Biogeochemical Cycles, Soils and Ecosystems
Cycles to know
- Carbon cycle: photosynthesis, respiration, decomposition, combustion, weathering and ocean exchange. Reservoirs: oceans (largest), rocks/fossil fuels, atmosphere, biosphere.
- Nitrogen cycle: nitrogen fixation, nitrification, assimilation, ammonification, denitrification. Nitrogen gas is abundant but unusable by most organisms until fixed.
- Phosphorus cycle: sedimentary, slow, no gaseous phase; often limiting in freshwater.
Soils
Soil forms from parent material, climate, organisms, topography and time. A profile has horizons (O, A, B, C, R). Key properties: texture (sand, silt, clay), structure, pH, organic matter and cation-exchange capacity. Clay and organic matter retain nutrients and water; sandy soils drain quickly.
Ecosystems
Energy flows through trophic levels with roughly 10% transfer efficiency (the '10% rule'). Nutrient cycling is largely closed in undisturbed systems; disturbances open them, causing losses.
Traps
- Saying nitrogen is unavailable because it is rare - it is abundant but inert (N₂).
- Assuming phosphorus has an atmospheric reservoir; it does not.
- Confusing soil texture (particle size) with structure (aggregation).
- Treating energy and nutrients as the same - energy flows, nutrients cycle.
Worked example
If producers capture 10 000 kJ, primary consumers receive about 1000 kJ and secondary consumers about 100 kJ. Rule: multiply by 0.1 per trophic level; explain why top predators are few.
Rule of thumb
When a question asks about limitation, identify which nutrient or factor is in shortest supply relative to demand - that is the limiting factor.
Environmental Change, Sustainability and Resource Management
Types of change
Environmental change can be gradual or abrupt, natural or human-induced. Distinguish forcing (the driver) from response (the observed change). Evidence comes from proxies (ice cores, tree rings, sediments, corals) and instrumental records. Natural drivers include orbital cycles (Milankovitch), volcanic eruptions and solar variation; human drivers include greenhouse gas emissions, land clearing and pollution.
Sustainability
Sustainability means meeting present needs without compromising future generations. It rests on environmental, social and economic pillars. Key ideas: carrying capacity, ecological footprint, renewable vs non-renewable resources, and precautionary principle. Resource management uses monitoring, regulation, restoration and adaptive management.
Managing impacts
- Mitigation reduces causes (emissions cuts, efficiency).
- Adaptation reduces vulnerability (levees, drought-resistant crops).
- Impact assessment predicts effects before development.
Traps
- Confusing mitigation with adaptation.
- Treating a correlation in proxy data as proof of causation.
- Assuming renewable resources are unlimited - many have use limits.
- Ignoring time lags between forcing and response.
Worked example
A city emits 2 million tonnes of CO₂ annually and aims to cut 40% by 2035. Target = 2 000 000 × 0.6 = 1.2 million tonnes, requiring a reduction of 800 000 tonnes. Rule: quantify the target, then choose mitigation or adaptation measures to match.
Rule of thumb
For any management question, state the resource, the pressure, the indicator used to monitor it, and the management response. This four-part structure answers most extended-response prompts.