Effects of soil inocula on the growth responses of native annual forbs and the invasive annual grass, Bromus diandrus

Background/Question/Methods

Invasive plants have the ability to alter the rhizosphere microbial community upon establishment and form either positive or negative feedbacks that affect their dominance.  Associated native forb species may also form positive or negative feedbacks, and the direction of feedbacks will affect competitive interactions between native and invasive plants. We tested plant-soil feedbacks as a potential mechanism altering native plant growth and diversity in a southern California annual grassland dominated by Bromus diandrus, an exotic annual grass. Four native forbs with varying sizes were chosen for study: Amsinckia menziesii, Layia platyglossa, Lasthenia californica, and Plantago erecta. A. menziesii is the largest of the forbs and may be equal in mass to B. diandrus but not as abundant.  A greenhouse study was conducted in which the native annuals and B. diandrus were grown from seed for seven weeks in monocultures and in competition in sterile soils, native inoculum, and invaded inoculum. Inoculum consisted of 15g of soil collected from the rhizosphere of native or invasive plants, and added to 800 g of sterile soil in pots.

Results/Conclusions

For plants grown in monocultures, the shoot biomass varied significantly (P<0.05) across soil inocula treatments for all species except L. californica.  P. erecta had lower shoot biomass and higher mortality when grown in sterile soils than with either native or invasive inoculum, suggesting high dependence on mycorrhizal fungi.  Conversely, both A. menziesii and B. diandrus had the greatest shoot biomass in sterile soils and the lowest shoot biomass in invasive soils.  When grown in competition with B. diandrus, P. erecta and L. platyglossa had the highest biomass in native inoculum, that was significantly greater than in sterile but not invasive inoculum, and A. menziesii had the highest biomass in sterile inoculum (P<0.05). Shoot biomass did not vary significantly across soil inocula treatments for B. diandrus in competition. Overall, three forb species showed a positive response to their own inoculum in mixture, but only one had a positive response in monoculture. In contrast, A. menziesii was indifferent to source of inoculum, but increased growth in sterile soil indicated a build-up of microbes that cause negative feedback. The lack of response of B. diandrus to soil inoculum in competition may in part explain its dominance in invaded annual grasslands.