The disruption of soil microbial communities is thought to be an important contributor to range expansion of several invasive plants. Previous research has focused on identifying how invasive plants change soil microbial diversity and composition. However, it is also important to recognize the different functional roles of genetically distinct microbial taxa to understand the mechanism of plant invasion and its impact on nutrient cycling and other ecosystem services. We examine changes in soil microbial functional groups associated with the invasion of Alliaria petiolata. Laboratory experiments suggest that A. petiolata can suppress arbuscular mycorrhizal fungi (AMF) to disrupt native plant communities. Despite mounting evidence of AMF suppression under controlled laboratory and greenhouse experiments, it is less clear if allelochemicals persist under natural field conditions. To determine how A. petiolata invasion alters soil bacterial and fungal diversity and function in the field, we assigned taxonomic and functional classification to the 16S and ITS rRNA sequences present in edaphically matched soil samples inside and outside ten naturally occurring populations of A. petiolata.  We also measured root health of plants co-occurring with A. petiolata to test the effect of soil microbial functional groups on plant health. In contrast to controlled experiments, we found no changes in mycorrhizal diversity, suggesting that mycorrhizal suppression is not a particularly strong mechanism of A. petiolata invasion. Instead, we recorded changes in pathogen community composition and an increase in root lesions for plants grown in A. petiolata invaded soils. These results support the ‘Enemy of my Enemy Hypothesis’ which predicts pathogen accumulation and spillover, decreasing native plant performance.

Synthesis

We did not find support for the suppression of AMF as a mechanism of invasion by A. petiolata in invaded field soils. However, changes in the community composition of pathogen and nutrient cycling microbes may be important forces underlying the invasion of A. petiolata and its impact on ecosystem function.

Methods are described in the manuscript. Files include data and annotated analysis files to recreate the published results.