Plant species can cultivate soil microbial communities which may then affect the fitness of neighboring or subsequent soil inhabitants. Previous work on black mustard (Brassica nigra), showed that an intransitive competitive hierarchy among plant species and plant genotypes was primarily mediated by allelochemical-induced changes in soil microbial communities. In this study, we evaluate the relative contributions of such previous soil history (i.e. legacy from previous plant selection on soil communities) and selection (i.e. the contemporary plant-soil interactions) on soil microbial community composition. We performed a 3x2 factorial greenhouse experiment with three soil innoculum treatments (soils obtained from field patches of black mustard with high or low secondary compound genotypes and from mixed non-brassica patches) and two plant selection treatments (non-brassica competitors, Malva parviflora or Sonchus oleraceus) and measured soil bacterial and fungal community composition and function by terminal restriction fragment length polymorphism and plant growth after four months.  

Results/Conclusions

Final bacterial and fungal communities differed in their reliance on initial innoculum source and subsequent plant selection. While bacterial community composition was significantly related to plant selection but not to the innoculum source, fungal community composition showed the opposite. While no treatment interactions were detected, innoculum source had a stronger affect on fungal communities cultivated with S. oleraceus as compared with M. parviflora. Further, while S. oleraceus growth was significantly affected by the soil innoculum treatment, no association was found between plant size and final fungal community composition. However, M. parviflora growth was not significantly associated with innoculum source but showed a strong relationship with the final fungal communities. Our results suggest that the relative importance of source pool and current selection for determining microbial community structure (and potentially function) not only differs between microbial taxa but also varies based on the specific selective environments.