FUNCTIONING OF THE SOIL SYSTEM - Soil Ecology - page 478

the annual production. In unburned savannas, ingestion increases up to
which represents 9 % of the annual litter biomass decomposed (Lavelle, 1978).

4.2.6 FUNCTIONING OF THE ANECIC DRILOSPHERE

Earthworm activities influence two major soil processes: (i) the dynamics of litter
and soil organic matter decomposition and (ii), the physical structure of the soil. Anecic
and endogeic drilospheres are distinguished, although they may sometimes have
comparable characteristics.

4.2.6.1

Effects of anecic earthworms on C and nutrient cycling

C and nutrient cycles in the anecic drilosphere are especially characterised by the:
1.
2.
3.
4.
5.

relatively low 'direct impact' of digestion on litter decomposition;
rapid turnover of C and N in the earthworm biomass;
important translocation of litter into the subsoil and mineral soil close to the surface;
accelerated turnover of SOM;
further transformation of organic matter mediated by microbial activity within casts.

Direct effects of digestion
Assimilation of litter by anecic earthworms seems to be highly variable. Maximum
assimilation rates between 30 and 75 % have been measured in Lumbricus rubellus ,
depending on litter quality and temperature (Dickschen and Topp, 1987; Daniel, 1991)
although much lower values (11.6-28.5 %) are reported by Crossley et al. (1971) for
a number of other species. The degree of previous litter decomposition, ambient
temperature and moisture conditions are other important determinants of litter assimila-
tion in earthworms, irrespective of species (Cooke and Luxton, 1980; Cooke, 1983;
Cortez et al., 1989). Specific rates of assimilation of N by anecic earthworms have not
been measured.
The flux of energy through anecic earthworms has rarely been estimated. Populations
of Lumbricus terrestris may assimilate ca. 10 % of litter fall in English woodlands
(Lakhani and Satchell, 1970). Based on only a few more estimates, the total assimilation
of litter by anecic earthworms is not considered likely to exceed 5 % of overall plant
production (Lee, 1985). The dramatic effects of anecic earthworms on litter decomposi-
tion and soil functioning are therefore mainly due to the reduction they effect in litter
particle size, its translocation into the soil and the large and selective stimulatory
influences they have on microbial communities.
The mineral soil that is mixed with litter is mostly derived from the upper 15 cm
and its organic matter is poorly assimilated; 95 % of the C assimilated by the anecic
worm Nicodrilus velox originates directly from ingested leaf litter, or after a period
of incubation in old casts (Bouché et al., 1983). This finding is corroborated by
observations based on measurements (Scharpenseel et al., 1989; Martin et al., 1991)
and microcosm experiments (Cortez et al., 1989) which show that anecic earthworms
only assimilate young organic matter from the litter.
As a result of digestion, a significant proportion of the organic matter present may be