Biogeochemical cycles are the natural pathways through which essential chemical elements and compounds—such as carbon, nitrogen, phosphorus, sulfur, oxygen, and water—move through the living (biotic) and non-living (abiotic) components of Earth. The term reflects the three realms these cycles traverse: biological organisms, geological materials (rocks, soil, sediment), and chemical processes in the atmosphere, hydrosphere, and lithosphere. Matter moves between reservoirs like the atmosphere, oceans, soil, rocks, and living organisms through processes including photosynthesis, respiration, decomposition, weathering, volcanic activity, precipitation, and combustion.
Major examples include:
- The carbon cycle: carbon moves between the atmosphere (as CO2), living organisms (via photosynthesis and respiration), oceans, and long-term storage in rocks and fossil fuels.
- The nitrogen cycle: atmospheric nitrogen gas is converted by bacteria into usable forms for plants, then cycled through organisms, soil, and back to the atmosphere via denitrification.
- The phosphorus cycle: primarily moves through rock weathering, soil, water, and organisms, lacking a significant atmospheric phase.
- The water (hydrologic) cycle: water moves between oceans, atmosphere, land, and living things through evaporation, condensation, precipitation, and transpiration.
These cycles play several critical roles in ecosystems. First, they ensure that finite elements are continuously recycled and made available to living organisms, since matter on Earth is essentially limited and cannot be created anew. Second, they link the physical environment with biological communities, allowing energy flow and nutrient availability to sustain food webs. Third, they regulate global climate and environmental conditions—for instance, the carbon cycle influences atmospheric greenhouse gas concentrations. Fourth, they maintain ecosystem productivity and stability by ensuring nutrients like nitrogen and phosphorus, which are often limiting factors for growth, remain accessible to plants and, in turn, animals.
Human activities—such as fossil fuel combustion, deforestation, and fertilizer use—can significantly disrupt these natural cycles, leading to consequences like climate change, eutrophication of waterways, and altered soil fertility, underscoring the importance of understanding and managing these processes sustainably.