Plant-soil feedbacks have been proposed as one potential mechanism to explain the success of exotic invasive plant species. Exotic plants have been shown to alter communities of soil bacteria, fungi, and soil fauna, often benefitting the exotic plant. However, trophic interactions among plants, bacteria, fungi, and soil fauna have the potential to affect plant success. We explored the role of belowground trophic interactions in the ongoing Leymus arenarius invasion in Great Lakes sand dunes in western Michigan. In a greenhouse experiment, we quantified differences in multi-trophic interactions among the exotic species Leymus arenarius, its native counterpart Ammophila breviligulata, arbuscular mycorrhizal fungi (AMF), and soil nematodes. Specifically, we asked (1) Do belowground multitrophic interactions influence invasive plant success? In a follow-up experiment, we further explored plant-soil feedbacks in the context of invasion by assessing native and non-native plant responses to soils conditioned by Ammophila or Leymus, paired with a nested competition treatment. For this experiment, we asked (2) What are the direct competitive and indirect soil-mediated effects of invasion on other plants?
Results/Conclusions
In our first experiment, manipulations of arbuscular mycorrhizal fungi and soil nematodes revealed that nematodes altered the outcome of AMF-plant interaction for the exotic Leymus. When nematodes were absent, exotic Leymus biomass had an inverse relationship with AMF root colonization. However, when nematodes were present, this relationship was reversed, indicating that soil nematodes may be enhancing the mutualism between Leymus and its AMF symbionts. This effect was not observed with the native Ammophila. In our second experiment, we found that direct competition from Leymus had stronger effects on other plants’ growth than indirect effects of soil conditioning, although Leymus-conditioned soils suppressed plant growth by ~9%. Overall, our results suggest that multi-trophic interactions involving nematodes can alter the outcomes of plant-soil interactions between an exotic plant and its AMF symbiont, but the soil-mediated effects are weaker than competitive plant-plant interactions.