1.6 The Phosphorus Cycle

Basics

• Movement of P atoms/molecules between sources and sinks.

• Major reservoirs: rocks/sediments containing P minerals.

• Very slow cycle compared to C/H2O/N cycles; no gas phase of P; limiting nutrient for plant growth.

• Necessary for DNA, ATP, and bone/tooth enamel in certain animals.

Sources

• Natural sources: weathering of rocks releases phosphate (PO4-3) that dissolves into soil/water.

• Synthetic sources: mining phosphate minerals for fertilizers/detergents, contributing to water contamination through runoff.

Assimilation/Decomposition

• P is absorbed by plant roots and assimilated into tissues; animals acquire P by eating plants/animals.

• Decomposers return phosphate to the soil.

Sedimentation/Geological Uplift

• Phosphate sediments form solid bits that settle at the bottom (sedimentation).

• Sediments can become sedimentary rock over time; geological uplift restarts the cycle.

Eutrophication

• Excess N & P from human/animal contamination leads to algal blooms, blocking sunlight, harming aquatic plants, and causing oxygen depletion, which kills fish.

Steps In Order:

1. Weathering: Natural sources release phosphate (PO4-3) from rocks into soil and water.

2. Assimilation: Plants absorb phosphates, which are assimilated into their tissues; animals acquire phosphorus by consuming plants or other animals.

3. Decomposition: Decomposers return phosphates to the soil when organisms die and decay.

4. Sedimentation: Phosphates settle at the bottom of water bodies, forming sediments.

5. Geological Uplift: Over time, sediments can become sedimentary rock; geological uplift can expose these rocks, restarting the cycle.