Garlic mustard (Alliaria petiolata) is a widespread, invasive plant in the Midwestern and Northeastern USA. This species secretes secondary compounds that interfere with mycorrhizal relationships in woody plants, inhibit native seed germination, and alter the activity of soil microbes. These attributes make garlic mustard a successful invader but also prevent garlic mustard from forming mycorrhizae itself, such that this plant may be particularly sensitive to soil nutrient availability. Field work in our lab showed that garlic mustard size, overwinter survival, and population density varied significantly among six forest patches in central Ohio, and environmental measurements suggested that high soil phosphorus availability was the probable factor associated with plant success. To explore this hypothesis, we conducted greenhouse studies in which we grew garlic mustard from seed in (1) field soils from the six sites to try to replicate field differences in plant size with soil type alone, and (2) compared garlic mustard responses to additions of phosphorus, nitrogen, phosphorus plus nitrogen, or nutrient-free water in two of the soils representing opposite ends of the phosphorus gradient. We measured dry weights of shoots and a subset of roots after thirty days. Data were analyzed using one-way or two-way analysis of variance.
In the comparison of garlic mustard sizes across the six different field soils, plants growing in the three high phosphorus soils were significantly larger than those in the low phosphorus soils, matching the pattern of plant size distributions observed in the field (p < 0.05). In the representative low phosphorus soil, phosphorus-treated shoots were heavier than the control and nitrogen-treated shoots (p < 0.001), and a similar pattern was seen for root weights (p < 0.001). No differences in weight were seen between phosphorus and phosphorus plus nitrogen shoots and roots. In the high phosphorus soil, nitrogen additions significantly increased final shoot biomass compared to the controls (p < 0.01), while phosphorus additions did not. These results indicate that garlic mustard is phosphorus-limited on the low phosphorus soils and nitrogen-limited on the high phosphorus soils, although pre-treatment measurements suggest that plants in the nitrogen addition treatment were significantly larger by chance. Our work suggests that phosphorus availability may be an important and under-appreciated factor for invasive success of garlic mustard in Ohio, and that an effective management strategy for this species may be the assessment of soil nutrients to prioritize monitoring and early removal in high phosphorus sites.