A metagenomic survey of soil microbial communities in old fields dominated by exotic plant species and native remnant grasslands in the Inland Pampa, Argentina

Background/Question/Methods

The reassembly of plant communities in old fields with strong cultivation legacy frequently leads to the establishment of exotic plant species. Such systems show little recovery towards native grasslands and may represent novel ecosystems. We hypothesize that exotic plant invasions produce shifts in soil microbial composition which might facilitate the persistence of a degraded vegetation state. Also, we expect bacterial communities from old fields to be homogenous owing to the fact that plant communities in successional grasslands are highly dominated by one or few species. By contrast, soil bacterial communities from natural remnants would show higher differences among sites. We used 454 pyrosequencing of the 16s rRNA gene to characterise and compare soil microbial communities of 5 exotic-dominated old fields and 5 native grassland remnants of the Inland Pampa, Argentina.

Results/Conclusions

We found that Acidobacteria and Planctomycetes were the most abundant bacterial phyla in these grasslands, accounting for 50% of sequences. The relative abundance of certain bacterial groups differed between old fields and native grasslands (Plactomycetes, Verrucomicrobia, and Chloroflexi were more abundant in old fields, whereas Nitrospirae was more abundant in remnants, p<0.05). Furthermore, in terms of specific composition, 60% of the identified species (OUT’s at 97%) were unique for old fields or native grassland remnants. Surprisingly, local diversity in old fields was higher than in remnants (equitability index in old fields: 0.85; in remnants: 0.79, p<0.05), opposite to the pattern shown by plant community.

The structure of bacterial communities of old fields was different from native grasslands when evaluated by two different metrics (ANOSIM: Bray Curtis: R=0.416, p=0.012; Weighted Unifrac: R=0.336, p=0.025). Additionally, bacterial communities in old field sites were more similar to one another than did the native grassland sites, demonstrating that old fields had more homogeneous soil bacterial communities. Our results suggest that the invasion of exotic plant species promote changes in the soil microbial community that might facilitate the persistence of exotic species in post agricultural grasslands.