Competitive outcomes between native and invasive plants are linked to shifts in the bacterial rhizosphere microbiome

Marina LaForgia, Ecology and Evolution, UC Davis, Davis, CA, Cassandra Ettinger, Genome Center, University of California, Davis, Davis, CA; Evolution and Ecology, University of California, Davis, Davis, CA and Hannah Kang, Plant Sciences, University of California, Davis, Davis, CA

Background/Question/Methods
Rhizosphere microbiomes have received growing attention in recent years for their role in plant health, stress tolerance, soil nutrition, and invasive species dominance. 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 plant species. California annual grasslands are a highly invaded system in which invasive annual grasses dominate over the less abundant native annual forbs. We investigated the composition and diversity of rhizosphere bacterial and fungal microbiomes of native annual forbs and invasive annual grasses using high-throughput amplicon sequencing of the bacterial 16S rRNA gene and the fungal ITS region. Plants were grown both individually and in competition in homogenized field soil in a screenhouse. We assessed how significant shifts in key microbial families correlate to plant competitive responses through changes in biomass (log competitive response ratios).
Results/Conclusions
We find that bacterial diversity and composition differ between invasive grasses and native forbs, but fungal diversity and composition do not. Further, bacterial communities under competition are distinct from both individual forb and grass bacterial communities. We identified four bacterial families (Burkholderiaceae, Methylophilaceae, Clostridiaceae_1, and Fibrobacteraceae) that varied in relative abundance between treatments and that were significantly correlated with plant competitive responses. Specifically, abundance of Burkholderiaceae, Clostridiaceae_1, and Fibrobacteraceae were all either positively correlated with grass competitive ability or negatively correlated with forb competitive ability. Conversely, Methylophilaceae was negatively correlated with grass competitive ability and positively correlated with forb competitive ability, however this family, which was abundant in forb microbiomes, was significantly lower in competition treatments. Invasive grass dominance may therefore be partially due to effects on the rhizosphere community of native forbs, with changes in specific bacterial families potentially benefiting grasses at the expense of native forbs. Our study underscores the importance of considering plant-rhizosphere interactions for understanding outcomes of plant invasion on grassland ecosystems.