Invasive Grass Dominance over Native Forbs Is Linked to Shifts in the Bacterial Rhizosphere Microbiome

Abstract

Rhizosphere microbiomes have received growing attention in recent years for their role in plant health, stress tolerance, soil nutrition, and invasion. Still, relatively little is known about how these microbial communities are altered under plant competition, and even less about whether these shifts are tied to competitive outcomes between native and invasive plants. We investigated the structure and diversity of rhizosphere bacterial and fungal microbiomes of native annual forbs and invasive annual grasses grown in a shade-house both individually and in competition using high-throughput amplicon sequencing of the bacterial 16S rRNA gene and the fungal ITS region. We assessed how differentially abundant microbial families correlate to plant biomass under competition. We find that bacterial diversity and structure differ between native forbs and invasive grasses, but fungal diversity and structure do not. Furthermore, bacterial community structures under competition are distinct from individual bacterial community structures. We also identified five bacterial families that varied in normalized abundance between treatments and that were correlated with plant biomass under competition. We speculate that invasive grass dominance over these natives may be partially due to effects on the rhizosphere community, with changes in specific bacterial families potentially benefiting invaders at the expense of natives.

Invasive plant species interact with drought to shift key functions and families in the native rhizosphere

“…While we did not find that here, it is possible that intra-group interactions (i.e., interactions between native forbs or between invasive grasses) in the field mask differences between natives and invasives as plants compete for limited nutrients (Hawkins & Crawford, 2018). Despite this, our findings that invasive competition shifts native rhizosphere microbes is generally well-supported (Komatsu & Simms, 2019;LaForgia et al, 2022;Rodríguez-Caballero et al, 2020), and we speculate that these invasive-associated microbial shifts play a role in the direction and magnitude of native-invasive interactions as seen elsewhere (Emam et al, 2014;Klironomos, 2002). The sole function that increased in abundance under invasive competition was OprP.…”

“…Invasive grass competition changed the overall community similarity between native rhizospheres, and led to enrichment in the abundance of specific families and functions. In previous work utilizing the same set of species, we found that taxonomic beta diversity significantly differed between invasive grasses, native forbs, and native-invasive pairs using 16S rRNA gene amplicon sequencing (LaForgia et al, 2022;Rodríguez-Caballero et al, 2020). While we did not find that here, it is possible that intra-group interactions (i.e., interactions between native forbs or between invasive grasses) in the field mask differences between natives and invasives as plants compete for limited nutrients (Hawkins & Crawford, 2018).…”

“…One of the least understood pathways contributing to invasive dominance over natives lies in the rhizosphere microbiome. While there is ample evidence that invaders can shift the rhizosphere microbiome, and some evidence suggesting that these invaderinduced changes may aid in the dominance of invasives over natives (LaForgia et al, 2022), how climate interacts with invasive species to alter the rhizosphere microbiome remains poorly understood.…”

“…In the presence of these invasives, however, the negative effects of drought are intensified and the beneficial effects of increased rainfall are dampened (LaForgia et al, 2020). Invasive grasses in this community also drive shifts in the rhizosphere microbial community composition (Batten et al, 2006) which may be contributing to invasive dominance over natives (LaForgia et al, 2022). In order to understand the effects of invasion on native plant communities under climate change, microbial shifts must be assessed under different levels of rainfall availability at the local level.…”

“…The application of SourceTracker, a Bayesian classifier, to metagenomic data is effective at predicting the proportion of a community assembled from known sources (Knights et al, 2011;McGhee et al, 2020), enabling better understanding of microbial community assembly under competing host plants. Utilizing amplicon data, these methods predicted that the joint rhizosphere microbiome of invasive and native plant pairs was equally sourced from both the native host and the invasive host in a well-watered greenhouse experiment (LaForgia et al, 2022). Neighbor-induced changes in the rhizosphere microbiome are greatest under strong competition (Ulbrich et al, 2022) and tend to shift towards those of the dominant competitor (Hortal et al, 2017;Lozano et al, 2019).…”