Agriculture Reference
In-Depth Information
CHAPTER III
SOIL ORGANISMS
III.1 GENERAL FEATURES
Soil is a dark, still, porous, semi-aquatic medium within which temperature and moisture
conditions are highly buffered. Soils were among the first terrestrial environments to
be colonised since they possess environmental conditions intermediate between aquatic
and aerial media. As such, soils have played a key role in evolution and the presence of
many primitive organisms in soils is considered a testimony of this phase of biological
evolution (see,
e.g.,
Ghilarov, 1983; Vannier, 1985).
The composition and structure of soil communities reflect both the spatial organisation
of soil and the major role this environment plays in decomposition processes.
Since decomposition is the main role of soil, many soil organisms are adapted to
feeding on decomposing organic materials which are relatively poor quality resources.
Soil organisms are subject to severe spatial constraints since they occupy a semi-
discontinuous network of pores filled with air and/or water, or water films which cover
solid particles.
Soil organisms have responded to these constraints through a variety of adaptive strategies
of which body size and respiratory patterns are the major characteristics (Table III.1).
the hydrobionts
are aquatic organisms which live and feed in the free water
and soil. They belong mainly to the
microflora
(Kingdoms Fungi and Bacteria,
including the Actinobacteria or Actinomycetes), and
microfauna
(protists and
nematodes). Their size rarely exceeds 10 to 50 and most are only a few
micrometres long. They are generally well adapted to occasional desiccation
and/or shortage of food in their micro-environment. The spatial scale at which they
operate is that of a few millimetres and their generation times are of the order of a
few hours to a few days.
hygrobionts
are invertebrates which possess an aerial system of respiration but still
depend on high moisture levels and often require free water in their environment.
Depending on their size, two main categories are defined. The
mesofauna
include
the microarthropods and the Oligochaeta (Enchytraeidae); these groups have body
widths of 0.1 to 2 mm, which allows them to move freely through the large pore
network within the surface litter and soil. They operate at scales of space and
time of, respectively, centimetres and weeks to months. The larger invertebrates,
the
macrofauna,
are much wider than most soil pores; they live in the surface
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