We selected three coniferous tree species (i.e. slash pine, hoop pine and kauri pine) and an eucalypt species to investigate the patterns of community structure of soil bacteria and eukaryotes. We found that soil pH and nutrient quality indicators like C : N and EOC : EON ratios were key factors determining the patterns of soil microbial communities. We introduced a key factor limitation hypothesis that gives a reasonable explanation for lower diversity indices under slash pine and Eucalyptus.