Plant-soil feedbacks and competitive interactions between invasive Bromus diandrus and native forb species

Feedback between plant and soil microbial communities plays a key role in plant invasions. We examined feedback in native and invasive plants growing in monoculture and mixture, to determine soil microorganisms' role in Bromus diandrus invasion. Four native forb species were grown in monoculture and in competition with Bromus and with different microbial inocula. Inoculum consisted of 20 g of soil collected from the rhizosphere of native or invasive plants used to create treatments of (1) whole soil, (2) filtrate containing non-mycorrhizal microbes, and (3) arbuscular mycorrhizal fungi (AMF) spores. Native species in monoculture experienced neutral to positive feedback with whole soil and filtrate inoculum. Feedback in Bromus grown in monoculture varied in direction and magnitude with different soil microbial fractions. Fine AMF (Glomus tenue) in filtrate inoculum appeared to cause observed positive feedback effect in native and invasive species, even with pathogenic fungi in roots. Feedback in mixture was more positive than in monoculture for some species. Our study highlights the difficulty of extending feedback results in monoculture to the community level, and the importance of fine AMF, which has received little attention, interacting with pathogens in plant invasion.