Do invasive plants alter soil microbial communities and does this influence plant competitive interactions

Background/Question/Methods

Soil microbial pathogens and mutualists can have a strong effect on plant community composition and have been shown to influence plant diversity and successional patterns.  Native and invasive plants may differ in their feedback with soil microbial communities.  For example, invasive plants can generate communities that increase their invasive success, whereas native plants often show negative feedback with the microbial community due to pathogen build-up.  The goals of this study were to characterize changes in microbial guilds using phospholipid (PLFA)/neutral lipid fatty acid (NLFA) analyses and to examine the functional consequences of these potential soil legacies on plant competition.  Soil was collected from remnant native communities as well as cheatgrass, knapweed and spurge invasions in March and October.  In order to measure the net effect of positive and negative feedback in soils cultured by different plant species, two native plant species (blanket flower and bluebunch wheatgrass) and cheatgrass,  knapweed, and leafy spurge were grown separately in a greenhouse for two generations. This trained soil was then used to inoculate sterile soil where a mixed community of these five species was planted and grown for 3 months. 

Results/Conclusions

The NLFA 16:1ω5, which is specific to arbuscular mycorrhizal fungi, differed significantly among plant communities in the field and across the season.  It was highest in knapweed and lowest in cheatgrass with native remnant soil being intermediate.  This roughly corresponds to the AM colonization observed in these communities.  Lipid analyses of greenhouse-trained soil indicated significantly higher AMF abundances overall compared with the field, and showed that forbs (blanketflower, knapweed, and leafy spurge) generated significantly higher AMF abundances than the two grasses (bluebunch wheatgrass an cheatgrass) after two generations.  Whether or not this resulted in higher AM colonization in subsequently grown plants is currently under investigation. Our mixed communities showed that plant-mediated shifts in microbial communities influenced plant growth and competitive interactions.  Bluebunch wheatgrass, blanketflower and cheatgrass all responded significantly to the soil training, whereas knapweed grew well regardless of soil history.  Overall, our results show that plant invasions alter soil microbial communities and that these changes can influence plant growth and competitive interactions for some, but not all plants.