Agriculture Reference
In-Depth Information
Biomantles may also be defined in terms of the activities of the three major groups
discussed below and in particular locations to the effects of certain surface and
soil-dwelling vertebrates. Other groups such as the Crustacea are also responsible for
considerable bioturbation in a few terrestrial environments (Butler,
1995) and their
effects are particularly clear in many tropical mangrove forest soils.
Earthworms
In the temperate grasslands of Europe, earthworms may mix and displace up to 500 Mg
of soil on a dry weight basis (Bouché, 1975) although only part of this soil is
deposited as casts on the surface. Darwin's (1881) estimates from English pastures
showed that casts deposited on the soil surface annually represented the approximate
equivalent of 0.25-0.50 mm of sub-surface soil and reflect the significant inversion and
mixing of the upper profile caused by these animals.
In the humid grass savannas of Africa, the estimated annual transit of soil through
the intestinal tracts of earthworms may be as high as 1,000 to 1,200 Mg of soil
also on a dry weight basis. Again, only a small amount (25 to 30 Mg or
ca.
2.5 %) of the
total is deposited on the soil surface as casts. These amounts are equivalent to layers of,
respectively, 8-10 cm overall and 1 -2 mm of fine particles deposited annually on the soil
surface (Lavelle, 1978).
Since earthworms have marked diurnal and seasonal vertical movements, soils from
different horizons may be mixed and in some cases, this may reduce or completely
offset the effects of leaching and eluviation. However, the activities of most earthworm
species are essentially concentrated within the A horizon and very little material is
normally brought to the surface from the deeper horizons.
Despite the above, the impacts of soil mixing by earthworms on soil function and
structure may be large. Three great groups of mollisols (vermustolls, vermudolls and
vermiborolls) are defined as having more than 50 % of the volume of their A horizons
(below any Ap horizon present) occupied by such earthworm constructs as galleries,
casts and infilled voids (Soil Survey Staff, 1999).
