1.6 The Phosphorus Cycle

Phosphorus has no atmospheric phase

Unlike carbon or nitrogen, phosphorus does not exist as a gas, so it doesn't cycle through the atmosphere. Its main reservoir is rock and sediment, where phosphorus exists as phosphate (PO4³⁻). Weathering of rocks slowly releases phosphate into soil and water, making this the slowest of the major biogeochemical cycles.

Phosphorus moves through the biosphere

Plants absorb dissolved phosphate from soil or water and incorporate it into organic molecules (DNA, ATP, cell membranes). Phosphorus moves up the food chain as animals eat plants, and returns to soil/water through decomposition and excretion. Because phosphate binds tightly to soil particles, it is often a limiting nutrient for plant and algal growth.

Human activity accelerates the cycle

Mining phosphate rock for fertilizers and detergents adds huge amounts of phosphorus to ecosystems quickly. Runoff carries excess phosphate into lakes and rivers, causing eutrophication: algal blooms, oxygen depletion, and dead zones. Sedimentation eventually buries phosphorus in ocean/lake sediments, where it can remain for millions of years until geologic uplift exposes it again.

Key terms

Main phosphorus reservoir

Rocks and sediments, since phosphorus has no gaseous atmospheric phase.

Phosphate (PO4³⁻)

The chemical form of phosphorus used by organisms and cycled through soil, water, and living things.

Eutrophication

Nutrient overload (often phosphorus) in water causing algal blooms and oxygen depletion, harming aquatic life.

Limiting nutrient

A scarce nutrient, like phosphorus, that restricts growth of organisms even when other resources are abundant.

Human impact on P cycle

Mining for fertilizers/detergents accelerates phosphorus release, increasing runoff and eutrophication risk.

Try an AP-style question

What makes the phosphorus cycle different from the carbon and nitrogen cycles?

1. It cycles very quickly
2. It moves mainly through the atmosphere
3. It doesn't involve living things
4. It has no significant gaseous phase