Phosphorus cycle

The phosphorus cycle is a biogeochemical cycle that moves phosphorus through the Earth's lithosphere, hydrosphere, and biosphere. Phosphorus is a vital element for all living things. It is a key component of DNA and RNA, which carry genetic instructions. It also forms ATP, a molecule used for energy storage in cells. Additionally, phosphorus is found in phospholipids, which make up cell membranes. In animals, inorganic phosphorus in the form of apatite helps build bones and tooth enamel.

Unlike the nitrogen or carbon cycles, the phosphorus cycle does not involve the atmosphere in a significant way. This is because phosphorus and its compounds do not easily enter a gaseous phase. The main source of gaseous phosphorus is phosphine, but it only forms under very specific, isolated conditions. Instead, most phosphorus remains on land, in rocks, or in soil minerals. It moves primarily as orthophosphate, which is the most common form found in the environment.

The cycle moves through several major physical and chemical stages. First, tectonic uplift brings phosphorus-bearing rocks, such as apatite, to the surface. Next, physical erosion and chemical weathering break these rocks down. Weathering releases phosphorus into the soil in a soluble form. Plants then assimilate this phosphate to create organic compounds. When animals eat these plants, the phosphorus enters their tissues. Finally, when organisms die, they decay and return phosphorus to the soil or water.

In aquatic ecosystems, phosphorus exists in four distinct pools. These include dissolved inorganic phosphorus (DIP), dissolved organic phosphorus (DOP), particulate inorganic phosphorus (PIP), and particulate organic phosphorus (POP). DIP mainly consists of orthophosphate and polyphosphate. DOP is made of substances like DNA and phosphoproteins. Particulate matter, like PIP and POP, does not pass through a 0.45 μm filter. POP includes both living and dead organisms, while PIP consists mainly of hydroxyapatite.

Phosphorus is often a limiting nutrient in many environments. This means that the amount of phosphorus available can control how much life can grow. In terrestrial systems, soil microorganisms act as both sinks and sources for available phosphorus. In aquatic systems, a sudden increase in phosphorus can lead to eutrophication. This process involves the over-production of organic matter, such as massive algae blooms. These blooms can occur in both freshwater and saltwater environments.

Human activities have significantly altered the natural speed and balance of this cycle. People mine phosphorus minerals to create fertilizers and industrial products. This helps increase crop yields in agriculture. However, this also leads to runoff that carries phosphates into lakes and streams. This excess phosphorus from sewage, detergents, and fertilizers can cause pollution. Large-scale fertilizer application is a primary way humans move phosphorus from rocks into the wider environment.

The phosphorus cycle is one of the slowest biogeochemical cycles on Earth. While phosphorus moves quickly through individual plants and animals, the movement through rocks and oceans takes a very long time. On land, phosphorus can become less available to plants over thousands of years due to runoff. In the ocean, only about 1% of the phosphorus carried to the deep sea is removed by burial in sediments. This slow movement between the Earth's crust and the biosphere defines the long-term rhythm of life.