Living things remove materials from the environment for growth and other processes.

These materials are eventually returned to the environment in waste materials, such as carbon dioxide and urine, or when living things die and decay.

Micro-organisms, such as bacteria and fungi, break down (digest) waste materials and dead organisms. They are decomposers.

Decay is faster in warm, moist conditions. Many micro-organisms are more active when there is plenty of oxygen.

Decomposers are important because they remove dead organisms and release nitrates and other minerals from the bodies into the soil. This maintains soil fertility and helps plants to grow, which in turn, provides food for animals.

When all waste materials and dead organisms in the ecosystem have been broken down and cycled, all the energy originally captured by plants has been transferred.

The decay processes carried out by micro-organisms are made use of by man.

They break down human wastes in sewage works.

They break down waste plant materials in compost heaps, which can then be used to fertilise the soil.

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The Carbon Cycle

In a stable community, the rate at which materials are removed from the environment is equal to the rate at which they are returned.

The materials are constantly being cycled.

All organisms need carbon for the compounds that make up their bodies.

Plants absorb carbon dioxide from the air for photosynthesis. The carbon is used to make proteins, fats and carbohydrates which make up the body of plants.

Carbon enters animals when they eat plants or other animals, and becomes part of compounds in their bodies.

When plants and animals respire, they produce carbon dioxide which is released back into the atmosphere.

Decomposers that break down dead organisms, release carbon dioxide as they respire.

Wood and fossil fuels contain carbon. When these are burnt, they release carbon dioxide.

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The Nitrogen Cycle

Nitrogen is an essential element to make proteins.

Although the air is about 80% nitrogen, most organisms cannot absorb it in this form.

Green plants absorb nitrogen in the form of nitrates from the soil and use it to make proteins.

Animals eat plants, or other animals which have eaten plants, and build up their own proteins, which also contain nitrogen.

Bacteria and fungi break down the waste products of animals and the protein from dead plants and animals. They produce ammonium compounds.

Nitrifying bacteria in the soil, convert ammonium compounds to nitrates, which plants can absorb.

There are some special bacteria, called nitrogen-fixing bacteria, which can use nitrogen directly from the air to make ammonium compounds. They live in the soil or in nodules on the roots of some plants.

Some unhelpful bacteria live in waterlogged soils. These break down nitrates back into nitrogen gas, so they are no longer available to plants.

They are called denitrifying bacteria.