Phosphorus is an important element for all life on Earth, playing a central role in biological processes. It is a building block for molecules such as DNA, RNA (which carry genetic information), and ATP, the energy currency within cells. It also forms a component of cell membranes and is a major mineral in bones and teeth. The phosphorus cycle describes the movement of this element through Earth systems, including the lithosphere, hydrosphere, and biosphere. Unlike other major biogeochemical cycles, such as carbon and nitrogen, the phosphorus cycle lacks an atmospheric gaseous phase, meaning it does not readily enter the atmosphere as a gas.
Release from Rocks
The primary reservoir for phosphorus on Earth is within rocks, particularly phosphate rocks like apatite. These minerals contain phosphorus as phosphate ions. The initial step involves the slow release of these ions from rocks through weathering and erosion. Physical weathering, caused by wind and water, breaks down rocks into smaller particles, while chemical weathering dissolves phosphorus compounds. This abiotic release is the slowest part of the short-term phosphorus cycle, continuously supplying dissolved phosphate to soils and water bodies.
Entry into Living Systems
Once released into the environment, dissolved inorganic phosphate ions become available for biological uptake. Plants absorb these ions directly from the soil through their root systems. Aquatic plants and algae similarly take up dissolved phosphate from water bodies. This absorption by producers is an important step, converting inorganic phosphorus into organic forms, making it accessible for the entire food web.
Movement Through Food Webs
After plants and algae absorb phosphorus, it integrates into their tissues as organic compounds. This phosphorus then moves through ecosystems as organisms consume one another. Herbivores obtain phosphorus by eating plants, and carnivores acquire it by consuming herbivores or other carnivores. Within living organisms, phosphorus is important for growth, development, and energy transfer, facilitating ATP synthesis.
Return to Soil and Water
Phosphorus is returned to the abiotic environment through the breakdown of organic matter. When plants and animals excrete waste or die, decomposers like bacteria and fungi break down their organic compounds. This decomposition, known as mineralization, converts organic phosphorus back into inorganic phosphate ions, which are released into the soil and water. Surface runoff can also carry phosphorus from terrestrial ecosystems into aquatic systems, further contributing to its return to water bodies.
Long-Term Storage and Geological Cycling
While much phosphorus cycles relatively quickly between living organisms and the soil or water, a portion can enter long-term storage. Some phosphorus in soil and water can settle and accumulate at the bottom of lakes and oceans, forming sediments. Over geological timescales, these phosphorus-rich sediments can be compacted and transformed into new sedimentary rocks. These rocks, containing stored phosphorus, can be uplifted to the Earth’s surface through tectonic activity. Once exposed, these uplifted rocks become subject to weathering and erosion, slowly releasing phosphorus back into the active cycle over millions of years, completing the slow, geological component of the phosphorus cycle.