Nutrient Cycling (AQA A Level Geography)

Mineral Nutrient Cycling

What are nutrients?

• Nutrients are chemical and mineral substances derived from:

  • Rock weathering

  • Decomposition

  • Precipitation

• Substances include:

  • Phosphorous (P)

  • Potassium (K)

  • Nitrogen (N)

  • Carbon (C)

  • Sulphur (S)

  • Trace elements (zinc (Zn), copper (Cu) etc.)

• These nutrients are necessary for the health and well-being of all living organisms and assist in the breakdown of food for energy

Mineral nutrient cycling

• These minerals are found in finite amounts and are therefore, recycled constantly between the atmosphere, soil, flora and fauna to sustain life in an ecosystem:

  • Grass takes nitrogen from the soil and carbon dioxide (CO2) from the air and is used to generate biomass

  • Sheep graze on the grass and the nutrients are passed along the food chain

  • At each stage, decomposers act on the waste produced, returning some of the nutrients to the soil

  • Respiration from all plants, animals and decomposers release CO2 into the atmosphere, thereby, returning carbon back into the cycle

  • When the sheep die, decomposers break down the whole and therefore act to return all nutrients back to the soil

  • This ensures that nutrients are not lost within the ecosystem

• This cycle is usually presented in a format called the Gersmehl diagram and is a simple system of:

  • Inputs - dissolved in precipitation and weathered rock (dissolved in soil water) - shown as proportional arrows

  • Stores - biomass, litter and soil - shown as proportional circles

  • Outputs - nutrients lost through runoff and leaching - shown as proportional arrows

  • Transfers - littering, decomposition and plant uptake - shown as proportional arrows

• Biomass - refers to the total weight of living organisms - plants and animals, of an area

  • Nutrients are stored in plants and animals

  • Plants uptake nutrients from the soil, along with the sun’s energy, to produce their own food

  • When animals eat plants, they take up the nutrients and convert plant tissues to their own food

• Litter - dead plants tissues, dead animals and animal wastes accumulate on the ground to form a layer of litter

  • Rain dissolves minerals, e.g. nitrogen and carbon in the air and add the nutrients to the litter

  • Some of the nutrients may be washed away by run-off

• Soil - when litter is decomposed, the nutrients are broken down into inorganic form and stored in the soil

  • Weathering of sub-surface parent rock contributes nutrients to the soil

  • Nutrients of soil may be lost through leaching

Nutrient cycles of different ecosystems

• In tropical rainforests, the nutrient store of biomass is huge with small stores of soil and litter 

  • Biomass is largest due to high temperatures, high rainfall and long growing seasons. Rapid absorption from soils means that the rate of photosynthesis is high

  • Soil store is small because high rainfall encourages leaching and uptake of nutrients is rapid

  • Litter is a small store because high temperatures and moisture levels result in fast decomposition

• Unlike the tropical rainforest's constant warmth and moist conditions, deciduous forests fluctuating climate (hot, wet, cold) means that the largest store is the soil

• Tropical savanna stores are more even; with less leaching due to less precipitation, although biomass is slightly larger as they have a continuous cover of perennial grasses

• Temperate grasslands have larger soil stores due to shorter growing seasons and colder temperatures

• Taiga ecosystems see the litter as the biggest store, as there is a slow transfer due to extreme cold

• Deserts have larger soil stores due to dry, hot/cold conditions that slow rate of transfers