In-Depth Notes on Organic Matter and Decomposition

General Topics

  • Exam Schedule: Exam on Friday; review session on Wednesday.
  • Main Topics Covered:
    • Global Pools of Carbon
    • Importance of Soil Organic Matter
    • Decomposition of Organic Matter
    • Carbon to Nitrogen Ratios in Organic Matter and Residues

Global Carbon Pools

  • Rank of carbon pools from greatest to least:
    • Ocean, Soil (1m), Atmosphere, Vegetation
    • Other rankings include:
    • Soil (1m), Vegetation, Ocean, Atmosphere
    • Vegetative plants, Soil (1m), Ocean, Atmosphere
  • Carbon Reserves:
    • Ocean: 40,000 Pg
    • Soil (1 meter depth): 2,000 Pg
    • Atmosphere: 800 Pg
    • Vegetation: 700 Pg

Fossil Fuel Emissions

  • Sources of Carbon:
    • Coal Deposits: 2,000 Pg
    • Oil & Gas Deposits: 500 Pg
    • Volcanic Activity: Minimal release
  • Sequestration: Carbon capture and storage practices critical in the fight against climate change.

Soil Organic Matter (Humus)

  • Definition: The smallest component of soil, significantly impacts physical, biological, and chemical properties.
  • Benefits of Soil Organic Matter:
    • Improves water retention
    • Increases cation exchange capacity (CEC)
    • Enhances soil structure and strength
    • Provides essential nutrients (especially Nitrogen)

Decomposition of Organic Matter

Aerobic Decomposition
  1. Process:
    • Organic compounds broken down by aerobic bacteria.
    • Release of CO2 and heat energy.
    • Nutrient release or immobilization (primarily Nitrogen).
    • Formation of humus, resistant to further decomposition.
Anaerobic Decomposition
  1. Process:
    • Occurs in low-oxygen environments (<10% oxygen).
    • Slower decomposition rates.
    • Production of methane (CH4) instead of CO2.
Factors Influencing Decomposition Rate
  • Environmental Conditions:
    • Temperature
    • Soil moisture
    • Soil aeration
  • Residue Placement:
    • Surface placement vs. incorporation below soil surface:
    • Surface: slower degradation, more exposure to elements.
    • Incorporated: enhanced microbial contact, stable moisture.
  • Carbon/Nitrogen Ratio:
    • High C/N ratio (>25) leads to Nitrogen immobilization.
    • Low C/N ratio (<25) allows for mineralization of Nitrogen.

Characteristics of Organic Soils (Histosols)

  • Organic Soil Definition:
    • Contains residues of plants and animals.
    • Typically >30% Organic Matter (OM).
  • Formation Conditions:
    • Wet depressional areas or cold, wet climates.
  • Types of Organic Soils:
    • Fibrists: higher visibility of plant parts.
    • Hemists: recognizable fibers
    • Saprists: highly decomposed, amorphous
      decay.

Unique Aspects of Organic Soils

  1. Lightweight: Low bulk density, often <0.2 g/cc.
  2. Erosion: Susceptible to wind erosion when dry.
  3. Burning Risks: Can burn when dry.
  4. Shrink and Decay: Lead to subsidence issues when drained and cultivated.

Historical Note: Tollund Man

  • Discovered in Denmark, preserved under organic bog soil, dating back 2,000 years.
  • The preservation state raised historical curiosity and significance in understanding anaerobic preservation.