Ecosystems and processes: Mineral nutrient cycling.

The idea

Mineral nutrient cycling describes the continuous movement of essential chemical elements, such as nitrogen and phosphorus, through an ecosystem. Using Gersmehl's model, this cycle is visualised through three main stores: biomass (living vegetation and animals), litter (dead organic matter), and soil. Nutrients transfer between these stores via flows: littering moves nutrients from biomass to litter, decomposition transfers them from litter to soil, and root uptake returns them to biomass. The system also experiences inputs, such as dissolved nutrients in precipitation or chemical weathering of parent rock, and outputs, including surface runoff and leaching.

For example: In a tropical rainforest, the biomass store is exceptionally large, and rapid decomposition ensures swift nutrient cycling despite heavily leached soils.

What earns the marks

• Identification of the three main nutrient stores: biomass, litter, and soil.
• Explanation of the flows between stores, including littering, decomposition, and root uptake.
• Description of system inputs (precipitation, weathering) and outputs (leaching, surface runoff).
• Application of Gersmehl's nutrient cycle model to specific biomes, such as the rapid cycling in tropical rainforests versus slow cycling in tundra.

Common mistakes and how to put them right

• Confusing leaching with surface runoff.

  Put it right: Leaching is the downward movement of soluble minerals through the soil profile, whereas runoff occurs over the surface.

• Assuming the soil is always the largest nutrient store.

  Put it right: In tropical rainforests, the biomass is the largest store due to rapid uptake and heavy leaching.

• Treating the nutrient cycle as a closed system.

  Put it right: Ecosystems have inputs like precipitation and weathering, and outputs like leaching, making them open systems.

Exam tips

• Draw and label a Gersmehl diagram to illustrate nutrient cycling in a specific biome, ensuring the size of circles and arrows reflects the relative size of stores and flows.
• Use specific examples of mineral nutrients, such as nitrogen, phosphorus, or potassium, rather than just referring to 'nutrients' generically.
• Link the speed of nutrient cycling to climatic factors, explaining how temperature and moisture control the rate of decomposition.