Forest ecosystems of Canada

Decomposition

Decomposition is the last stage in the ecological cycle, when basic elements return to the soil (e.g. the nitrogen from proteins) or to the atmosphere (carbon in the form of CO₂) and are once again available to plants.

The energy initially amassed through photosynthesis is used by decomposers to survive. Through their respiration, this energy is finally dissipated as heat.

Barely 1% of the plant matter in a forest ecosystem is consumed by herbivores. Decomposition therefore takes place primarily in the plant litter on the ground, which consists of dead leaves, plants or trees, animal droppings and the bodies of dead animals.

The players involved in decomposition

Fungi and bacteria are by far the most active decomposers. They are remarkably efficient, and the smaller the pieces to be decomposed, the faster these micro-organisms are able to do their job.

Organic waste, such as leaf matter and the droppings of herbivores, first feeds a host of small animals including insects, earthworms and other small invertebrates living in the plant litter.

This microfauna breaks up the organic matter, digesting part of it, and thus facilitating the task of the micro-organisms that complete the process of decomposition.

Decomposition energy

Decomposition is a feeding process. Micro-organisms and microfauna feed on organic waste, taking from it the energy and the nutrients they need to live. To accomplish this, decomposers respire, just as we do. Respiration consumes oxygen and releases energy, which is ultimately transformed into heat and dissipates into the atmosphere.

Since respiration is the "tool" of decomposition, it is affected by the same factors:

• temperature and humidity; the higher the temperature and humidity level, the faster the decomposition process will be;
• the oxygen level; if oxygen is lacking, for example in a saturated soil, respiration is not possible and decomposers cannot survive, although some micro-organisms can thrive without air (fermentation).
  
  

Features of a cold climate

In a northern climate, decomposition is limited by the long months of winter. Summer after summer, the plants of a forest ecosystem succeed in manufacturing an impressive quantity of biomass, much of which finds its way into the forest litter.

Because decomposers are only active during the summer months, they cannot manage to break down everything in the soil. The litter builds up and can reach up to 10 cm in thickness. Underlying layers remain half decomposed, even if the temperature and moisture conditions are favourable, because oxygen becomes increasingly scarce, which restricts micro-organism activity. This is one of the reasons why our forest soils are such significant carbon pools.

At this point, the carbon could only be liberated by a disturbance of the soil, such as a fire.

Last Modified: 2007-10-09