Congeneric invasive versus native plants utilize similar inorganic nitrogen forms but have disparate use efficiencies

Aims Soil inorganic nitrogen (N) has long been recognized to play an important role in plant invasions. Whilst comparing the N use strategies of multiple invasive versus native plant congeners along an entire N gradient is key to understanding plant invasion success, there are few related studies. Methods We conducted a potted experiment with six invasive and native congeneric pairs, which were subjected to 11 nitrate/ammonium (NO3−/NH4+) ratios (i.e. 100% NO3− at one end and 100% NH4+ at the other end), each with low- and high-N levels. Each species–N combination was replicated eight times, and thus there were 2112 pots in total. We measured the following traits: the total biomass, growth advantage, biomass allocation, leaf chlorophyll content and low-N tolerance. Important Findings Invasive and native congeners grew well at any NO3−/NH4+ ratios, and their responses of growth, allocation and tolerance were approximately parallel along the 11 NO3−/NH4+ ratios across two N levels. Plant invaders grew larger and had greater chlorophyll contents, higher root biomass allocation and stronger low-N tolerance than their congeneric natives. These findings suggest that invasive and native plant congeners may utilize similar inorganic N forms (i.e. NO3− and NH4+) across an entire N composition gradient and that higher N use efficiencies could favor alien plants to invade new plant communities where congeneric natives are dominants.