Competition shifts the advantage of the invasive plant Bidens alba to a disadvantage under soil ammonia nitrogen

Invasive plants often benefit from changes in soil nitrogen (N) availability and from positive feedback of soil microbes, but we know little about the effects of soil inorganic N forms on the competition between invasive and native species and the role of ammonia-oxidizing archaea (AOA) and ammonia-oxidizing bacteria (AOB). We conducted a common garden experiment and grew the invasive species Bidens alba and its native congener, Bidens biternata, in monoculture and mixed culture with four N treatments (NH 4 + , NH 4 NO 3 , NO 3 − and no N addition) with and without the nitrification inhibitor dicyandiamide (DCD). We added a soil microbial suspension containing AOA and AOB to sterilized soil while excluding mycorrhizal fungi. The results showed that B. alba had a growth advantage over B. biternata under the NH 4 + treatment in the monoculture, whereas competition with B. biternata led to a disadvantage for B. alba under the NH 4 + treatment. However, B. alba had a competitive tolerance of B. biternata advantage under NO 3 − . The NH 4 + treatment reduced the competitive suppression ability of B. alba , while the NO 3 − and NH 4 NO 3 treatments strengthened it. The AOB abundance was more sensitive than the AOA abundance to N addition, but the abundances of AOA and AOB did not significantly differ across all N treatments. The application of DCD significantly influenced the soil net nitrification rate but had little effect on the soil NH 4 + and NO 3 − . Our results provide evidence that competition eliminates the growth advantage of B. alba under NH 4 + . This study highlights the importance of N form in competitive outcomes between B. alba and B. biternata as well as the role of AOA and AOB.