Biogeochemical cycles

DEFINITION: Movements of chemical elements among parts of the Earth, including the atmosphere, the Earth’s crust, oceans, and living things

The movement and location of chemical elements among the various systems that make up the Earth affect the planet’s climate, the diversity and range of species, the impact and intensity of geological events, and a host of other matters related to the Earth as a whole and subsystems within it.

Modern geoscientists consider the Earth to comprise a set of interacting open systems: the atmosphere (the layers of air that envelop the Earth), the biosphere (the areas that support and are filled with living things), the lithosphere (the Earth’s crust), and the hydrosphere (the water found on the Earth). These various spheres contain within them important chemical elements. The six most abundant elements are carbon, nitrogen, oxygen, hydrogen, phosphorus, and sulfur; these elements, alone or in combination with others, serve as critical macronutrients to living things. Other important macronutrients are potassium, calcium, iron, and magnesium. So-called micronutrients are present in smaller amounts and play critical roles in sustaining life on Earth: boron (used by green plants), copper (critical to the functioning of some enzymes), and molybdenum (vital for the functioning of nitrogen-fixing bacteria). Other chemical elements, such as ammonia, are also involved in biogeochemical cycles.

The atmosphere and the oceans serve as important reservoirs where the key gas elements (carbon, nitrogen, oxygen, and hydrogen) are mainly stored. Soils and sedimentary rocks serve as principal reservoirs for the storage of phosphorus and sulfur. Throughout the Earth, these various chemicals are moved about by different physical, chemical, and human-induced processes that alter the respective amounts of elements in various places and within both living and nonliving things.

An example of a biogeochemical cycle is the hydrologic (water) cycle that moves oxygen and hydrogen, along with other chemicals, from reservoir to reservoir, principally at or near the surface of the Earth. This includes the proportionally large amounts of water residing within living things as well as the water found in the oceans, the atmosphere, surface water, groundwater, glaciers, and soils. During various periods of the planet’s history, the relative percentages of oxygen, hydrogen, and other chemical elements found in the hydrosphere have changed dramatically. These changes have, in turn, affected the overall temperature and climate of the Earth and the types of environments available for living things; they have also sometimes caused physical changes to the other spheres (atmosphere, biosphere, and lithosphere).

Another biogeochemical cycle is the carbon cycle. Most of the original carbon in the Earth’s systems came out of the Earth’s mantle in the form of carbon dioxide gas released by volcanoes. The carbon dioxide in the atmosphere is taken up by various means, including dissolution in seawater in the form of bicarbonate ions and absorption by photosynthetic organisms such as algae and plants that convert it into sugar and other organic chemicals. As this carbon enters the food chain, it ultimately finds its way into the tissues of animals. It is estimated that some 100 billion tons of carbon move every year worldwide from the atmosphere into life-forms. In this sense, the carbon that has been so transformed has moved from the reservoir of the atmosphere into the reservoir of the biosphere.

Human beings’ burning of fuels that are rich in carbon has resulted in increases in the amounts of carbon dioxide and other greenhouse gases in the Earth’s atmosphere, beyond the rate that would have occurred via natural processes. These greenhouse gases contribute to global climate change, which has led to more extreme weather events.

Bibliography

Bashkin, Vladimir N., with Robert W. Howarth. Modern Biogeochemistry. Kluwer Academic, 2003.

Edwards, Thea M., et al. "Ammonia and Aquatic Ecosystems – A Review of Global Sources, Biogeochemical Cycling, and Effects on Fish." Science of The Total Environment, vol. 907, 2024. ScienceDirect, doi.org/10.1016/j.scitotenv.2023.167911. Accessed 17 Sept. 2025.

Libes, Susan M. Introduction to Marine Biogeochemistry. 2nd ed., Academic, 2009.

Moseman, Andrew, and Daniel Rothman. “How Much Carbon Dioxide Does the Earth Naturally Absorb?” Massachusetts Institute of Technology Climate Portal, 26 Jan 2024, climate.mit.edu/ask-mit/how-much-carbon-dioxide-does-earth-naturally-absorb. Accessed 17 Sept. 2025.

"What Is the Carbon Cycle?" National Ocean Service, 16 June 2024, oceanservice.noaa.gov/facts/carbon-cycle.html. Accessed 17 Sept. 2025.