Biogeochemical Cycles

What are biogeochemical cycles?

In Earth Sciences or Geosciences, biogeochemical cycles refer to a set of interconnected or independent natural pathways through which nutrients and other essential elements of living matter are circulated between the abiotic and biotic components of the Earth System. The abiotic or non-living components include the atmosphere – the layer of gases surrounding the Earth, the lithosphere – the solid part of the Earth, and the hydrosphere (+cryosphere) – all the water present on the Earth, including the polar icecaps. The biotic component is the biosphere or the zone of life on the Earth. As the name suggests, all of the biological, geological and chemical aspects of the cycle are considered.

The most popular biogeochemical cycles include the water or hydrological cycle, oxygen cycle, carbon cycle, nitrogen cycle, weathering in the rock cycle.

What are nutrients and nutrient cycle?

Any living being, plant or animal or otherwise, is made up of complex molecules that are essentially aggregates of some major elements such as C, H, O, N, P, S and several other minor elements. These elements and their simple molecules used by living beings for growth and development are what we call nutrients. During their lives, the organisms capture these elements from the environment around them, combine them, form complex compounds, and ultimately release them into the system upon death and decay. The various processes that alter the forms of these elements and their simple compounds make up the nutrient cycle. The nutrients are circulated within the system, and their total mass (all forms included) remains constant, making the system a closed system.

Components of biogeochemical cycles:

Every biogeochemical cycle must have three essential components:

  1. Reservoir pool – A large store of nutrients at any stage of a biogeochemical cycle, typically abiotic but can be biotic in some cases.
  2. Exchange or cycling pool – A smaller but active stage of a biogeochemical cycle responsible for the rapid exchange of nutrients between the various components involved in the cycle. These are generally biotic.
  3. Nutrient Sink – A small or large stage of a biogeochemical cycle where the nutrients settle out of circulation indefinitely. The occurrences of certain unique reactions and processes bring the elements back into circulation.

Classification of biogeochemical cycles:

According to reservoir phases involved in cycling, biogeochemical cycles can be broadly categorized into two groups: Gaseous cycles and sedimentary cycles.

  • Gaseous cycles – A cycle was the primary reservoir involved in the atmosphere (air) or hydrosphere (ocean). Typical examples include the carbon cycle, oxygen cycle and water cycle. The involvement of a large fluid reservoir results in the rapid circulation of elements. This gives the system a strong capacity for self-adjustment against the changes occurring in the biosphere, which forces the circulation.
  • Sedimentary cycles – A cycle where the primary reservoir involved is the lithosphere (solid Earth). Typical examples include the sulphur cycle, phosphorus cycle, and rock cycle. Each segment of the cycle consists fundamentally of a solution phase and a rock phase. Weathering of minerals from the Earth’s crust releases some elements into the solution phase, which pass through a series of intermediate stages, and ultimately reach the deep seas (sink). In the rock phase, the insoluble elements are deposited as sediments in the shallow seas and eventually becomes a rock, only to be weathered and recycled again.

By, Rahul Subbaraman (Department of Geological Sciences, IISER Kolkata)

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