Mutualisms are widespread in nature but the strength of these interactions can vary with environmental conditions. Under certain circumstances, mutualisms can erode into parasitism as conflict arises between the two species. In the plant-arbuscular mycorrhizal fungi (AMF) mutualism, AMF external hyphae provide plants with nutrients/water, and plants supply AMF with carbon. This mutualism is critical for many plants; ~70% of forest understory species are obligately dependent on AMF. Garlic mustard (Alliaria petiolata) produces allelochemicals that are toxic to the nutrient-providing AMF external hyphae, yet can leave the carbon-absorbing AMF arbuscules intact, potentially setting up a conflict. Thus, the recent invasion of forests by garlic mustard may be altering conditions that favor this widespread mutualism. Because AMF external hyphae of forest plants should die upon exposure to garlic mustard allelochemicals, we hypothesized that reduced soil respiration rates would be seen in treated plants relative to controls. To test this hypothesis, we used the forest native Maianthemum racemosum in a pot experiment in the field. Plants were treated with either fresh garlic mustard litter or the controls of fresh Hesperis matronalis litter or no litter for one week and then soil respiration was measured. H. matronalis is a relative of garlic mustard that is largely naturalized and allelochemical-free. 

Results/Conclusions

We found a significant overall treatment effect on soil respiration (ancova, p = 0.04, covariates: plant root mass and temperature). Soil respiration was significantly lower around plants in the garlic mustard treatment compared to plants in the H. matronalis treatment (pairwise comparison, p = 0.035). This finding suggests that garlic mustard litter decreases the abundance/activity of microbes in our experiment. The no litter treatment had the lowest soil respiration, which was expected due to the lack of nutrient input from litter for microbial digestion. In addition, our previous work shows that M. racemosum treated with fresh garlic mustard litter display significantly reduced physiological function (i.e. photosynthetic rate, transpiration rate, stomatal conductance) compared to control plants while retaining AMF arbuscules in their roots. Coupling these two results strongly suggests that garlic mustard can kill AMF external hyphae in the soil and decrease the nutritional benefits received by native plants from these mutualists, while leaving the cost of the interaction (arbuscules) intact. This could shift the interaction, at least temporarily, to parasitism. We propose that the creation of conflict within important native mutualisms may be a critical mechanism that allows invaders to succeed in novel habitats.