1.6 The Phosphorus Cycle

Living things need phosphorus to make DNA, RNA, ATP (the molecule cells use to carry energy) and cell membranes. Vertebrates also use it in bones and teeth.

Unlike carbon and nitrogen, phosphorus has no major gas form, so the atmosphere isn't an important reservoir. Most phosphorus is locked in rocks and in sediments on the ocean floor. It enters ecosystems mainly as phosphate (PO₄³⁻), released slowly when rocks weather.

Because the main reservoir is rock and the return path depends on uplift, the phosphorus cycle is the slowest of the major nutrient cycles. Within an ecosystem, however, phosphorus is recycled fairly quickly between soil, plants and decomposers.

• Weathering: wind, rain and chemical reactions break down phosphate-containing rock, releasing phosphate into soil and water.
• Uptake: plants absorb phosphate from soil through their roots, and algae absorb it from water. Animals get phosphorus by eating plants or other animals.
• Decomposition: when organisms die or produce waste, decomposers return phosphate to the soil or water.
• Runoff: phosphate washes from land into rivers, lakes and eventually the ocean.
• Sedimentation: in water, phosphate can combine with other elements, like calcium, and settle out as sediment. Over long times, sediments become rock.
• Geologic uplift: tectonic forces raise old seafloor rock onto land, where weathering can start the cycle again. This takes millions of years.

Since weathering is slow and phosphate tends to bind to soil particles or settle out of water, very little phosphorus is dissolved and available at any moment. In many freshwater lakes it's the nutrient in shortest supply, so it limits how much algae can grow. Add phosphorus and algae take off.

A famous experiment at Canada's Experimental Lakes Area in the 1970s divided a lake in two with a curtain. Scientists added carbon and nitrogen to both halves, but phosphorus to only one. Only the half that got phosphorus developed a large algal bloom, and photos of the result became famous evidence for limiting phosphates in detergents.

People mine phosphate rock to make fertilizer, which speeds up the cycle by moving phosphorus out of rock far faster than weathering would. Phosphorus also enters water from sewage, animal manure and, in the past, laundry detergents.

Extra phosphate in lakes and streams feeds algal blooms. When the algae die, decomposers use up dissolved oxygen, which can kill fish. This process is called eutrophication, covered in Unit 8. Because phosphate binds to soil, a lot of it reaches water attached to eroded soil, so controlling erosion also reduces phosphorus pollution.