Effects Of Invasion Degree On Of Xanthium Stumarium On Native X. Sibiricum,Soil Nitrogen Transformation And Nitrifying Bacteria

With entering a new era,the problem of alien plant invasion has been becoming more and more serious,posing a lot of damage to economics and natural environment.Nitrogen is an indispensable nutrient element for plant growth.The interactions between invasive alien plants and soil nitrogen transforming microorganisms plays important roles in the process of alien plant invasions.Previous studies in our laboratory showed that the aqueous extract of the whole plant of Xanthium strumarium,an invasive alien plant that prefers nitrate relative to ammonium nitrogen,can significantly promote the rate of soil nitrification and increase the content of soil nitrate nitrogen.However,the aqueous extract of the native plant X.sibiricum cannot,which prefers ammonium nitrogen.These results indicate that the invasion of X.strumarium may affect soil nitrogen transformation and nitrogen forms,and then contribute to its invasiveness.The more serious the invasion degree is,the greater the impact is.In order to explore the effects of invasive alien plants with different degrees of invasion on growth of co-occurring native plants,soil nitrogen transformation and forms,and related microorganisms X.strumarium and X.sibiricun were grow in monoculture and mixed cultures in pots under two levels of nutrients.Biomass and biomass allocation,and characteristics of rhizosphere soils such as nitrogen transformation,nitrogen forms,and the diversity and community structure of ammonia oxidizing bacteria were determined for the two species.We also analyzed the relationships among invasion degrees of the invader,soil nitrogen transformation and soil nitrifying bacteria.The main conclusions are as follows:Regardless of cultivation methods and soil nutrients,the aboveground biomass,underground biomass,total biomass,and stem biomass fraction of X.strumarium were significantly higher than those of X.sibiricum.Regardless of soil nutrients,with the increase of invasion intensity of X.strumarium,the aboveground biomass and total biomass of each Xanthium species decreased significantly,and the fruit biomass of X.sibiricum also decreased significantly.The increase of soil nutrient content significantly increased the aboveground biomass,underground biomass and total biomass of X.strumarium and X.sibiricum,and the fruit biomass of X.sibiricum and stem biomass fraction of X.strumarium,but significantly reduced the root biomass fraction of X.strumarium.Regardless of soil nutrients,when grown in monoculture the urease activity of X.strumarium was significantly higher than that of X.sibiricum,while the nitrate nitrogen content in rhizosphere soil of X.strumarium was significantly lower than that of X.sibiricum.Regardless of soil nutrients,with the increase of invasion intensity of X.strumarium,the inhibitory effect of X.strumarium on X.sibiricum increased,and the net nitrification rate in rhizosphere soil of X.sibiricum showed a downward trend.With the increase of soil nutrients,the content of ammonium nitrogen and net nitrification rate increased significantly in rhizosphere soil from the two Xanthium species.With the increase of soil nutrients,the diversity of ammonia oxidation bacteria(AOB)and the ammonia oxidation potential,decreased significantly in rhizosphere soils of the two Xanthium species,while the nitrification potential increased significantly.The AOB community structure was also influenced significantly by soil nutrients,but that of ammonia oxidation archaea(AOA)was not.In our study,AOB dominated the nitrification process in rhizosphere soil,and its dominant role did not alter with the change of soil nutrients levels and invasion degrees of X.strumarium.In conclusion,with the increase of invasion intensity of X.strumarium the growth and reproduction of X.sibiricun were inhibited more seriously.Xanthium strumarium gradually excluded X.sibiricun through competition and occupied the ecological niche of X.sibiricun.Xanthium strumarium has a stronger ability to absorb nitrate nitrogen.The invasion of X.strumarium significantly altered the diversity and structure of soil ammonia oxidizing bacteria community in the invaded area,creating a soil environment beneficial to its own growth,which could promotes its further invasion.Our study is helpful to reveal the relationship between plant invasion and soil nitrifying bacteria,and provides evidence for revealing the plant-soil feedback mechanism,and the scientific basis for control of X.strumarium.