Introduction to Ecological Risk Assessment - Dealing with Contaminated Sites: From Theory Towards Practical Application - page 588

plant's components. Here, the plant components are intermediates in the different
cycles; for example, proteins contain carbon, hydrogen, oxygen and nitrogen, and
so proteins are intermediates in the four corresponding cycles.
Green plants and trees use carbon dioxide to produce plant material using CO 2 as
a carbon source and light as an energy source (carbon fixation by photosynthesis).
Plant material is used by soil organisms for growth and reproduction, resulting in
carbon fixation in soils within the bodies of organisms and in the humus fraction of
soil. The degradation of fresh organic material into carbon dioxide and of the humic
fraction of the soil by soil organisms is an important part of the carbon cycle.
In the carbon cycle, organisms such as mites, springtails and earthworms mix
and consume dead plant material left on top of the soil, and thereby redistribute
the organic matter throughout the topsoil. Other organisms, such as potworms and
nematodes consume plant litter further in the soil.
Thereafter, bacteria and fungi become the major players in the decomposi-
tion of organic matter in soils. This decomposition takes place in many stages.
Fungi convert hard-to-digest and larger organic substances slowly into forms
that other organisms can use (Ingham 2000b ). They are able to degrade highly
complex and resistant compounds such as cellulose, gums and lignins (Conklin
2002 ). Actinomycetes also help decompose complex organic matter (Conklin 2002 ;
Sullivan 2004 ). Actinomycetes are abundant in soils in which the easily decomposed
organic matter has already been decomposed and only the more resistant compounds
remain. Actinomycetes and fungi are the most important decomposers of tough plant
materials such as bark and woody stem material. Moreover, they are very effective
in attacking tough, raw plant tissues such as cellulose and lignin.
Bacteria are heavily involved in nutrient cycling. Firstly, they are responsible for
the production of ammonia and ammonium through the decomposition of organic
material. Subsequently, other bacteria perform the process of nitrification, that is, the
transformation of ammonium into nitrate. Nitrate is an important nitrogen source for
plants. Under anaerobic conditions (in the deeper layers of the soil) denitrification
(conversion of nitrate to molecular nitrogen) takes place. This process leads to the
loss of available soil nitrogen and a subsequent loss in soil fertility. However, chem-
ically bound nitrogen usually circulates many times between the soil and organisms
before denitrification returns it to the atmosphere. Bacteria play important roles
in the rapid consumption and degradation of easily degradable substances such as
proteins, lipids and sugars, producing nutrients for plants and other organisms in
the soil. Bacteria are also performers in the other nutrient cycles, making nutrients
available to plants and other organisms by releasing sulphur, phosphorous and trace
elements from organic matter or from soil minerals such as potassium, phospho-
rous, magnesium, calcium and iron (Sullivan 2004 ). Phosphate solubilizing bacteria
( Bacillus megateriumi ) (PSB) and earthworms ( Pheretima guillelmi and Eisenia
fetida ) contribute to the phosphorus turnover and transformation in soil (Wan and
Wong 2004 ).
Another important part of the nitrogen cycle concerns nitrogen fixation , that
is, the conversion of nitrogen from the atmosphere into nitrogen-containing
organic substances (Lindahl et al. 2007 ). This important process is performed by