Phosphorus cycle

The importance of phosphates

Phosphate ions are key components of DNA, RNA, ATP and phospholipids, meaning it is important for phosphorus to cycle within an ecosystem. These ions move through rocks, water, soil, sediments and organisms 

The phosphorus cycle process

• phosphates in rocks are released into the soil and into water sources in the form of phosphate ions, due to chemical weathering

• phosphate ions may be washed into the soil by rain, and are taken up by plants, or absorbed from water by algae

• phosphate ions are transferred to consumers during feeding 

• phosphate ions in waste products and dead organisms are released into the soil or water during decomposition by saprobionts (when aquatic organisms die they sink to the bottom of the water source and sediment)

• the phosphate ions can be taken up and used again by producers, or may be trapped in sediments that may turn into phosphorus-containing rock once again

What is the role of saprobionts?

Following the death of plants and animals, saprobionts break down the organic compounds in the dead matter, as well as those in any urine and faeces, releasing phosphate ions into the soil. Phosphate ions are released into the soil and are assimilated by plants by active transport

What is guano?

Guano is the waste product of sea birds, and contains a high proportion of phosphate ions. Guano plays a key role in the phosphorus cycle, as excretion directly onto soil, as well as runoffs from covered cliffs, returns a significant amount of PO₄³ ions in the soil, particularly in coastal areas. Due to its high proportion of phosphate ions, guano can be used as a natural fertiliser to replace the ions lost when crops are harvested, and livestock removed.  

Impact of human activity upon the phosphorus cycle

Fertiliser use: the use of fertilisers can increase the amount of phosphorus in the soil, leading to over-fertilisation and runoff into water bodies  

Land use changes: changes in land use, such as deforestation and urbanisation, can reduce the amount of phosphorus in the soil and increase the amount of phosphorus in runoff and leaching  

Sewage release: sewage can increase the amount of phosphorus in water bodies, leading to eutrophication and changes in aquatic ecosystems