Soil biota can play a critical role in interactions between native and invasive plants. For example, invaders may accumulate soil pathogens that inhibit native species, suppress mutualists of native competitors, or alter microbial communities that drive ecosystem processes such as decomposition and 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 four-year factorial field experiment manipulating cogongrass (Imperata cylindrica) invasion and precipitation and used high-throughput sequencing to assess changes in soil microbial communities. Subsequently, we used soils collected from the field 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 to test the overall effect of microbes, in soil from each of the four invasion by drought treatment combinations, and alone or in competition with the other species.
Results/Conclusions
We found major changes in soil microbial communities in response to invasion and drought treatments. Drought significantly reduced bacterial richness and cogongrass cover increased fungal richness. Furthermore, fungal functional groups, including plant pathogens and mycorrhizal fungi, differed in relative abundance in the invasion and drought treatments. In the greenhouse, we found that soil microbes had a positive effect on the invader (12% increase in biomass) but a negative effect on both native species (20-30% decrease) when grown alone. Surprisingly, when cogongrass grew in competition with native species, the effect of soil microbes switched from positive to negative (38% decrease) and for longleaf pine in competition switched from negative to positive (25% increase). Soil legacy of drought had no effect on any species. Only wiregrass was affected by soil legacy 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 legacy had species specific effects 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.