Shrub encroachment on hill prairies alters soil microbial community composition

Background/Question/Methods

Hill prairies can be found on erodible, southwest-facing slopes in otherwise forested river valleys and bluffs of the Midwestern United States. Their dry microclimates inhibit forest establishment, but recent decades of fire suppression have allowed for shrub and tree encroachment, endangering these unique ecosystems. Shrub encroachment is known to alter soil microbial activity rates, but it is unclear how shrub encroachment affects the composition of soil microbial communities. We surveyed the community composition of soil bacteria and fungi in eleven isolated hill prairie patches along a 40-mile stretch of Mississippi River bluffs in southwestern Illinois, running transects from the prairie core, through the shrub-encroached border, and into the surrounding forest. We characterized community composition using a DNA-based fingerprinting approach, and we tested the hypothesis that shrub encroachment alters microbial community composition using nested permutational multivariate analysis of variance.

Results/Conclusions

Both bacterial and fungal communities varied significantly across the prairie-to-forest continuum (p < 0.001 for both tests). We found large-scale beta diversity along the bluffs, which accounted for ~32% of the total variance in microbial community composition. However, in spite of this patch-to-patch variability, we were able to identify microbial communities that typified the three surveyed habitats, and prairie microbial communities were always distinct from those of the surrounding forest. The majority of shrub-associated fungal communities were intermediate between those of the prairie core and those of the forest, indicating that shrub encroachment gradually alters soil fungal communities to be more “forest-like.” Roughly half of the shrub-associated bacterial communities were intermediate between those of the prairie core and those of the forest, while the other half of the shrub-associated bacterial communities were indistinguishable from those of the forest. This indicates that bacterial communities may shift much more rapidly to a “forest-like” state in the face of shrub encroachment. These shrub-induced changes in soil microbial communities may have lasting consequences for soil ecosystem functioning or for plant-microbe interactions that affect plant community dynamics.