Plants can change the characteristics of the soil in which they grow, including the structure and function of the soil microbial community.  We performed a greenhouse experiment to determine if the expanding native plant species, Smilax rotundifolia, has altered the soil microbial communities of an Eastern serpentine grassland in such a way as to impact the growth of native grasses, preventing grasses from re-colonizing areas overtaken by S. rotundifolia.

We examined how Schizachyrium scoparium seedling growth was affected by different microbial fractions from soil collected in the grassland or in S. rotundifolia thickets.  Seedlings were grown in either whole field soil or sterilized soil from each location with some microbial fraction extracted from whole field soil added back.  Seedlings grown in sterilized soil received an arbuscular mycorrhizal (AM) fungi fraction, a fraction containing small microbes (bacteria and other fungi), a combination of both fractions, or neither.  We analyzed total aboveground biomass as a means of quantifying the soil treatments on plant growth.  

Results/Conclusions

The results show significant variation in growth depending upon both type of microbial fraction and soil location.  S. scoparium grew larger in whole field soil collected in S. rotundifolia thickets than in whole field soil from the grassland.  Seedlings growing in any treatment with AM fungi were much larger than those in treatments without.  The grassland AM fungi fraction resulted in the greatest aboveground biomass in the entire experiment. The grassland small microbes fraction reduced aboveground biomass, while the S. rotundifolia small microbe fraction did not.

These results expand the evidence for negative plant-soil feedback in this system.  Feedback occurs not only among grass species, whereby some grass species grow less well in their own soils than in soils cultured by other grass species, but there is also feedback between S. scoparium and S. rotundifolia.  Although S. rotundifolia is expanding into the grassland, it improves the soil for S. scoparium.