Recent studies have shown that exotic plant species can have wide-ranging effects on soil ecosystem processes and biotic diversity, and those effects may be more extensive than previously thought. Garlic mustard [Allliaria petiolata (M. Bieb.) Cavara and Grande] is an exotic, invasive herb that poses a threat to North American forests by reducing native plant performance and recruitment. However, little is known about potential impacts to belowground communities and processes. The object of this study was to determine the effects of A. petiolata on soil physical, chemical, biogeochemical, and biotic properties. Soil samples were collected between October 2006 and February 2008 from plots with A. petiolata present or absent in three Ohio forests: the University of Toledo Stranahan Arboretum (Arboretum), Fuller Preserve (Fuller), and South Park (South). Variables measured included moisture content, pH, organic matter (SOM), nutrient dynamics [dissolved organic carbon (DOC), microbial biomass carbon (Cmic), dissolved organic nitrogen (DON), microbial biomass nitrogen (Nmic), ammonium, nitrate, and phosphate], microbial community function [activities of acid phosphatase (PHOS), α-glucosidase (AG), β-glucosidase (BG), N-acetyl-β-glucosaminidase (NAG), β-D-cellobiohydrase (CBH), β-xylosidase (BXYL), leucine amino peptidase (LAP), urease, phenol oxidase (phenox), and peroxidase (perox)], and microbial community structure [community DNA profiles, Simpson index (D), Shannon index (H’), and richness (S)].
Results showed that A. petiolata presence had a significant affect on only approximately one third of the measured variables. Generally, the variables affected and the direction of effects (positive or negative) were inconsistent both spatially and temporally. Soil moisture, SOM, Nmic, DOC, phosphate, BG activity, ratio of C- to P-acquiring enzyme activities (C:P), and bacterial richness and diversity were lower with A. petiolata present at Arboretum, but most treatment differences were observed at only one sampling date, Fall 2006. Moisture, Cmic, LAP activity, C:P, and bacterial richness and diversity were impacted at Fuller with all but C:P lower in soils with A. petiolata present. At South, BXYL and PHOS activities and ratio of C- to N-acquiring enzyme activities were lower in soils from A. petiolata stands, but pH, Nmic, nitrate, phosphate, CBH activity, and C:P were higher.
Despite inconsistent effects of A. petiolata presence on bacteria community richness, diversity, and extracellular enzyme production, bacterial community composition shifts were found between treatments at all three sites. Generally, communities from soil with A. petiolata present were only approximately 50 percent similar to communities from non-invaded soils. Unfortunately, it is not known which species of bacteria were impacted or if the species impacted were similar between sites. However, the observed community changes strongly suggest a pervasive and consistent impact of A. petioloata on soil bacteria.
It is uncertain if A. petiolata altered these properties or if soil properties influenced A. petiolata colonization. However, A. petiolata was associated with differences in a broad range of soil properties, most notably bacteria community composition. These changes may potentially lead to impacts on ecosystem processes such as nutrient turnover and decomposition and on plant community composition. If exotic plants benefit from these modifications, it could create a feedback loop that promotes A. petiolata growth or additional exotic plant invasions.