Microorganisms and soil physico-chemical conditions in the drilosphere of Lumbricus terrestris

The influence of organic matter input into the soil by Lumbricus terrestris on the spatial distribution of microorganisms and on physico-chemical soil characteristics was examined. An experiment was carried out with L. terrestris in soil cores of about 25 cm depth. Lettuce was supplied to the worms as a food source. In the control soil without earthworms, lettuce was manually mixed with the soil. The numbers of total bacteria, siderophore producing bacteria and fluorescent pseudomonads were greatest in the casts and burrowing walls of the 0–5 cm soil layer (1.5 × 109, 1.6 × 108 and 4.4 × 107 CFU g−1 of dry soil, respectively). Numbers in the uningested soil from the same soil layer were 60–320 times smaller. In the deeper soil layers (5–22 cm), bacterial numbers in the uningested soil were 6–32 times smaller than in the burrow walls. The bacterial counts for the control soil were intermediate between the counts for the casts + burrow walls and the counts for the uningested soil. Over the depth of soil studied, the NO3−N content of the casts and burrow walls was 1.2- to 2.1-fold greater than in the uningested soil, but it was only greater than the control soil (notably 1.66-fold) in the top layer (0–5 cm). The distribution of microorganisms and nutrients along the earthworm burrows was associated with good water availability and improved aeration. Based on these observations, the drilosphere of actively feeding L. terrestris is considered to be one of the most important microbial niches in soil.