The carbon cycle lies at the heart of the all biological activities whether it’s the formation of complex carbon molecules by metabolic activity (originally from carbon dioxide fixation from the atmosphere by photosynthesis & chemosynthesis), respiration or the decomposition and release of carbon dioxide and organic molecules from organisms. Most of the world’s carbon is however locked-up in sediments and storage products such as carbonates in rocks with only a fraction being circulated by living organisms.

In soil, plants and to some extent algae are mainly responsible for the assimilation of C and the production of complex molecules. Three basic forms of C are found in soil: insoluble, soluble and biomass carbon (Killham, 1994). Insoluble C accounts for about 90% total soil organic C and is mainly plant derived cellulose and lignin and chitin from animal exoskeletons and fungi.  Some of this material forms humic material which is very difficult to degrade with some estimates suggesting 250-1500 years but may serve to regulate decomposition (Campbell, 1983). Soluble C such as sugars is more readily available and is rapidly assimilated by the microbial population. Biomass  C is that contained in the living organisms. All organic material passes, as some stage, through the soil microorganisms. Their activity therefore regulates the cycle. Microorganisms are themselves, in turn, regulated by the environment .

External influences may have some effect on the global C cycle. Direct effects include the application of biocides and toxins which may kill parts of the microbial community. Indirectly, the application of agricultural products such as animal effluents may affect soil structure and alter the C:N ratio affecting microbial activity. Globally, atmospheric levels of carbon dioxide are affected by the burning of fossil fuels to affect the climate and photosynthesis.

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