FUNCTIONING OF THE SOIL SYSTEM - Soil Ecology - page 491

In fresh casts of the pantropical species Pontoscolex corethrurus kept in standardised
laboratory cultures, concentrations of ammonium-N varied from soil.
Maximum absolute values were found in an andisol with a high N concentration and
minimum values in a much poorer inceptisol from Mexico. In the Martiniquan andisol,
ammonium-N contents of casts decreased when the worms were fed soil from increasingly
deeper soil strata with lower organic-N contents. The maximum absolute mineralisation
rate between ingested soil and casts was found in a vertisol (29.5 %) while most other
values were in the range 4-10 %. Finally, comparison of N concentrations in
control soils (in which N mineralisation was artificially enhanced by sieving the soil to
less than 2 mm and keeping it at field capacity for the duration of the cultures) and casts
indicates that the effect of earthworms becomes increasingly important in soils with a
low organic nitrogen content.
In laboratory cultures of the earthworms Pontoscolex corethrurus and Polypheretima
elongata in Martiniquan andosols, 45 to 65 % more ammonium was found in casts of
the former species and suggests that N mineralisation rates differ between species
(Lavelle et al. unpublished data). Other results suggest that tropical native species have
greater mineralisation capabilities than exotics (Barois et al ., 1999).