This study was designed to clarify the relationship between the distributions of soil organisms and roots under conventional tillage (CT) and reduced tillage (RT) systems. We hypothesized that the distributional relationship would be changed by the stratified organic matter under RT. To test the hypothesis, we compared the vertical and horizontal distributions of soil organisms within and between rows at depths of 0-10 and 10-20 cm in a wheat field. With the exception of nematodes, the spatial distributions of microorganisms and microarthropods were correlated with root biomass. The ratio of microbial activity or the number of organisms in the soil within rows to the same parameter between rows at a depth of 0 and 10 cm, differed among different soil organisms (ranging from 0.9 to 8.2). The ratios for microorganisms and microarthropods were greater under CT than under RT. This suggests that the root effect is small or is offset by high organic matter content in the surface layer under RT, and that the magnitude of the effect was greater under CT. (C) 2008 Elsevier Masson SAS. All rights reserved.