27.10:
The Phosphorus Cycle
In the phosphorous cycle, phosphorous atoms exist primarily in rock, not a gaseous state. When rock erodes, the phosphorous is released and becomes dissolved in streams, lakes and ground water.
Plants and algae use these free inorganic phosphates in the soil or water to produce organic molecules. Then by consuming the plants, heterotrophs tap into the phosphorous stores to build their own compounds. When organisms die, decomposers break up the phosphorous molecules and release inorganic phosphate to be used again by plants and algae, a process called phosphate mineralization.
While phosphates cycle quickly through organisms in the food chain, they have a longer residence time in water. Dissolved phosphate ions react to form insoluble compounds that precipitate in the ocean and become sediment, then rock, and as result of tectonic uplift, eventually return to the environment.
However, since the amount of phosphorous is limited in the environment, it must be provided to agricultural crop plants as fertilizer to get maximum yield. Excess phosphorous becomes run-off in aquatic ecosystems resulting in a variety of environmental problems such as algal blooms.
27.10:
The Phosphorus Cycle
Unlike carbon, water, and nitrogen, phosphorus is not present in the atmosphere as a gas. Instead, most phosphorus in the ecosystem exists as compounds, such as phosphate ions (PO43-), found in soil, water, sediment and rocks. Phosphorus is often a limiting nutrient (i.e., in short supply). Consequently, phosphorus is added to most agricultural fertilizers, which can cause environmental problems related to runoff in aquatic ecosystems.
Biological Phosphorus Cycle
Phosphorus is present in many important biological structures, such as DNA, cell membranes, bones and teeth. It is not present in the atmosphere in a gaseous form, but is found in minerals, sediment, volcanic ash, and aerosols. As rocks and sediment weather over time, they release inorganic phosphate, which gradually reaches soil and surface water. Plants absorb and incorporate these phosphates into organic molecules. Animals obtain and incorporate phosphates by consuming plants and other animals. When plants and animals die or excrete waste, organic phosphates return to the soil and are broken down by bacteria—in a process called phosphate mineralization—into inorganic forms that can again be used by plants.
Geochemical Phosphorus Cycle
Natural runoff can transport phosphates to rivers, lakes, and the ocean, where they can be ingested by aquatic organisms. When aquatic organisms die or excrete waste, phosphorus-containing compounds may sink to the ocean floor and eventually form sedimentary layers. Over thousands of years, geological uplift can return phosphorus-containing rocks from the ocean to land.
Human Impacts on the Phosphorus Cycle
Like nitrogen, phosphorus is often a limiting factor in plant growth in natural environments, which has led to the agricultural practice of adding phosphorus to fertilizers in order to increase crop yield. However, agricultural runoff from this practice can stimulate the rapid growth of aquatic producers, causing a variety of environmental problems.
Suggested Reading
Watson, Andrew J., Timothy M. Lenton, and Benjamin J. W. Mills. “Ocean Deoxygenation, the Global Phosphorus Cycle and the Possibility of Human-Caused Large-Scale Ocean Anoxia.” Philosophical Transactions. Series A, Mathematical, Physical, and Engineering Sciences 375, no. 2102 (September 13, 2017). [Source]
White, Angelicque, and Sonya Dyhrman. “The Marine Phosphorus Cycle.” Frontiers in Microbiology 4 (May 21, 2013). [Source]