Lesson: Biogeochemical Cycles: Carbon, Nitrogen, Phosphorus

Background Context and Historical Significance:

Long before humans began to study Earth’s systems, nature had established complex cycles to transport vital elements throughout the planet. These cycles, known as biogeochemical cycles (combining ‘bio’ for life, ‘geo’ for Earth, and ‘chemical’ for the substances that move), are nature’s way of recycling essential nutrients. The understanding and study of these cycles have become increasingly important as human activities have begun to significantly alter them, with potential consequences for Earth’s climate, ecosystems, and human societies.

Detailed Content:

  1. What are Biogeochemical Cycles?:
    • Definition: Biogeochemical cycles refer to the natural processes that recycle essential nutrients from the environment, to living organisms, and back.
    • Importance: They ensure the sustainability of life by providing essential elements in a usable form to living organisms.
  2. Carbon Cycle:
    • Process:
      • Photosynthesis: Plants and algae absorb carbon dioxide (CO2) and sunlight to produce glucose and oxygen.
      • Respiration: Organisms use oxygen to break down glucose, releasing energy and emitting CO2.
      • Decomposition: Decomposers break down dead organisms, releasing CO2 back into the atmosphere.
      • Combustion: Burning of fossil fuels or wood releases stored carbon as CO2.
      • Ocean Uptake and Release: Oceans absorb and release CO2, acting as a significant carbon sink.
    • Human Impact: Increased combustion of fossil fuels is elevating atmospheric CO2 levels, contributing to global warming.
  3. Nitrogen Cycle:
    • Process:
      • Nitrogen Fixation: Bacteria convert atmospheric nitrogen (N2) to ammonia (NH3).
      • Nitrification: Bacteria convert ammonia to nitrites (NO2-) and then to nitrates (NO3-).
      • Assimilation: Plants absorb nitrates to make proteins.
      • Ammonification: Decomposers convert organic waste into ammonia.
      • Denitrification: Bacteria convert nitrates back to N2.
    • Human Impact: Overuse of nitrogen-based fertilizers can lead to water pollution, affecting aquatic life.
  4. Phosphorus Cycle:
    • Process:
      • Weathering: Rain and weathering release phosphates from rocks into the soil and waters.
      • Assimilation: Plants absorb phosphates and animals obtain them by consuming plants or other animals.
      • Decomposition: Decomposers release phosphorus back into the soil.
      • Sedimentation: Over time, phosphates are carried to the ocean and can be incorporated into new rock formations.
    • Human Impact: Mining for phosphate fertilizers and detergents disrupts the natural cycle and can cause water pollution known as eutrophication.

Patterns and Trends:

The modification of these cycles by human activities has led to:

  • Climate Change: Elevated CO2 levels are causing a greenhouse effect.
  • Ocean Acidification: Increased CO2 absorption by oceans is making them more acidic.
  • Dead Zones: Overfertilization leads to oxygen-depleted zones in aquatic ecosystems.

Influential Figures or Works:

  • James Lovelock: Proposed the Gaia hypothesis, which views Earth as a self-regulating system where biogeochemical cycles play a critical role.
  • Charles David Keeling: Developed the Keeling Curve, which documents the rise of CO2 levels in the atmosphere.


Biogeochemical cycles are essential for life and have been finely tuned over millennia. However, rapid human development is altering these cycles, resulting in unforeseen and potentially dire consequences for our planet. Understanding and respecting these cycles is paramount for our sustainable future.

Further Reading & Activities:

  1. Documentaries: “Our Planet” narrated by Sir David Attenborough offers insights into the delicate balance of nature.
  2. Activities: Organize or participate in tree planting activities or local clean-up drives to help restore natural balances.

These cycles, as intricate as they are, underscore the delicate balance and interdependency of life on Earth. Recognizing their importance and our role in them is fundamental to understanding the larger picture of planetary health.