Unit 5: Biogeochemical Cycles

1. Introduction to Biogeochemical Cycles

Unlike energy, which flows one-way, matter (chemical elements) is cycled.

Biogeochemical Cycle: The pathway by which a chemical element moves through the bio- (living), geo- (earth), and chemo- (chemical) components of an ecosystem.

Types of Cycles:

1. Gaseous Cycles: Reservoir is the atmosphere/ocean (e.g., Carbon, Nitrogen, Sulphur, Water).
2. Sedimentary Cycles: Reservoir is the Earth's crust (e.g., Phosphorus).

2. The Hydrological (Water) Cycle

The continuous movement of water (H₂O), driven by solar energy.

Key Processes:

• Evaporation: Liquid to gas (from oceans, lakes).
• Condensation: Gas to liquid (forming clouds).
• Precipitation: Water released from clouds (rain, snow).
• Transpiration: Evaporation of water from plants.
• Runoff: Water flowing over the land surface.
• Infiltration: Water soaking into the ground (groundwater).

3. The Carbon Cycle (Gaseous)

Carbon is the basis of all organic molecules. The main reservoir is CO₂ in the atmosphere.

Key Processes:

1. Photosynthesis: Plants pull CO₂ from the atmosphere to make organic compounds (sugar).
2. Respiration: Organisms (plants, animals, decomposers) break down organic compounds, releasing CO₂.
3. Decomposition: Decomposers break down dead organic matter, releasing CO₂.
4. Combustion: Burning of fossil fuels and wood releases stored carbon into the atmosphere as CO₂ (major human impact).

4. The Nitrogen Cycle (Gaseous)

Nitrogen is essential for proteins and DNA. The main reservoir is N₂ gas in the atmosphere (~78%), but this form is unusable by most organisms.

Key Processes (driven by bacteria):

1. Nitrogen Fixation: Conversion of unusable N₂ gas into usable ammonia (NH₃).
   • Biological: By bacteria (e.g., Rhizobium in legume roots).
   • Atmospheric: By lightning.
2. Nitrification: A two-step process by bacteria in the soil.
   • Step 1: Ammonia (NH₃) → Nitrite (NO₂⁻) (by Nitrosomonas).
   • Step 2: Nitrite (NO₂⁻) → Nitrate (NO₃⁻) (by Nitrobacter).
3. Assimilation: Plants absorb usable nitrogen (nitrates or ammonia) from the soil.
4. Ammonification: Decomposers convert organic nitrogen (from dead organisms/waste) back into ammonia (NH₃).
5. Denitrification: Bacteria (e.g., Pseudomonas) in anaerobic (low-oxygen) conditions convert nitrates (NO₃⁻) back into N₂ gas, which returns to the atmosphere.

5. The Phosphorus Cycle (Sedimentary)

Phosphorus is essential for DNA, RNA, and ATP.

Main Reservoir: Phosphate rock (sedimentary rock). There is no significant gaseous component.

Key Processes:

1. Weathering: Rain and erosion slowly break down phosphate rocks, releasing phosphate (PO₄³⁻) into the soil and water (this is the bottleneck, making it a limiting nutrient).
2. Assimilation (Uptake): Plants absorb dissolved phosphate from the soil.
3. Decomposition: Decomposers return phosphate to the soil.
4. Sedimentation: Phosphate washes into oceans, settles, and forms new sedimentary rock over millions of years.
5. Geological Uplift: Tectonics brings the rock back to the surface.
Diagram: A diagram *without* an atmospheric component. Show "Weathering" from "Rock" to "Soil." "Assimilation" to "Plants" and "Animals." "Decomposition" back to "Soil." "Runoff" to "Ocean" and "Sedimentation" to new "Rock."

6. The Sulphur Cycle (Gaseous)

Sulphur is important for some proteins. This cycle has both gaseous and sedimentary components.

Reservoirs: Rocks, soils, and the atmosphere (as Sulphur Dioxide, SO₂).

Key Processes:

1. Weathering/Decomposition: Releases sulphur from rocks and organic matter.
2. Volcanic Activity: A major natural source of atmospheric SO₂.
3. Human Impact (Combustion): Burning of fossil fuels (especially coal) releases massive amounts of SO₂, the primary cause of acid rain.
4. Atmospheric Conversion: SO₂ reacts with water to form sulphuric acid (H₂SO₄), which falls as acid rain.
5. Assimilation: Plants absorb sulphate ions (SO₄²⁻) from the soil.

7. Nutrient Cycling in Ecosystems

This describes the movement and storage of nutrients *within* an ecosystem.

Ecosystem Input of Nutrients

How nutrients enter the ecosystem:

• Atmospheric Input:
  • Wet deposition: Nutrients dissolved in rain (e.g., acid rain).
  • Dry deposition: Nutrients falling as dust.
  • Gaseous fixation: e.g., Nitrogen fixation.
• Weathering: Breakdown of parent rock releases P, Ca, K, etc.

Biotic Accumulation

The process of nutrients being taken up by organisms and stored in their biomass.

• Plants absorb nutrients (uptake).
• These nutrients are stored in leaves, wood, etc.
• In a tropical rainforest, decomposition is very fast, so most nutrients are stored in the biotic accumulation (the living plants) and not in the soil.

Nutrient Supply and Uptake

• Nutrient Supply: This is controlled by decomposition (mineralization). Decomposers break down organic detritus and release inorganic nutrients.
• Nutrient Uptake: This is assimilation. Plant roots re-absorb the inorganic nutrients, completing the "internal" cycle.