Agriculture Reference
In-Depth Information
lacking and the temperature is very high close to the soil surface (Athias, 1976).
Similar overall patterns occur over a variety of situations with large interspecific,
seasonal and even daily variations, as observed in leaf litter of North American desert
ecosystems (McKay et al., 1987).
In relatively bare soils such as savannas after fire and in cultivated soils, populations
are distributed much more deeper than in soils with a surface covering of vegetation and
litter (Lagerlöf and Andrén, 1989; Athias, 1974).
Seasonal variation
Acarine communities comprise a large number of species with diverse life cycles.
Their generation times average about one year and populations occasionally show clear
seasonal abundance patterns ( e.g., Asikidis and Stamou, 1991; Lagerlöf and Andrén,
1989). Quite often, no clear pattern of seasonal variation may be detected, probably due
to the overwhelming influence of spatial variability which tends to mask temporal
variation (see e.g., Athias, 1976 for a discussion of variability in savanna ecosystems).
4.2.3
ENCHYTRAEIDAE
4.2.3.1
General Biology
Enchytraeidae are small white-coloured Oligochaeta, 1 to 50 mm long and possessing
fasciculate setae. They live in aquatic and, particularly, terrestrial environments. Respiration
is cutaneous and these animals require a constantly wet integument for efficient gas
exchange. Enchytraeidae have limited ability to move and work the soil and thus they
live in the litter and organic horizons of the upper few centimetres of the soil. Some species
have been reported to make burrows in the sand and transport material as faeces to
a depth of 4 cm. Others move vertically and tunnel through earthworm casts, or conifer
needles (Standen, 1984). In agroecosystems in the Netherlands, 21 to 35 % of the enchy-
traeid population had mineral grains in the gut (Didden, 1990) although they have rather
limited consequences for the soil physical structure (Springett et al., 1970). In contrast
to earthworms, they have no gizzard, which implies an inefficient comminution and
mixing of decomposing material (Bal, 1982).
They are rather homogenous in their anatomy and are divided into 21 genera and
ca. 600 species (Dash, 1990) (Figure III.39).
Enchytraeidae do not have an adequate suite of enzymes to digest complex polysac-
charides (Dash et al., 1981) and may therefore rely on the 'external rumen' type of
digestion. As a result, they are principally saprophagous and first order decomposers.
They preferentially ingest parenchymatous tissues rather than the veins of decomposing
leaves, fungal mycelia and faecal pellets of Collembola or earthworm casts, but do not
feed on purely microbial material. They are attracted by plant debris with high N but low
tannin contents, low C:N ratios (Latter and Howson, 1978; Dash, 1990; Wolters, 1988)
and lignin contents. Consequently, they grow better when ingesting bleached leaves
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