Nutrient cycling and the nutrient cycling model

Overview

Nutrient cycle, three stores in the nutrient cycling model and inputs to and outputs from the nutrient cycling model.

What you need to know

Answer each one before you reveal it.

Questions from this subtopic

Each one with the model answer underneath.

1. 1

   What is a nutrient cycle?

   Model answer

   A nutrient cycle is the repeated circulation of the chemical elements that plants and animals need - such as nitrogen, phosphorus, potassium, calcium and carbon - between the living organisms of an ecosystem and its physical environment. Nutrients are taken up from the soil by plants, pass into animals through feeding, return to the ground as dead organic matter, are released by decomposition and are taken up again. Because the elements are not destroyed, the same nutrients can be used over and over, which is why a mature ecosystem can be highly productive with only very small external inputs.

2. 2

   Why do nutrients cycle while energy does not?

   Model answer

   Because nutrients are matter and energy is not. Nutrients are chemical elements that simply change their location and their chemical combination as they move between soil, plant, animal and litter, so they can be returned to the start of the loop indefinitely. Energy is degraded every time it is used: at each transfer the great majority is converted to heat during respiration and radiated away, and heat cannot be recaptured and refixed by plants. An ecosystem therefore needs a continuous new supply of solar energy, but it can recycle the same store of nutrients.

3. 3

   What are the three stores in the nutrient cycling model?

   Model answer

   Biomass, the nutrients held in all the living material of the ecosystem, chiefly the vegetation; litter, the nutrients held in dead organic matter lying on and in the surface of the ground, such as fallen leaves, dead wood and animal remains; and soil, the nutrients held within the soil in a form that plant roots can take up. In the model these are drawn as three circles whose areas are drawn in proportion to the quantity of nutrients each holds, so the diagram shows at a glance where the nutrients of a particular ecosystem are concentrated.

4. 4

   Describe the transfers between the three stores in the nutrient cycling model.

   Model answer

   Litterfall transfers nutrients from the biomass store to the litter store, as leaves, branches, dead organisms and waste fall to the ground. Decomposition transfers nutrients from the litter store to the soil store, as bacteria, fungi and detritivores break the dead material down and release the nutrients in soluble form. Uptake transfers nutrients from the soil store back into the biomass, absorbed in solution by plant roots. These three arrows form the closed loop of the cycle, and in the model each is drawn with a width proportional to the size of the flow.

5. 5

   What are the inputs to and outputs from the nutrient cycling model?

   Model answer

   There are two inputs. Precipitation brings dissolved nutrients from the atmosphere into the litter store, and the weathering of the parent rock releases mineral nutrients into the soil store. There are two outputs. Runoff carries nutrients out of the litter store when surface water washes material away, and leaching carries nutrients in solution downwards out of the soil store beyond the reach of roots. In the diagram inputs are drawn as arrows entering these stores from outside and outputs as arrows leaving them, again with the width of each arrow proportional to the size of the flow.

6. 6

   How would you read a nutrient cycling diagram for a particular ecosystem?

   Model answer

   You compare the sizes of the three circles to see where the nutrients are concentrated, and the widths of the arrows to see which transfers dominate and how fast the cycle is turning. A very large biomass circle with thick transfers and heavy leaching tells you the system cycles its nutrients fast and holds them in the vegetation rather than in the ground and would be crippled if that vegetation were removed. A very large litter circle tells you that decomposition is slow and that nutrients are locked up in dead material. A large soil circle tells you that nutrients are being retained in the ground rather than leached. In every case the explanation lies in the climate and the soil, because temperature and moisture control the rate of decomposition, and rainfall controls the rate of leaching.

Other ways to work this

The same content, four other ways.

What is inside Levo

Six ways to work the same material.