FUNCTIONING OF THE SOIL SYSTEM - Soil Ecology - page 492

Extrapolation of these results to the scales of field populations and a year shows that
mineral-N released by earthworm populations may represent a significant proportion
of plant requirements in grasslands. In the moist savanna of Lamto (Côte d'Ivoire),
the overall release of mineral-N in casts of the endogeic earthworm Millsonia anomala
was estimated at by using the DRILOTROP model which simulates
the dynamics and effects on soil of populations of this species (Martin, 1990). With the
addition of N from decomposed dead earthworms and cutaneous mucus, a total amount
of 21 to 39 kg assimilable is annually released in this savanna. This represents a
significant proportion of the 70 kg of annually incorporated into the plant
biomass. Nonetheless, the proportion of earthworm-derived N actually incorporated into
plant biomass is not known.
In the absence of plants, ammonium-N released in casts rapidly disappears, either
through nitrification or reorganisation into the microbial biomass (Syers et al., 1979;
Lavelle et al ., 1992b)(Figure IV.51 and IV.59). Eight days after deposition, casts of M.
anomala had a mineral-N content equal to uningested soil control values. In soils with
higher clay contents, reorganisation is slower and a few weeks may be required to
achieve values equivalent to the control soils (Lavelle et al., 1992 op. cit.). In those soils
where nitrification does not occur, e.g., the savanna soils at Lamto, ammonium may be
taken up by plants or directly transformed to microbial tissues.
Phosphorus availability is also influenced by endogeic earthworm activities. In casts
of P. corethrurus , exchangeable and water-extractable inorganic P increase over time
with maximum values, respectively, 2-8 times and 3-8 times those of a control after one
to four days (Table IV.19) (López-Hernández et al., 1993). In a vertisol from Martinique
(West Indies), fresh casts of Polypheretima elongata contained 43 % more exchangeable
P than non-ingested soil, mainly due to digestion processes. Extrapolation of these