Nutrient Recycling

Energy flows through an ecosystem in one direction: it enters as light and leaves as heat, and it is never recycled. Nutrients such as carbon, nitrogen and phosphorus are different. The Earth has a fixed amount of each, so they must be recycled between living organisms and the environment.When organisms die, or produce waste such as faeces, urine and shed leaves, the nutrients are locked up in complex organic molecules (proteins, nucleic acids, carbohydrates, lipids). Plants cannot absorb these, so they must be broken down.Decomposers are mainly bacteria and fungi. They are saprobionts: they secrete enzymes onto the dead material, which digest it outside their cells. This is extracellular digestion. They then absorb the soluble products, such as sugars and amino acids, and use them in respiration and growth.As a result:
- carbon is returned to the air as carbon dioxide when decomposers carry out respiration, and can be taken up again by plants in photosynthesis
- nitrogen in proteins is released as ammonium ions, and phosphorus as phosphate ions, which can be taken up by plant roots.Detritivores, such as earthworms, woodlice and millipedes, feed on dead organic matter (detritus) and break it into smaller pieces. This increases its surface area, so decomposers can break it down faster.Without decomposers, dead material would build up, nutrients would stay locked in it, and plant growth would eventually stop, because the soil would run out of the mineral ions plants need.