When nonnative species are introduced to a new range, their success is dependent on the relative strengths of interactions with new species. Soil biota can play a critical role in competition between native and invasive plants if invaders accumulate soil pathogens that inhibit native species, suppress mutualists of native competitors, or alter microbial communities that drive ecosystem processes such as decomposition or nutrient cycling. Furthermore, plant-soil interactions are likely to change under altered abiotic conditions associated with climate change. The objectives of this study were to determine the role of soil microbial communities in performance of invasive versus native plant species and to evaluate how legacy soil effects of invasion and drought influence the competitive interactions between invasive and native species. We conducted a long-term factorial field experiment manipulating cogongrass (Imperata cylindrica) invasion and precipitation and used soils collected from this experiment as inoculum in a greenhouse study to evaluate plant performance under different soil legacies. The invader (cogongrass) and two dominant native species (longleaf pine and wiregrass) were grown in either sterile or live soil inoculum, alone or in competition, and in soil from each of the four invasion by drought treatment combinations.
Results/Conclusions
We found that the effect of soil microbes on plant growth was positive (12% increase in biomass) for the invader but negative for both native species (20-30% decrease) when grown alone. Surprisingly, the effect of soil microbes switched from positive to negative (38% decrease) for the invader when grown in competition with native species and switched from negative to positive (25% increase) for longleaf pine when grown in competition. The effect of soil microbes on wiregrass remained negative under competition but the magnitude of the effect was dampened. Soil legacy had smaller but significant effects on plant growth, and these effects varied by species. Both cogongrass and pine growth was improved by soil legacy of drought, while only wiregrass was affected by soil history of invasion, where wiregrass growth was inhibited in invaded soils. Our results show that competition can fundamentally alter the role of soil microbes in plant invasion, and that soil biotic and abiotic legacies have more moderate but significant effects on plant performance with implications for plant invasion under climate change. More broadly, understanding the roles of soil microbial communities in plant competition could improve the success of restoration efforts for native ecosystems.