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Phosphorus Cycle Overview

Aug 29, 2025

Overview

This lecture explains the phosphorus cycle, detailing how phosphorus moves between reservoirs and the environmental impacts of human activity on this cycle.

The Phosphorus Cycle: Overview

  • The phosphorus cycle describes the movement of phosphorus between sources (rocks, sediments) and sinks (soils, water bodies).
  • Phosphorus movement is much slower than in the carbon or nitrogen cycles.

Major Reservoirs and Processes

  • Main reservoirs of phosphorus are rocks and phosphorus-containing sediments.
  • Weathering releases phosphate from rocks and sediments through wind and rain.
  • Phosphate is carried by runoff into soils and aquatic ecosystems.
  • There is no gaseous phase of phosphorus, which slows its cycle.

Biological Importance of Phosphorus

  • Phosphorus is essential for ATP (energy), DNA, and bone and teeth strength in animals.
  • Plants absorb phosphorus from soil; animals obtain it by eating plants or other animals.

Human Impact and Synthetic Sources

  • Humans mine phosphate rocks for fertilizers and detergents.
  • Runoff from agriculture and household wastewater adds phosphorus to water bodies.

Recycling and Return to Soil

  • Decomposition and waste return phosphorus from dead matter and excretion to the soil.
  • This forms a mini-loop similar to that in the nitrogen cycle.

Sedimentation and Geological Uplift

  • Excess phosphate in water precipitates out, forming sediments on the waterbed (sedimentation).
  • Over time, sediments are compressed into sedimentary rock.
  • Geological uplift exposes new phosphorus rocks to weathering, restarting the cycle.

Environmental Impact: Eutrophication

  • Excess phosphorus (and nitrogen) in water causes eutrophication, leading to algal blooms.
  • Algal blooms block sunlight, killing underwater plants.
  • Bacteria decomposing dead plants and algae use up oxygen, causing "dead zones" with little aquatic life.
  • Positive feedback loop: less oxygen leads to more deaths, more decomposition, and further reduced oxygen.

Key Terms & Definitions

  • Phosphorus Cycle โ€” movement of phosphorus through rocks, soils, water, and living things.
  • Weathering โ€” breakdown of rocks, releasing phosphate.
  • Assimilation โ€” absorption of phosphorus by plants.
  • Sedimentation โ€” formation of solid phosphate deposits at the bottom of water bodies.
  • Geological Uplift โ€” movement of rock layers to the surface by tectonic activity.
  • Eutrophication โ€” nutrient enrichment of water, causing excessive algae growth and oxygen loss.
  • Positive Feedback Loop โ€” a process that accelerates itself due to its outcomes.

Action Items / Next Steps

  • Practice: Use a model of the phosphorus cycle to describe how phosphorus moves between two reservoirs.
  • Review phosphorus cycle diagrams and the effects of human activities on nutrient cycles.