Introduction to Ecological Risk Assessment - Dealing with Contaminated Sites: From Theory Towards Practical Application - page 587

earthworms can process 9,000 kg (20,000 pounds) of topsoil per year (Edwards and
Burrows 1988 ). Earthworms also contribute considerably to the redistribution of
metals from the deeper layers to the upper soils (Zorn et al. 2008 ).
To a lesser extent, the actions of smaller organisms such as enchytraeids and
collembola are also important in soil structuring (Langmaack et al. 1999 ).
13.4.3.3 Humus Formation
Humus is a complex organic colloidal material with a dark colour. It is the end prod-
uct of organic matter decomposition and originates from detritus, dead leaves and
other plant materials, dead (micro)organisms and faecal material. It is very resistant
to decay, sometimes lasting as long as 1000 years (Conklin 2002 ). Under natural
conditions, humus is found on the soil surface and in the upper layers of the soil. It
is intensively colonized by all kinds of organisms. Generally, old and young dead
organic material mixes together on top of and in the soil, resulting in mixtures of
different organic materials, partly resistant to decay and partly in the progress of
decomposition.
Humus has excellent qualities with regard to the storage of moisture, and organic
and inorganic compounds added to soil. It provides a habitat for plant roots and
organisms. Therefore, it is extremely useful for plant growth and, hence, improves
agriculture, horticulture, the development of nature, and the potential for plant and
tree growth in urban settings. For this reasons it is often used as a soil conditioner
and fertilizer, and also as a man-made humus called compost.
It is interesting to note that soil organisms contribute to humus formation, and
thereby adapt their environment so that it functions optimally. An example of this
phenomenon is the important role of earthworms in humus formation, given that the
optimal habitat for these earthworms is, by the same token, a humus-rich soil layer.
From this perspective, it can be concluded that, in addition to feeding relationships,
one species or organism may profit from another species or organism.
13.4.3.4 Element Cycling and Nutrient Supply
All chemical elements are cycled within the biosphere and therefore are present
in different forms. In the carbon cycle, gaseous carbon dioxide is incorporated in
complex organic material by the process of photosynthesis in plants and algae. Plant
material is used by organisms to feed upon, and after excretion the organic material
is degraded by organisms into its elemental components and the whole cycle starts
all over again. Plants use different elements for growth such as nitrogen (in the form
of ammonium or nitrate), sulphur (mostly in the form of oxides), phosphate and
other elements (nutrients). All the nutrients are intermediates in cycles: the nitrogen
cycle, sulphur cycle, the phosphorus cycle, etc. (Stevenson and Cole 1999 ).
The cycles mentioned are performed by plants and organisms, and soil organisms
especially play an important role. These cycles are tightly coupled. For example,
when plants take carbon dioxide from the air, they are active within the carbon cycle.
To make plant material, however, they need nitrogen and other nutrients to make the