Unit 5: Biogeochemical cycles
Introduction to Biogeochemical Cycles
Unlike energy, which flows, matter (nutrients) cycles within the biosphere. A biogeochemical cycle is the pathway by which a chemical element (e.g., Carbon, Nitrogen) moves through the biotic (bio-) and abiotic (geo-) components of an ecosystem.
Cycles are classified by their main reservoir:
- Gaseous Cycles: The main reservoir is the atmosphere (e.g., Carbon, Nitrogen). These cycles are global and fast.
- Sedimentary Cycles: The main reservoir is the Earth's crust (rocks and soil) (e.g., Phosphorus, Sulphur). These cycles are local and very slow.
The Hydrological (Water) Cycle
The movement of water (H₂O) on, above, and below the surface of the Earth. It is a physical cycle, not a chemical one, driven by solar energy.
[Image of a diagram of the Hydrological (Water) Cycle]
- Evaporation: Liquid water turns into water vapor (gas), primarily from oceans and lakes.
- Transpiration: The release of water vapor from plants into the atmosphere.
- Evapotranspiration: The combined process of evaporation and transpiration.
- Condensation: Water vapor in the air cools and turns back into liquid, forming clouds.
- Precipitation: Water falls from clouds as rain, snow, sleet, or hail.
- Infiltration: Water soaks into the soil to become groundwater.
- Runoff: Water flows over the land surface into rivers, lakes, and oceans.
The Carbon Cycle
The cycle of carbon, the basic building block of all organic life. Its main reservoir is the atmosphere (as CO₂).
[Image of a diagram of the Carbon Cycle]
- Photosynthesis: Plants and algae take CO₂ from the atmosphere and "fix" it into organic sugars.
- Consumption: Animals get carbon by eating plants or other animals.
- Respiration: All living organisms (plants, animals, decomposers) release CO₂ back into the atmosphere through cellular respiration.
- Decomposition: Decomposers (bacteria, fungi) break down dead organisms, returning carbon to the soil and air.
- Fossil Fuels: Over millions of years, some dead organic matter is buried and converts to coal, oil, or gas (fossil fuels).
- Combustion: Burning fossil fuels (and wood) releases large amounts of CO₂ into the atmosphere, causing global warming.
The Nitrogen Cycle
The cycle of nitrogen, a key component of proteins and DNA. The main reservoir is the atmosphere (as N₂ gas), but this form is unusable by most organisms.
This is the most complex cycle, driven entirely by bacteria. Focus on the names of the five key processes.
- Nitrogen Fixation: The conversion of unusable N₂ gas into usable ammonia (NH₃). This is done by:
- Biological: Nitrogen-fixing bacteria (e.g., *Rhizobium* in legume roots).
- Atmospheric: Lightning.
- Ammonification: Decomposers (bacteria, fungi) break down organic nitrogen (from dead organisms, waste) back into ammonia (NH₃), which becomes ammonium (NH₄⁺) in soil.
- Nitrification: A two-step process by nitrifying bacteria in the soil.
- Ammonium (NH₄⁺) is converted to Nitrite (NO₂⁻).
- Nitrite (NO₂⁻) is converted to Nitrate (NO₃⁻).
- Assimilation: Plants absorb usable nitrogen (Nitrate NO₃⁻ or Ammonium NH₄⁺) from the soil and incorporate it into their tissues (proteins, DNA).
- Denitrification: Denitrifying bacteria (in anaerobic, or oxygen-poor, conditions) convert Nitrate (NO₃⁻) back into N₂ gas, which returns to the atmosphere.
The Phosphorus Cycle
A sedimentary cycle. Phosphorus (a key part of ATP and DNA) has no major atmospheric component. Its main reservoir is in rocks and soil minerals.
[Image of a diagram of the Phosphorus Cycle]
- Weathering: Rain and erosion slowly break down phosphate-rich rocks, releasing phosphate (PO₄³⁻) into the soil and water.
- Assimilation (Uptake): Plants absorb phosphate from the soil. Animals get it by eating plants.
- Decomposition (Mineralization): Decomposers break down dead organisms, returning phosphate to the soil.
- Sedimentation: Phosphate in water washes into rivers and oceans, where it settles as sediment. Over millions of years, this sediment forms new rock.
Because this cycle is so slow and relies on weathering, phosphorus is often the primary limiting nutrient in freshwater ecosystems. Human use of P-rich fertilizers causes runoff, leading to eutrophication (algal blooms).
The Sulphur Cycle
A cycle that is both sedimentary (in rocks) and gaseous (in the atmosphere).
[Image of a diagram of the Sulphur Cycle]
- Reservoir: Most sulphur is stored in rocks (e.g., gypsum, pyrite) and in the ocean as sulfate (SO₄²⁻).
- Natural Gaseous Release: Volcanoes release sulfur dioxide (SO₂). Anaerobic bacteria release hydrogen sulfide (H₂S, the "rotten egg" smell).
- Assimilation: Plants absorb sulfate (SO₄²⁻) from the soil to make proteins.
- Human Impact: Burning coal (which is high in sulfur) releases massive amounts of SO₂.
- Acid Rain: SO₂ in the atmosphere reacts with water to form sulfuric acid (H₂SO₄), the main component of acid rain.
Nutrient Cycling in Ecosystems
This describes the overall movement of nutrients within a single ecosystem, focusing on the inputs, outputs (losses), and internal cycling.
- Input of Nutrients: How nutrients enter the ecosystem.
- Gaseous: From the atmosphere (e.g., N-fixation, CO₂).
- Weathering: From the parent rock (e.g., P, Ca, K).
- Wet/Dry Deposition: Nutrients carried by rain (wet) or dust (dry).
- Ecosystem Losses: How nutrients leave the ecosystem.
- Leaching/Runoff: Nutrients dissolve in water and are washed away (a major problem in agriculture).
- Gaseous: Nutrients return to the atmosphere (e.g., denitrification).
- Harvest: Humans removing crops or timber from the land.
- Nutrient Supply and Uptake: The process of nutrients being made available (e.g., by decomposition) and then taken up by plants. A healthy ecosystem has a "tight" nutrient cycle, where nutrients are recycled quickly and losses are minimal.