Phosphorus cycle

Phosphorus Cycle Study Guide

What is the Phosphorus Cycle?

The phosphorus cycle is the movement of phosphorus through the biosphere, lithosphere (rocks), hydrosphere (water), and organisms. Unlike carbon and nitrogen, phosphorus does not have a gaseous form in the atmosphere. Instead, it cycles mainly through soil, water, and living things.

Importance of Phosphorus

• Essential for DNA, RNA, and ATP (energy storage in cells).

• Important for cell membranes (phospholipids).

• Vital for plant growth and root development.

• Supports strong bones and teeth in animals.

Major Components of the Phosphorus Cycle

1. Weathering of Rocks

• Phosphorus is stored in rocks as phosphate minerals.

• Weathering (rain, wind, erosion) releases phosphate (PO₄³⁻) into soil and water.

2. Absorption by Plants

• Plants absorb phosphate from the soil through their roots.

• Phosphates are used to build organic molecules like DNA and ATP.

3. Consumption by Animals

• Herbivores eat plants, obtaining phosphorus.

• Carnivores get phosphorus by eating herbivores.

4. Decomposition & Return to Soil

• When plants and animals die, decomposers (bacteria and fungi) break down organic matter, returning phosphorus to the soil.

• Waste (urine, feces) also returns phosphorus to the environment.

5. Sedimentation & Geological Processes

• Some phosphorus washes into rivers, lakes, and oceans.

• Phosphates settle to form new sedimentary rock.

• Over millions of years, geological uplift brings phosphorus-rich rocks back to the surface.

Short-Term vs. Long-Term Phosphorus Cycle

Human Impact on the Phosphorus Cycle

1. Fertilizers & Agriculture

• Farmers use phosphate-based fertilizers to boost crop growth.

• Excess phosphorus from fertilizers washes into rivers and lakes, causing eutrophication.

2. Eutrophication (Nutrient Overload in Water)

• Excess phosphorus causes rapid algae growth (algal blooms).

• Algae die and decompose, using up oxygen.

• Low oxygen kills fish and disrupts ecosystems.

3. Deforestation & Soil Erosion

• Clearing forests removes plants that hold phosphorus in the soil.

• Erosion washes phosphorus away, reducing soil fertility.

Key Terms to Remember

✅ Phosphate (PO₄³⁻): The main form of phosphorus in the cycle.
✅ Weathering: Process that releases phosphorus from rocks.
✅ Decomposition: Returns phosphorus to soil and water.
✅ Eutrophication: Overgrowth of algae due to excess nutrients.
✅ Sedimentation: Phosphorus settles and forms new rocks.
✅ Geological Uplift: Rocks containing phosphorus are pushed to the surface.

Phosphorus Cycle Diagram

(Include a labeled diagram showing phosphorus moving between rocks, soil, plants, animals, water, and sediments.)

Review Questions

1. Why is phosphorus important for living organisms?

2. How does phosphorus naturally enter the soil?

3. What is eutrophication, and how does it affect aquatic ecosystems?

4. How do human activities impact the phosphorus cycle?

5. Why is the phosphorus cycle different from the carbon and nitrogen cycles?