coal is a resevoir of carbon, but the human use of fossil fuels has released carbon into the atmosphere, increasing the amount of carbon in circulation. Likewise, phosphorous and nitrogen are extracted from geological reservoirs and used in phosphorous, and excesses of these elements have caused the overgrowth of plant matter and the disruption of many ecosystems.