THE ROLE OF THE EARTHWORM LUMBRICUS-TERRESTRIS IN THE TRANSPORT OF BACTERIAL INOCULA THROUGH SOIL

Two laboratory experiments were used to investigate the effect of the earthworm Lumbricus terrestris on transport of genetically marked Pseu domonas fluorescens inocula through soil microcosms. The microcosms co mprised cylindrical cores of repacked soil with or without earthworms. Late log-phase cells of P. fluorescens, chromosomally marked with lux genes encoding bioluminescence, were applied to the surface of soil c ores as inoculated filter paper discs. Ln one experiment, 5 and 10 day s after inoculation, cores were destructively harvested to determine c oncentrations of marked pseudomonads with depth relative to the initia l inoculum applied. Transport of the bacteria occurred only in the pre sence of earthworms, In a second experiment cores were subjected to si mulated rainfall events 18 h after inoculation with lux-marked bacteri a at 3-day intervals over a 24-day period. Resulting leachates were an alysed for the appearance of the marked bacteria, and after 28 days co res were destructively harvested. Although some marked cells (less tha n 0.1% of the inoculum applied) were leached through soil in percolati ng water, particularly in the presence of earthworms, the most importa nt effect of earthworms on cell transport was through burial of inocul ated litter rather than an increase in bypass flow due to earthworm ch annels.