Confirming soil phosphorus as an important driver of invasive garlic mustard (Alliaria petiolata) success
Garlic mustard (Alliaria petiolata), a widespread, invasive, herbaceous plant in the Midwestern and Northeastern United States, possesses unique physiology. This species interferes with mycorrhizal relationships in woody plants and secretes secondary compounds that deter herbivory, inhibit native seed germination, and alter the activity of soil microbes. These attributes make garlic mustard a successful invader; however, as a non-mycorrhizal species itself, garlic mustard may be particularly sensitive to soil nutrient availability. Previous field work in our lab showed that garlic mustard plant size and population density varies significantly among forest patches in central Ohio, USA. Investigations into this pattern showed soil phosphorus availability to be the most probable factor associated with plant success. To confirm this hypothesis, we used a greenhouse study to compare garlic mustard responses to nitrogen and phosphorus additions. Garlic mustard plants were grown from seed in a common field soil and treated with phosphorus, nitrogen, phosphorus plus nitrogen, or no nutrient additions. As a non-destructive assessment of plant growth, we measured the third leaf length before treatment and eleven days after treatment. We also measured dry weights of shoots and a subset of roots after thirty days of treatment. Data were analyzed using two-way analysis of variance.
There were no differences in leaf size among the four treatment groups before treatments were applied. After eleven days, garlic mustard treated with phosphorus had larger leaves than the control and nitrogen-treated plants (22.6mm, 17.8mm, and 14.5mm respectively, p<0.01). Plants treated with phosphorus and phosphorus plus nitrogen did not differ significantly in leaf size (22.6mm and 21.4mm respectively). Shoot and root weight were also significantly affected by phosphorus additions. Phosphorus-treated shoots were heavier than the control and nitrogen-treated shoots (0.2417g, 0.0710g, and 0.0719g respectively, p<0.001), and a similar pattern was seen for root weights (0.0322g, 0.0132g, and 0.0133g respectively, p<0.001). No differences in weight were seen between phosphorus and phosphorus plus nitrogen shoots (0.2417g and 0.2705g respectively) and roots (0.0322g and 0.0321g respectively). These results indicate that garlic mustard growth is more strongly limited by phosphorus than by nitrogen in this soil, and sensitivity to phosphorus is consistent with garlic mustard being a non-mycorrhizal plant. Future studies will examine garlic mustard sensitivity to nutrient additions in other soil types. Our work suggests that an effective management strategy for invasive garlic mustard may be the assessment of soil nutrients to prioritize monitoring and to ensure early removal in high phosphorus areas.