Rainforest Nutrient Cycle
How nutrients cycle through a tropical rainforest using the Gersmehl model - the biomass, litter and soil stores, the transfers between them, and why the soil is surprisingly poor.
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Lush forest, poor soil
A tropical rainforest is bursting with life, so you might expect very rich soil beneath it. In fact the soil is surprisingly poor. To see why, geographers follow the nutrient cycle. The Gersmehl model shows nutrients moving between three stores. In this module you will learn the stores, the transfers between them, and why clearing the forest quickly ruins the soil.
The three stores
In the Gersmehl model, nutrients are held in three stores, drawn as circles:
Match each transfer
- Uptake
- Litterfall
- Decomposition
- Leaching
- Roots absorb nutrients from the soil into the biomass
- Dead leaves fall from the biomass to the litter
- Decomposers break down litter, returning nutrients to the soil
- Water washes nutrients down out of the soil and away
Go round the cycle
Follow the nutrients around the cycle. Put these stages in order, starting from the soil store.
- Nutrients are held in the soil store
- Roots take up the nutrients into the plants (uptake)
- The nutrients become part of the living biomass
- Dead leaves and plants fall as litter (litterfall)
- Decomposers return the nutrients to the soil (decomposition)
The biggest store
In a tropical rainforest, which nutrient store is the largest?
- Biomass
- Litter
- Soil
- All three are the same size
Inputs and outputs
Nutrients also enter and leave the whole cycle, not just move between stores.
Spot the losses
Select the TWO processes that are outputs, taking nutrients out of the cycle.
- Surface runoff
- Leaching
- Uptake by roots
- Decomposition of litter
Why the soil is so poor
It seems strange that such a green forest grows on poor soil. The reason is the speed of the cycle. The hot, wet climate makes decomposition and uptake very fast, so nutrients are pulled into the biomass almost as soon as they are released. Heavy rain also causes strong leaching, washing nutrients out of the soil. So most nutrients stay locked in the living biomass, and the soil store stays thin.
The cycle in words
In the Gersmehl model, nutrients are held in three stores. Plants absorb nutrients from the soil through their roots, a transfer called _____. When leaves and plants die, they drop to the ground to form _____. In the hot, wet climate, decomposers break down the dead material quickly, a transfer called _____. These nutrients return to the _____. The largest store in a tropical rainforest is the _____.
Transfer, input or loss?
A transfer moves nutrients BETWEEN two stores. Tap the TWO processes below that are transfers.
- Uptake
- Precipitation
- Litterfall
- Leaching
- Weathering
Clearing the forest
Loggers clear an area of rainforest. Work out what happens to the nutrient cycle.
- The trees are cut down and taken away. What happens to the biomass store?
- With no trees dropping leaves, what happens to the litter reaching the soil?
- Heavy tropical rain now falls straight onto the bare soil. What happens to its nutrients?
Explain the poor soil
Explain why the soil in a tropical rainforest is poor in nutrients, even though the forest above it is so lush. Use the Gersmehl stores and transfers in your answer. Write in clear, full sentences.
- Name the three stores and say which is largest in a rainforest
- Explain how the hot, wet climate speeds up decomposition and uptake
- Explain how leaching removes nutrients from the soil
- Explain why clearing the forest then leaves the soil even poorer