Influence of barley straw and the lumbricid earthworm Aporrectodea trapezoides on Rhizobium meliloti L5-30R, Pseudomonas corrugata 2140R, microbial biomass and microbial activity in a red-brown earth soil

In greenhouse experiments, the ability of barley straw and the earthworm Aporrectodea trapezoides to influence the persistence of Pseudomonas corrugata 2140R and Rhizobium meliloti L5-30R, previously inoculated separately into soil, was examined. The addition of barley straw (0.62% w/w), significantly increased the numbers of both introduced bacteria ca. 1000- to 3000-fold after 29 d incubation and ca. 25-to 100-fold after 63 d incubation in soil. In the absence of barley straw, there was a significant positive linear relationship between the number of A. trapezoides (at densities equivalent to 0, 105, 315 or 525 m−2) and the numbers of both introduced bacteria after 29 d, but not after 63 d incubation. In contrast, in the presence of barley straw, there was a significant negative linear relationship between the number of A. trapezoides and the numbers of both introduced bacteria after 29 and 63 d incubation. By combining data from both sampling times, there was a significant linear relationship between the persistence of both introduced bacteria and changes in microbial biomass only in the presence of added barley straw. This would suggest that A. trapezoides had a selective effect upon the persistence of both introduced bacteria in the absence of barley straw, which was not manifest upon the whole microbial community.