The stimulatory effect of earthworms (Lumbricus terrestris is L.) on s
oil microbial activity was studied under microcosm-controlled conditio
ns. The hypothesis was tested that microbial stimulation observed in t
he presence of a soil invertebrate would be due to the utilization of
additional nutritive substances (secretion and excretion products) tha
t it provides. Changes in microbial activity were monitored by measuri
ng simultaneously CO2 release and protozoan population density. The in
crease in CO2 released in the presence of earthworms was found to resu
lt from both earthworm respiration and enhanced microbial respiration.
The stimulation of microbial activity was confirmed by a significant
increase in protozoan population density, which was 3-19 times greater
in the presence of earthworms. The respiratory rate of L. terrestris
was estimated to be 53 mu l O-2 g(-1) h(-1). Earthworm respiration sig
nificantly correlated with individual earthworm weight, but there was
no correlation between the increase in microbial respiration and earth
worm weight. This finding does not support the hypothesis given above
that enhanced microbial respiration is due to utilization of earthworm
excreta, A new hypothesis that relationships between microbial activi
ty and earthworms are not based on trophic links alone but also on cat
alytic mechanisms is proposed and discussed.