Phosphorus is a fundamental element for all known life forms on Earth. It forms the structural backbone of DNA and RNA, and is a major component of adenosine triphosphate (ATP). Beyond genetic material and energy transfer, phosphorus is found in phospholipids, the main building blocks of cell membranes. In animals, calcium phosphate salts contribute to the structural integrity of bones and teeth. Unlike elements such as carbon or nitrogen, phosphorus does not readily exist in a gaseous form, making its cycle primarily focused on movement through the lithosphere, hydrosphere, and biosphere.
Decomposition and Phosphorus Release
When plants and animals die, their organic matter contains phosphorus in complex forms. Microorganisms like bacteria and fungi drive decomposition, breaking down complex organic molecules within dead organisms. Through their metabolic activities, these decomposers transform organic phosphorus into inorganic phosphate. This process, known as mineralization, releases phosphorus in soluble inorganic forms that plants can utilize. The efficiency of this release depends on factors like soil moisture, temperature, and the carbon-to-phosphorus ratio of the decomposing material.
Phosphorus Forms and Availability
Once released from decaying organic matter, phosphorus primarily exists as orthophosphate ions, such as dihydrogen phosphate and hydrogen phosphate. These are the main forms plants absorb from the soil. Soil pH significantly influences the availability of these ions, with an optimal range generally between 6.0 and 7.0. At a pH below 7.0, dihydrogen phosphate is more prevalent, while at higher levels, hydrogen phosphate dominates. Outside this optimal range, phosphate ions react with other minerals, reducing their availability; for instance, binding with aluminum and iron in acidic soils, or precipitating with calcium in neutral or alkaline soils.
Re-entry into the Living World
Inorganic phosphate ions are absorbed by plants through their root systems, utilizing specialized transport systems. Once absorbed, phosphorus is incorporated into various organic molecules within the plant’s tissues. This phosphorus then moves through the food web as herbivores consume plants, and carnivores obtain phosphorus by consuming other animals. Organisms utilize phosphorus for growth and other metabolic functions. It is eventually returned to the soil or water through waste products or when organisms die.
The Long-Term Phosphorus Journey
Phosphorus not immediately taken up by plants or the food web can embark on a much longer journey. Dissolved phosphates are carried by water through leaching and runoff into rivers, lakes, and eventually the oceans. In aquatic environments, phosphorus settles and accumulates on the seabed, forming sediments. Over vast geological timescales, these sediments transform into sedimentary rocks, like phosphate rock, representing a long-term reservoir. Geological uplift brings these rocks back to land, where weathering slowly releases phosphate ions back into soils and water, making them available for biological uptake.