Nutrient Cycling (AQA A Level Geography)

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

Gersmehl model shows the transfer of nutrients between stores in the nutrient cycle. In the diagram above, all the stores and flows are shown equal, however, each and every ecosystem will show proportionality in relation to its unique make-up.

• 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