Soil carbon cycling

Carbon pools

The soil carbon cycle can be represented as a number of pools (forms in which C is stored) and fluxes (pathways by which C is converted from one form to another). Some of the main pools of the soil carbon cycle are shown in the diagram.

POLYSACCHARIDES - Polysaccharides (including carbohydrates) are the most abundant component of plant residues and are polymers of sugar molecules. Polysaccharides vary in their resistance to decay. Starch may be relatively rapidly decomposed, but cellulose decomposes more slowly (months).

SOIL MICROORGANISMS - The main groups of heterotrophic soil microorganisms are bacteria, fungi, actinomycetes, and protozoa. They obtain energy and nutrients from the decomposition of residues in the soil and also (more slowly) from humus. A fresh plant residue is rapidly colonized by microorganisms which excrete enzymes that decompose its complex molecules into small soluble molecules that can be absorbed and metabolized.

HUMUS - Humus comprises a complex mixture of compounds that are produced as residues are decomposed. One component of humus is altered lignin, and the acidic phenol and carboxyl groups of this component give soil organic matter its acidity and cation exchange capacity. Humus also contains residues of dead microorganisms. Humus decomposes more slowly than plant residues, partly because of chemical resistance to decay, but more significantly because the organic compounds are physically protected by their association with clay particles.

CARBON DIOXIDE - A large part of the carbon contained in residues is converted to carbon dioxide by microbial respiration. Respiration provides the energy necessary for microbial metabolism, including that required to decompose the plant residues. Similarly, when the microorganisms use the more stable humus as a substrate, part of its carbon is also converted to carbon dioxide.

Residue decomposition

A new phase of C cycling starts when plant residue is added to the soil. The residue is rapidly colonized by microorganisms which start to decompose it, the most reactive components disappearing first. Much of the C is lost as CO₂, but some is converted to humus. In warm, moist soil, this process can take a few months.

Humus decomposition

Even if there is no further addition of plant residue to the soil, microbial activity does not cease. Humus itself is decomposed, but at a slower rate than for the original plant residues. For example, with soils continuously cultivated for cereals, with little annual residue return, soil humus levels will fall significantly within years to decades.