Understanding Key Ecosystem Cycles: Nitrogen, Carbon, Fire & Soil

Posted on Feb 7, 2025 in Geology

Understanding Key Ecosystem Cycles

The Nitrogen Cycle

Nitrogen constitutes 78% of the air. While abundant, it’s largely unusable in its gaseous form. It exists in small amounts in water and is part of mineral salts or ammonia. Very few organisms can directly use atmospheric nitrogen. The nitrogen cycle in nature proceeds as follows:

  1. Plants absorb dissolved nitrogen compounds from water and, through photosynthesis, incorporate them into organic compounds like proteins.
  2. Certain bacteria can convert atmospheric nitrogen into compounds that plants can use.
  3. These bacteria live freely in the soil or symbiotically within plant roots.
  4. Nitrogen-rich organic compounds, such as proteins, pass to consumers through the food chain. Nitrogen returns to the soil through death and decomposition or via feces.
  5. Decomposing bacteria transform detritus nitrogen into nitrates, which producers then utilize. Other bacteria break down soil nitrates, releasing nitrogen back into the atmosphere.

Inorganic fertilizers can also introduce nitrogen into the natural cycle.

The Carbon Cycle

Carbon is a crucial chemical element, abundant in both living and non-living matter.

  1. Plants, algae, and photosynthetic bacteria capture carbon dioxide from the air or water through photosynthesis, converting it into organic compounds.
  2. Carbon moves through trophic levels via consumption.
  3. Carbon returns to the atmosphere as carbon dioxide through the respiration of living organisms.
  4. The remains of buried organisms transform into coal or oil over time.
  5. Atmospheric CO2 is released from burning plant materials, volcanic activity, and the combustion of coal and oil.
  6. Marine organisms use dissolved carbon in water to form their shells. Upon death, this carbon returns to the earth.

Ecological Changes After a Fire

Fire is a frequent occurrence in Mediterranean forests. Some plants in this ecosystem are pyrophytes, meaning they regenerate quickly after a fire.

  1. After a fire, only underground elements and pyrophytic seeds survive. The soil is damaged, and humus disappears.
  2. If the soil isn’t completely destroyed, pasture begins to regenerate with herbaceous plants.
  3. Over the next 10-15 years, burned trunks decompose, enriching the soil. Shrubs become dominant, and animal populations increase.
  4. Larger shrubs and pines become predominant after 30-35 years. Mammals, such as foxes, appear.
  5. The forest fully recovers in about 50-60 years.

The Soil as an Ecosystem

Soil is a complex ecosystem with the following components:

  • Inorganic matter: Bits of rock, air, water, and pore spaces.
  • Organic matter: Bacteria, fungi, small animals, undecomposed remains of organisms, and humus (partially decayed organic material).

Soil Formation

Soil formation is a slow and complex process. Mature soil exhibits distinct layers called soil horizons, each with a different composition. A cross-section of these horizons is called a soil profile.

Stages of Soil Formation

  1. Bedrock: Changes in temperature, air, and water break down the rocks, altering their composition. Lichens colonize the surface of the bedrock.
  2. Young Soil: Hundreds of years later, the combined action of water, air, and living organisms (lichens, mosses, and microorganisms) allows the formation of young soil, supporting undemanding herbs and grasses.
  3. Mature Soil: Several thousand years after the initial disturbance, deeper layers of the bedrock are affected, resulting in thicker soil enriched with organic matter. Mature soil horizons have formed.