The most commonly cited mechanism for exotic plant invasions is the Enemy Release Hypothesis, which until recently, has focused primarily on aboveground interactions. However, escape from belowground natural enemies, such as soil-borne pathogens, is now being investigated by researchers. Invasive plants may take advantage of novel plant-soil biota interactions and exploit feedback relationships that native species do not. We conducted three greenhouse experiments to test for plant-soil feedback interactions as a mechanism for the success of the invasive grass Phalaris arundinacea. Two experiments tested the plant-soil feedback responses of P. arundinacea and the native sedge Carex stricta. The first was a reciprocal transplant experiment and the second was a soil training experiment that grew P. arundinacea or C. stricta in one of three soil treatments (P. arundinacea trained soil, C. stricta trained soil, and sterilized soil/control). Our third experiment tested for allelopathic effects between P. arundinacea and C. stricta by growing each species in competition with a conspecific or heterospecific neighbor, in treatments with or without activated carbon.

Results/Conclusions

In the reciprocal transplant experiment, biomass of both P. arundinacea and C. stricta was greater when grown in their own soil than when grown in the soil of the other species (i.e., positive plant-soil feedbacks). The results of our soil-training experiment indicated that the effects of a plant-soil feedback interaction may be dependent on time. Biomass of P. arundinacea when grown in P. arundinacea trained soil was not different than when P. arundinacea was grown in control soil (a neutral plant-soil feedback). However, biomass of C. stricta was reduced when grown in C. stricta trained soil compared to when it was grown in control soil (a negative plant-soil feedback). These results suggested that C. stricta was inhibited by its own soil biota, whereas P. arundinacea was not. The results of our competition experiment indicated that P. arundinacea did not inhibit the growth of C. stricta via allelopathy. Additionally, we found that activated carbon had no effect on the growth of either species. Overall, our results suggest that P. arundinacea may benefit from novel plant-soil biota interactions and does not inhibit the growth of native North American species via allelopathy. The beneficial (positive and neutral) plant-soil feedback interactions demonstrated by P. arundinacea may contribute to its success as an invader in North America.