Plant competition and insect herbivory mediate the impact of invasive plants on soil bacterial community and function

Exotic plant invasion is an important driver of biodiversity loss. However, little is known regarding how exotic species influence biodiversity belowground and whether such impact can be regulated by other biotic stresses, such as plant competition and insect herbivory. In this study, we used the invasive plant Alternanthera philoxeroides (Mart.) Griseb, native congener Alternanthera sessilis (L.) DC, biocontrol agent Agasicles hygrophila (Selman & Vogt), and native insect Cassida piperata (Coleoptera:Cassididae) as a study system. We established three plant combinations (monoculture of Alternanthera philoxeroides, monoculture of Alternanthera sessilis, and mixture of Alternanthera philoxeroides and Alternanthera sessilis) and four insect herbivory treatments (no insect herbivory, Agasicles hygrophila herbivory, C. piperata herbivory, and Agasicles hygrophila + C. piperata herbivory). We examined how plant competition and insect herbivory mediates the impact of invasive plants on soil bacterial community using high throughput sequencing. We found that plant competition, herbivory treatment, and their interaction did not affect the observed sub-operational taxonomic unit (OTU), Shannon, Phylogenetic diversity, and Evenness indices. These results indicated that neither Alternanthera philoxeroides nor Alternanthera sessilis affected alpha diversity of soil bacteria, and plant competition and herbivory did not exert a mediating effect. In contrast, plant competition strongly affected the composition and structure of the soil bacterial community, decreasing the relative abundance of Chlorobacteria and chemoheterotrophic bacteria while increasing the relative abundance of Acidobacteria. Moreover, plant competition and its interaction with herbivory further regulated the impact of Alternanthera philoxeroides on soil bacterial community composition and structure as well as dominant and functional phyla. Our study indicated that such impact was further regulated by other biotic stresses. Therefore, biotic factors should be included when evaluating the impact of invasive species on biodiversity.