The role of the earthworm Lumbricus terrestris Linnaeus in the transport of bacterial inocula in soils
The effect of L. terrestris on the transport of marked bacterial inocula in the absence of percolating water was investigated using repacked soil microcosms over a 10 day period. The microcosms consisted of cylindrical cores containing loamy sand. The bacterial inoculum was applied in filter paper disks to the surface of soil cores containing L. terrestris and to control cores. Destructive analysis of cores was carried out 5 and 10 days after inoculation to facilitate enumeration of marked bacterial inocula at 3 depths in the soil. Significantly greater vertical transport of marked bacteria occurred in cores containing L. terrestris. The effect of L. terrestris on the dispersal of marked bacteria in the presence of percolating water was investigated using both repacked soil cores and larger intact cores over a one month period. Bacterial inocula were again applied in filter paper disks to the surface of cores and the cores subjected to simulated rainfall events at 3 day intervals. Concentrations of marked bacteria in leachate were determined and destructive harvests of cores carried out after 24 days. The results from repacked soil cores demonstrated that cores containing earthworms were associated with early breakthrough, higher percentage recovery and greater numbers of marked bacteria in the leachate than control cores without earthworms. Results from intact soil cores demonstrated that cores containing earthworms were associated with different bacterial leaching patterns, higher percentage recovery and greater numbers of bacteria in the leachate than control cores. For both repacked and intact cores, the presence of earthworms in cores led to transport of marked bacteria to greater depths in the soil. Plaster of Paris impregnation of repacked and intact cores highlighted the importance of earthworm burrowing in providing pathways for bypass water flow and hence bacterial transport through soil. Results from the use of interdisciplinary methods developed and adapted in this study identify the importance of earthworms in the fate of microbial inocula in soil and the potential for use of earthworms in biotechnology.