Rucete ✏ Campbell Biology In a Nutshell
Unit 8 ECOLOGY — Concept 55.4 Biological and Geochemical Processes Cycle Nutrients and Water in Ecosystems
Unlike energy, which flows one-way through ecosystems, chemical elements like carbon, nitrogen, and phosphorus cycle continuously. These biogeochemical cycles are driven by both biological and geochemical processes, and their rates are heavily influenced by climate, decomposition, and human activities.
1. Decomposition and Nutrient Recycling
- Decomposers (fungi, prokaryotes) recycle nutrients from organic matter to the environment
- Decomposition rates depend on temperature, moisture, and nutrient levels
- Tropics: fast decomposition → poor soils
- Temperate zones: moderate rate → nutrient-rich detritus
- Cold/wet: slow decomposition → organic matter builds up
- Aquatic sediments decompose slowly under low oxygen, acting as nutrient sinks unless upwelling occurs
2. Biogeochemical Cycles: Global vs Local
- Global cycles: involve atmospheric gases (carbon, nitrogen)
- Local cycles: confined to soil/water (phosphorus, calcium, potassium)
- Four nutrient reservoirs:
- A: Organic available (e.g., living organisms)
- B: Organic unavailable (e.g., fossil fuels)
- C: Inorganic available (e.g., soil, water)
- D: Inorganic unavailable (e.g., rocks)
3. The Water Cycle
- Reservoirs: oceans (97%), ice (2%), freshwater (1%)
- Processes: evaporation → condensation → precipitation → runoff
- Transpiration by plants contributes to atmospheric water
- Essential for primary production and decomposition
4. The Carbon Cycle
- Carbon is the basis of organic molecules
- Photosynthesis removes CO₂; respiration returns it
- Burning fossil fuels and deforestation add excess CO₂ → global warming
- Largest carbon reservoir = sedimentary rock (e.g., limestone)
5. The Nitrogen Cycle
- Nitrogen is crucial for proteins and nucleic acids
- Atmospheric N₂ must be fixed to NH₄⁺ or NO₃⁻ by bacteria/lightning/fertilizers
- Cycle processes: nitrification, ammonification, denitrification
- Excess nitrogen → eutrophication and air pollution
6. The Phosphorus Cycle
- Needed for DNA, ATP, bones, and teeth
- Exists as phosphate (PO₄³⁻); no gaseous phase
- Released by rock weathering, taken up by plants
- Local cycling dominates → returns via waste and decay
7. Case Study: Hubbard Brook Experimental Forest
- Measured nutrient flow in forested watershed
- Intact forest: tight nutrient retention
- Deforestation caused:
- ↑ water runoff (30–40%)
- 60× nitrate loss
- Showed vegetation is critical for nutrient retention and ecosystem health
In a Nutshell
Chemical elements cycle through ecosystems via complex biological and geological pathways. Decomposers, climate, and ecosystem structure all influence the rate and direction of nutrient cycling. Disruptions—like deforestation or pollution—can rapidly unbalance these cycles, leading to nutrient loss, ecosystem degradation, and downstream effects like algal blooms or climate change.