Garlic mustard (Alliaria petiolata) is an invasive plant that can out-compete native flora by forming a thick monoculture in the forest ground cover.  This species produces allelochemicals that have the potential to alter microbial community composition and function.  The objectives of this study were to determine the effects of garlic mustard on microbial community composition, function, and soil fertility in comparison to native ground cover.  The study site is a 60 year old mixed-mesophytic forest on Ultic Hapludalfs in southeast Ohio.  Twelve, 150 m² plots were established in 2007.  Six plots served as the control (native ground cover) and the other six were dominated by garlic mustard.  On June 2009, five soil cores were harvested (5 cm deep) for subsequent analysis.  We determined microbial community composition using phospholipid fatty acid (PLFA) analysis, and we quantified microbial function by measuring the activity of extracellular enzymes involved in the depolymerization of cellulose (β-glucosidase), lignin (peroxidase), chitin (chitinase), and organic P (acid phosphatase).  Soil fertility was determined by measuring total soil C, N and available P.

Results/Conclusions

Results showed that garlic mustard significantly (P < 0.01) reduced microbial biomass by 55% in comparison to the control plots. Using multi-response permutation procedures, PLFA biomarkers did not differ (P = 0.58) between soil from the garlic mustard and control plots.  We observed garlic mustard significantly (P < 0.05) increased the activity of β-glucosidase, chitinase, and phosphatase by approximately 35%.  However, garlic mustard significantly (P < 0.01) suppressed the activity of peroxidase by 25%.  Garlic mustard soil contained around 20% more total C and N, and 100% more P than the control plots.  Increased soil C in the garlic mustard plots had a significant negative correlation (r = 0.65; P < 0.05) with peroxidase activity.  Results suggest that garlic mustard can decrease microbial biomass, which correlates with a change in soil function.  Specifically, there is an increase in the decay of labile organic matter, but a suppression in the decay of recalcitrant organic matter.  These results are consistent with the observed increases in surface soil C, N, and P.  It appears that one of the consequences of garlic mustard colonization is increases in soil fertility and the capacity of soils to store carbon in mixed-mesophytic forests.