Vegetation variables and rhizosphere bacterial communities from ten paired sites in southwestern Finland, 2021

Plant invasions can have major impacts on ecosystems not only in aboveground but also in belowground through changes in microbiota. Particularly legumes, which often host nitrogen-fixing symbionts (rhizobia), can be expected to modify soil bacterial communities. We examined the effect of the invasive herbaceous legume, Lupinus polyphyllus, on the alpha diversity and community composition of soil bacteria. We also explored whether vegetation cover, the cover of other (non-invasive) legumes or number of vascular plants was associated with soil bacterial communities. We sampled rhizosphere soil and surveyed vegetation from ten paired sites (invaded more than ten years ago and uninvaded locations) in southwestern Finland, and identified bacterial DNA by using 16S rRNA gene amplicon sequencing. The plant invader and the three vegetation variables considered had no effect on soil bacterial alpha diversity in terms of bacterial richness, Shannon and Inverse Simpson diversity indices. The composition of soil bacterial communities differed between invaded and uninvaded soils at four out of the ten sites. However, the differences in the relative abundances of top bacterial families between the two soil origins were inconsistent across sites, including rhizobia in the family of Bradyrhizobiaceae that the study species is hosting. Moreover, vegetation cover, legume cover after excluding the study species, and number of plant species explained a small proportion of the variation in the bacterial community composition. Our findings indicate that L. polyphyllus has the potential to modify the soil bacterial community composition locally, at least at sites where it has been present for more than a decade.

Methods
To examine the impact of the herbaceous legume Lupinus polyphyllus on soil bacterial communities, vegetation variables and rhizosphere samples were collected from ten paired sites (ten invaded and ten uninvaded locations) around the Turku area in southwestern Finland in June 2021. The uninvaded locations were at least 30 m apart from the invaded locations, and the distances between the ten sample sites varied from 1.7 to 32.7 km. Each location contained four sampling points of 1 × 1 m.

Bacterial DNA was identified by using 16S rRNA gene amplicon sequencing, resulting in 9897 amplicon sequence variants (ASVs) for analysis. To characterise soil bacterial diversity, bacterial richness (i.e., number of bacterial ASVs per sample), Shannon and inverse Simpson diversity indices were calculated based on the bacterial ASVs.