Background/Question/Methods   While Nitrogen is abundant in the atmosphere, plants must obtain N from the soil, as mineral Ammonium (NH4+) or Nitrate (NO3-). Although many plants take up only one form or N or the other, the exotic invasive Alliaria petiolata has been shown to be able to take up both, a property that may contribute to its invasiveness. We investigated whether this ability influences the competitiveness of A. petiolata under greenhouse conditions when the form of N available is manipulated, especially against species that have been shown to specialize on NH4+ or NO3-. One might expect a NH4+ specialist to compete fairly well with A. petiolata when all N is provided as ammonium, but not do as well under mixed or exclusively NO3- N. We grew an ammonium specialist (Ageratina altissima) a nitrate specialist (Hordeum vulgare) and a plant that does best under mixed conditions (Lythrum salicaria) as singletons, in high density monospecific plantings, and as singletons surrounded by A. petiolata. Plants were grown in sand culture with the quality but not quantity of N manipulated by adding Ammonium Sulphate, Sodium Nitrate or both. We measured response to competition by monitoring plant size through the growth period and dry above-and below-ground biomass, root:shoot ratio and leaf specific area at harvest.

Results/Conclusions   Results showed that when all three specialist species were in competition either with its own species or with Alliaria petiolata under the preferred N conditions, it was suppressed by roughly the same degree. However, when a specialist was grown in a nonpreferred N condition, the suppressive effects of A. petiolata were exaggerated beyond the intraspecific competition effects. We also found that species differed in their responses to growing in nonpreferred N conditions in the absence of competition. Native species grew smaller both in competition with A. petiolata and with itself, while exotic Lythrum adjusted its root:shoot ratio, producing more roots, possibly in a search for favorable N sources. These findings suggest that A. petiolata's ability to exploit multiple forms of N may enhance its competitive abilities when encountering species that specialize on only one form, thus expanding its invasiveness in a community context.