Carbon, strictly speaking, is not a nutrient, but it is the driving force for the nutrient
cycling process. It forms an energy source for the population of soil organisms. Carbon is also
incorporated into forms organic substances that resist decomposition and accumulate in the form
of humus. Humus is chemically active and plays a role in the retention of metals and nutrient
elements in soil. It also plays role in the physical structure of soil fostering aeration and root
THE CARBON CYCLE
The global carbon cycle includes the terrestrial cycle and the sea (Fig. 1). The amount
of C contained in the organic matter of terrestrial soil (about 2.5 X 10'5 kg) is three to four times
the C content of the atmosphere (7 X 10"' kg) and five to six times the land biomass (4.8 X 10"4
kg). Carbon preserved in sediments is about ten million times that of the terrestrial biomass.
About 15 percent of the atmospheric C mass is returned to the soil each year, while an equivalent
amount is released from the soil to the atmosphere.
C residues returned to soil are chemically complex and require numerous species of
microorganisms to effect their decomposition. Some of the C is converted to CO2, some is
incorporated into microbial tissue, and some is converted into stable humus. While humus is
being created, other existing humus is being decomposed, so there may not be a significant net
gain or loss. Most upland ecosystems maintain a relatively steady inflow of C and outflow of
CO2. As soils become wetter, there is a tendency to accumulate C in the form of humus. Thus,
over tens of thousands of years or more, peat and muck soils are formed.
The initial decomposition process is carried out by multicelled animals such as earthworms
and beetles. Molds and spore-forming bacteria are active in consuming proteins, starches, and
cellulose. Byproducts include ammonia, hydrogen sulfide, carbon dioxide, organic acids, and
other partially oxidized substances. The more easily degradable components are decomposed
first, and resistant components such as lignin are slowly decomposed by actinomycetes and fungi.
The discussion under the Nitrogen Cycle section will illustrate one of the functions of
residual C in soil as a mediator of N transformations. Humic substances coat mineral particles
allowing the formation of structural units in soil. This provides for friability that is important
in maintaining soil porosity, aeration, and ability to be mechanically worked by farm implements.
The capacity for organic matter decomposition in soil has been taken advantage of by the use of
animal manures as fertilizers and the addition of organic wastes from sewage or manufacturing
A commonly utilized technique for utilizing the soil's capacity for degrading organic
matter is the so-called soil farming of petroleum-contaminated soil. The soil is aerated by tillage
or injected air and N, P, and K nutrients are added to stimulate the soil organisms. A few weeks
to a few months of incubation are usually sufficient to accomplish renovation.