The belowground mechanisms facilitating invasion and proliferation of non-native plant species into natural communities are of critical importance for understanding plant invasions. Research on allelopathy suggests that some exotic plant species produce compounds via root exudates that may suppress or inhibit the growth of neighboring plants, but the specific mechanisms and consequences of these plant-chemical interactions remain elusive. In an effort to understand the abiotic and biotic factors governing allelopathic activity, a two-part greenhouse experiment was designed to examine plant-soil interactions of the invasive euphorb, Phyllanthus urinaria, and the residual soil effects of these interactions on subsequent growth of a target species, Brassica rapa. Our results confirmed that plant-soil feedback from Phyllanthus negatively influenced growth of Brassica and the effect was proportional to the Phyllanthus biomass. However, it is unclear whether the mechanism responsible for this negative effect is associated with chemical suppression (i.e., allelopathy) or with depletion of soil nutrients (i.e., competition). In addition to screening for phyto-toxic compounds, soil analysis of macro- and micro- nutrient levels may be necessary to differentiate between plant strategies that create chemical interference vs. those that influence resource availability. Description and definition of these plant mechanisms may provide useful insight into understanding the factors that enable alien species to invade and successfully coexist in natural communities.

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