The phosphorus cycle describes the movement of phosphorus through Earth’s various systems, including the lithosphere, hydrosphere, and biosphere. This element is fundamental for all life, forming a structural component of DNA, RNA, and cell membranes. It also plays a central role in energy transfer within organisms as part of adenosine triphosphate (ATP). Unlike cycles of other elements such as carbon and nitrogen, the phosphorus cycle does not involve a significant atmospheric gaseous phase, making it primarily a terrestrial and aquatic process.
Earth’s Initial Phosphorus Reservoir
The phosphorus cycle begins with rocks, primarily phosphate-rich sedimentary rocks like phosphorite and certain igneous rocks. Apatite, a group of phosphate minerals, is the main source of phosphorus in these geological formations. This phosphorus is locked within the rock structure until it is released through a process called weathering.
Weathering involves both physical and chemical breakdown of rocks. Physical weathering, caused by factors like freeze-thaw cycles and wind, fragments rocks. Chemical weathering, through reactions with natural acids or microbial activity, alters the rock’s composition, freeing phosphate ions. This gradual process makes phosphorus available in the soil and water, initiating its journey through ecosystems.
Movement Through Terrestrial Ecosystems
Once phosphorus is released into the soil, it becomes accessible to plants. Plants absorb these dissolved phosphates through their roots. The concentration of plant-available phosphorus in soil is typically quite low, often ranging from 0.001 to 1 mg/L. Within plant tissues, phosphorus is incorporated into organic molecules, including nucleic acids and phospholipids.
As plants are consumed by herbivores, phosphorus is transferred through the terrestrial food web. This transfer continues as carnivores consume herbivores, moving the element higher up the food chain. When plants and animals die or excrete waste, decomposers such as bacteria and fungi break down the organic matter. Decomposition returns phosphorus to the soil in an inorganic, plant-available form, completing the terrestrial cycle.
Phosphorus’s Journey to Aquatic Environments
From terrestrial systems, phosphorus moves to aquatic environments. Rainwater runoff carries dissolved phosphates and phosphorus-laden sediment from land into streams, rivers, and eventually lakes and oceans. This transport occurs naturally as water flows over and through the landscape, picking up the readily available phosphorus. Human activities, such as agricultural runoff and wastewater discharges, can significantly increase phosphorus in water bodies.
In aquatic ecosystems, dissolved phosphates are absorbed by aquatic plants and algae, forming the base of the aquatic food web. These primary producers are consumed by aquatic herbivores, and phosphorus moves through the food chain to higher trophic levels like fish. Some phosphorus also settles to the bottom of water bodies as sediment, becoming part of a temporary or long-term reservoir.
Long-Term Storage and Geological Uplift
Phosphorus accumulates in marine sediments over geological timescales. This occurs as dead aquatic organisms, waste, and particulate matter sink to the ocean floor. Over millions of years, these sediments undergo compaction and lithification, forming new sedimentary rocks, particularly phosphorite. This process effectively locks away phosphorus from active biological cycling.
The cycle completes through geological uplift, a slow process driven by tectonic plate movements. These forces push the deep-seated sedimentary rocks back to the Earth’s surface. Once exposed, these uplifted rocks are subjected to weathering, releasing phosphorus and restarting the cycle. This geological component highlights the extremely slow nature of the overall phosphorus cycle, with individual phosphorus ions potentially remaining in ocean sediments for tens of thousands of years before uplift.