Productivity and decomposition
Primary productivity and the process of decomposition.
The essentials
Productivity: A constant input of solar energy is the basic requirement of any ecosystem. Primary production is the amount of biomass or organic matter produced per unit area over a time period by plants during photosynthesis. It is expressed as weight (g m⁻²) or energy (kcal m⁻²). The rate of biomass production is called productivity, expressed as g m⁻² yr⁻¹ or (kcal m⁻²) yr⁻¹.
- Gross primary productivity (GPP): the rate of production of organic matter during photosynthesis.
- A considerable part of GPP is used by plants in respiration (R).
- Net primary productivity (NPP) = GPP − R. NPP is the biomass available to heterotrophs (herbivores and decomposers).
- Secondary productivity: the rate of formation of new organic matter by consumers.
Primary productivity depends on the plant species in an area, environmental factors, availability of nutrients and the photosynthetic capacity of plants, so it varies between ecosystems. Although oceans occupy about 70 per cent of the earth's surface, their productivity is relatively low compared with land.
Decomposition: Decomposers break down complex organic matter into inorganic substances such as CO₂, water and nutrients. Dead plant remains (leaves, bark, flowers) and dead animal remains including faecal matter form detritus, the raw material for decomposition. The steps are:
- Fragmentation: detritivores such as earthworms break detritus into smaller particles.
- Leaching: water-soluble inorganic nutrients go down into the soil and get precipitated as unavailable salts.
- Catabolism: bacterial and fungal enzymes degrade detritus into simpler inorganic substances.
- Humification: accumulation of a dark, amorphous substance called humus, which is highly resistant to microbial action and decomposes very slowly; it is a reservoir of nutrients.
- Mineralisation: humus is further degraded by some microbes, releasing inorganic nutrients.
Decomposition is largely oxygen-requiring. Its rate is controlled by the chemical composition of detritus and by climate. It is slower if detritus is rich in lignin and chitin, and faster if it is rich in nitrogen and water-soluble substances such as sugars. Warm and moist conditions favour decomposition; low temperature and anaerobiosis inhibit it, leading to build-up of organic matter.