Species invasions are a significant element of global change. There are several
mechanisms in which invasive species dominate the natives in their invaded range. One
of those mechanisms is plant-soil feedback. In my master’s thesis I assessed the role and
importance of soil microbiota in the interactions between the herbaceous invader Lupinus
polyphyllus (garden lupin) and three native herbs commonly found in human-affected
habitats representing different attributes: Trifolium repens (white clover), Centaurea
cyanus (cornflower) and Taraxacum spp. (dandelion). I evaluated the effects of
competition and the presence of soil microbes by growing L. polyphyllus together with
each of the three native plant species in a common garden pot experiment. A soil
inoculum crafted from around lupin roots was used to introduce soil microbes into the
sterilized substrate. The results of this study suggest that competition (the presence of L.
polyphyllus) has an effect on the studied native herbs’ growth and these effects vary
among species. The effects were not all negative, as T. repens grew more shoots when
growing with L. polyphyllus and C. cyanus was unaffected. Taraxacum spp. however,
showed a significant decrease, especially in root growth. Soil microbes associated with
L. polyphyllus also affected the growth of the studied natives, but species responded to
the microbe addition similarly, growing less roots. Also, in turn, the species L. polyphyllus
was growing with affected its growth. The biomass of L. polyphyllus was negatively
affected by the presence of C. cyanus. Lupinus polyphyllus nodule growth was tentatively
affected by microbes and competition together, as with Taraxacum spp. the microbes
seemed to affect the growth of the nodules negatively. I conclude that the benefits of
mutualistic soil microbes may vary with the species the invader is competing with. I also
suggest that the effects of soil microbes, both positive and negative, may differ in different
competitional settings. Hopefully, these results can add to the knowledge of mechanisms
of invasions, important in planning conservational efforts and screening for possible
invaders
