Growing density interacts with competitor identity to modulate nitrogen form preference of an invasive plant

Invasive plants often alter the relative availability of soil nitrogen (N) forms to benefit their fitness over that of native species, while co-occurring plants may also use alternative N forms to avoid competition for the same resources. However, it remains unclear whether interspecific competition affects N acquisition by a plant invader and the role of community composition in modulating its impacts on native species. In this study, we aimed to assess how plant interactions affect the strength and direction of such a nutrient-dependent pattern and the relative importance of soil resources and plant community composition in determining the outcome of competition between an invasive plant and neighboring species. We used a screenhouse (wire mesh-covered) experiment to compare the performance of an invasive species, Flaveria bidentis, co-occurring non-native plant Amaranthus retroflexus and native species Bidens maximowicziana either grown alone or in competition with a gradient of growing proportions in which plants had access to different N forms. We found that elevated soil N availability was generally beneficial to F. bidentis, and competition from neighboring plants enhanced its growth and reproduction. While F. bidentis retained the preference for NH4+-N over other N forms in competition, this pattern was more pronounced when it grow at lower proportion and competed with B. maximowicziana. Consequently, this invader’s impacts may be the strongest at low density during the earliest stages of invasion and dependent on recipient species. Our results highlight the species-specific responses in N utilization to soil N availabilities under different competitive regimes and the feasibility of manipulating soil N forms and quantities, as well as selecting competitive resident plants for ecological restoration.