Abstract: Global warming shows a diurnal asymmetrical trend. Invasive plant species have significant impacts on soil nitrogen (N) transformation. Ammonia-oxidizing bacteria (AOB) and archaea (AOA) are the key microorganisms regulating N transformation. However, the effects of invasive plants on soil N transformation under day and night-warming and its relationship with ammonia-oxidizing microorganisms remain unclear. Two invasive alien plants (Bidens alba and Praxelis clematidea) and a native plant (Emilia sonchifolia) were selected. A common garden experiment with three warming treatments (day warming, night warming and whole day warming) and a control (no warming) was set up. We investigated the effects of different plants on soil N transformation under day and night-warming and the relationship between soil N content and ammonia-oxidizing microorganisms. The results showed that day warming increased the soil NH₄⁺ of B. alba and the soil NO₃^- content of E. sonchifolia, while night warming increased soil NO₃^- content of B. alba but decreased soil NH₄⁺ content of P. clematidea. Those results indicate that different warming had different effects on soil nitrogen content, which depended on plant species. Day and night-warming increased the abundance of AOB of P. clematidea, while soil AOB abundance of B. alba was only increased under night warming, and warming had no significant effects on abundance of soil AOA. There was a signi-ficant positive correlation between the abundance of AOB and NO₃^- of the two invasive species, indicating that the promotion effect of invasive plants on soil NO₃^- content was mainly related to the increases of AOB abundance under warming. This study is helpful for understanding the effects of day and night-warming on soil N transformation with plant invasion, and the role of AOA and AOB in soil N transformation.

Key words: invasive plant, warming, nitrogen transformation, ammonia-oxidizer