Changes in soil nutrients and microbial communities following understory shrub invasion by Berberis thunbergii

Background/Question/Methods

Berberis thunbergii, or Japanese barberry, is a clonal shrub that has invaded numerous forests in the northeastern United States.  Research we carried out over the past decade has shown that B. thunbergii alters soil properties and microbial communities, and that many of these effects persist after B. thunbergii is removed.  These studies were performed in the field or greenhouse, and focused on characterizing soil nitrogen availability and mineralization, as well as microbial community structure (through phospholipid fatty acid analysis) and function (through extracellular enzyme activities).  Most recently, we have investigated root architecture and functional traits, as well as belowground competitive dynamics in B. thunbergii and other shrub species.  Our aim is to synthesize research on B. thunbergii interactions and effects belowground so as to more fully characterize the ecological legacies of its invasion for both microbial and plant communities.

Results/Conclusions

The most consistent edaphic changes that we have found in association with B. thunbergii colonization are increases in nitrate availability, nitrification rate, and pH.  In addition, we have observed shifts in microbial community composition (primarily increases in bacteria:fungi) as well as decreases in the activity of several extracellular soil enzymes.  Evidence from the field and greenhouse indicate that these changes persist for at least three years, though actual durations are likely much longer.  Although several of our studies implicated leaf litter in raising soil nitrogen and initiating microbial community shifts, several lines of indirect evidence suggest that root litter and exudates also contribute to the observed shifts.  In one study, we found that B. thunbergii roots have low C:N and morphological traits associated with rapid turnover, and in another study we found greater legacy effects in rhizosphere soil than in bulk soil.  Our research has yielded some insight into the implications of B. thunbergii’s belowground legacies.  Potentially as a result of increased nitrate availability, B. thunbergii and competing species both grew more rapidly in greenhouse soils that were previously conditioned by the invader.  However, the stand structure and distribution of roots beneath B. thunbergii stands suggest that it can capitalize disproportionately on elevated soil nitrogen.  In addition to leaving legacies that persist after plants are removed, B. thunbergii can thereby capitalize on soil feedbacks, ultimately contributing to its ability to form monocultures in forest understories.