SOIL ORGANISMS - Soil Ecology - page 350

Within the tropical latitudes, certain earthworms have developed the capacity to live on
soils with low organic matter contents. They comprise more than 70 % of earthworm
biomass in the humid savannas of Lamto (Côte d'Ivoire) and can digest equally all
particle-size fractions of SOM, including the finest and most decomposition-resistant
fractions (Martin et al., 1991). Thus, with increased temperatures, the resource base
of earthworm communities is considerably enlarged even though alpha diversity is not
significantly increased. However, functional diversity is greatly increased and niche
overlap between species significantly decreased (Lavelle, 1983c).
There is a striking parallel between roots and earthworms straightforwardly expressed
by Janzen (1985) who asserted that 'plants wear their guts on the outside'. Roots produce
exudates, a mixture of assimilable carbohydrates and proteins, which trigger microbial
activity and subsequently nutrient release in the rhizosphere (see Chapter IV.3). In the
earthworm gut, intestinal mucus is the functional equivalent of root exudates (Lavelle
et al ., 1983b; Martin et al ., 1987), and the movement of soil through the gut may be
considered analogous to root elongation.
The digestive system described in these worms is analogous to the mechanisms of
nutrient release in the rhizosphere postulated by Trofymow and Coleman (1982) and
Clarholm (1983) and further demonstrated by Billes et al. (1986); earthworms disperse
the ingested soil with large amounts of water (100 % of the weight of dry soil) and
mucus (5-16 %). This leads to an intense microbial activity in the anterior part of the
gut at the expense of the intestinal mucus, a mixture of highly assimilable compounds.
By the time that the digesta has moved to the median part of the gut, the mucus has been
entirely metabolised and the ingested microflora has become able to digest the more