Decay and Decomposition

I

Introduction

Decay and Decomposition, breakdown of dead plants or animals or other organic matter, such as animal or plant wastes. The reason the Earth is not knee-deep in dead vegetation, excreta, and animal corpses is due to the processes of biological decay and decomposition. This is distinct from chemical decomposition, by which compounds spontaneously break down to their constituent parts.

During biological decomposition, nutrients, which are assimilated into organic compounds—initially by plants (as primary production) and recycled through the foodchains—are ultimately returned to the biosphere, which is maintained in a more or less steady state. This cycle of matter, a process known as mineralization, is entirely mediated by micro-organisms, although fire can also make a small contribution, and is the route by which all the biologically important elements within the biosphere are recycled.

II

Bacteria and Fungi

Bacteria and fungi are the principal agents of decomposition and are collectively known as decomposers. They act on dead organic plant material and the excretion products or corpses of higher animals. Organisms growing on dead material are called saprophytes. Living organisms can protect themselves from decomposition; therefore, the cycle of materials is related to the lifespan of the plant or animal.

Microbial decomposition is the major pathway by which carbon dioxide, taken up initially by plants during photosynthesis, is returned to the atmosphere. It is also fundamental to the disposal of sewage—which is vital to the health of all people.

Saprophytes are diverse and the versatility of their metabolism shows the capability of each kind of saprophyte to degrade specific organic compounds. Every naturally occurring organic compound can be decomposed, either by single micro-organisms, or by different species working in combination. Some organic constituents of plants are more resistant to microbial decomposition than others and accumulate in the environment. This plant matter, known as humus, is the major organic component of soil and is important for soil fertility by affecting drainage and oxygen penetration into the soil.

A

Fungi

Fungi are usually the initial colonists of organic material, as they have the metabolic capability to degrade cell walls and release the more readily degradable cellular contents. Specific bacteria can also degrade cellulose in plant cell walls; communities of these bacteria thrive in the gut of herbivorous animals and are entirely responsible for cellulose decomposition in the rumen (stomach) of many important domestic animals. The decomposition of wood can be accelerated through the activity of wood-eating insects, such as termites, which depend on specialized microbial communities maintained in the gut to make nutrients in wood available to them.

III

Soil and Water

Micro-organisms are abundant in soil and water. A teaspoonful of unpolluted natural water will contain approximately 1 million bacteria, while the top 15cm (6 in) of well-fertilized soil may contain more than five tonnes of bacteria and fungi per hectare. The decomposition of organic material provides energy for the micro-organisms to grow and divide. These large populations are utilized as a food source by protozoa, whose metabolic processes rapidly recycle the nutrients assimilated by the bacteria. This process, which is known as the microbial loop, short-circuits the usual food chain and is becoming recognized as an important recycling process in surface waters. The grazing pressure by protozoa is essential for controlling the numbers of bacteria, as the progeny of one bacterium dividing every 20 minutes would be in excess of 1,000 in a little over 3 hours.