Phosphorus Cycle

Phosphate compounds are found abundantly in sedimentary rocks. Over long periods, weathering releases phosphorus into surface water and soils. Volcanic ash and fertilizers are also significant sources of phosphates.

Phosphorus is a crucial nutrient for all living organisms, forming building blocks like DNA and cell membranes. It is also vital for plant growth. In nature, phosphorus primarily exists as phosphates, which are phosphorus atoms bonded to four oxygen atoms.

Plants absorb phosphate compounds from the soil. Animals then acquire phosphorus by consuming these plants. When organisms excrete waste or die, detritivores like worms take up phosphates, or they return to the soil. Phosphorus can also be carried by surface runoff to rivers, lakes, and oceans, where aquatic organisms such as phytoplankton absorb it. Marine organisms' waste and bodies sink to the ocean floor, forming new sedimentary layers. Over long geological timescales, these phosphorus-containing sedimentary rocks can be uplifted from the ocean back to land, restarting the cycle, though this process is very slow.

Phosphorus is often a limiting factor in ecosystems. However, excess phosphorus from fertilizers, carried by surface runoff into aquatic ecosystems, can lead to eutrophication. This phenomenon involves the excessive growth of algae or microbes that were previously limited by low phosphorus levels.

Eutrophication is harmful because when the large quantities of algae die and are decomposed by microbes, significant amounts of oxygen are consumed. This sharp drop in dissolved oxygen levels can kill aquatic organisms. Areas in lakes and oceans depleted of oxygen due to nutrient influx are known as dead zones. For example, fertilizer runoff from the Mississippi River basin created a dead zone over 8,000 square miles off the coast of Mexico.