Effects Of Nitrogen Levels And Nitrate To Ammonium Ratios On The Invasive Plant Xanthium Strumarium And Its Native Congener X.sibiricum

Globalization is increasing the spread and diffusion of invasive plants,which brings serious consequences in society,economy and environment.Nitrogen deposition facilitates the invasion of exotic plants and reduces species diversity in local habitat.However,most studies on the effect of nitrogen deposition on invasive plants only considered nitrogen deposition level,and very few studies considered the effects of nitrogen forms on plant invasion.The responses of invasive plant Xanthium strumarium and its native congener X.sibiricum to different nitrogen levels and nitrate to ammonium ratios were investigated using hydroponic culture in this study.The main purpose of the study is to determine the preferences for inorganic nitrogen forms of these two species.The results showed that:(1)The biomass,specific leaf area,stem diameter,leaf nitrogen content,enzyme activity and photosynthetic rate of X.strumarium under nitrate to ammonium ratio of 9 : 1 were significantly higher than those under nitrate to ammonium ratio of 1 : 9 at the same nitrogen level in monochlture.Beside the activities of the root and leaf nitrate reductase activitiy,the variations in X.sibiricum showed an opposite response.Compared with nitrate to ammonium ratio of 1 : 9,under nitrate to ammonium ratio of 9 : 1 were significantly increasing the interspecific difference of biomass,specific leaf area,stem diameter and photosynthetic rate of two plants,which showed that X.strumarium preferred nitrate and X.sibiricum preferred ammonium nitrogen.(2)The biomass,stem diameter,leaf nitrogen content,chlorophyll content,photosynthetic rate and glutamine synthetase activity of X.strumarium and X.sibiricum under high nitrogen level were significantly higher than those under low nitrogen level at the same nitrogen ratio in monochlture.The biomass,specific leaf area,stem diameter,leaf nitrogen content,photosynthetic rate and glutamine synthetase activity of X.sibiricum under high nitrogen level were significantly higher than those under low nitrogen level in nitrate to ammonium ratio of 1 : 9.While the effect of nitrogen level on the stem diameter,leaf nitrogen content,photosynthetic rate and nitrate reductase activities of X.strumarium was significantly different,and the other characters had no obvious influence.the nitrogen level significantly increasing the interspecific difference of biomass,stem diameter,chlorophyll content and photosynthetic rate of two plants under nitrate to ammonium ratio of 1 : 9,which showed that X.sibiricum preferred ammonium nitrogen.the nitrogen level significantly increasing the interspecific difference of biomass,stem diameter,chlorophyll content and photosynthetic rate of two plants under nitrate to ammonium ratio of 9 : 1,which showed that X.strumarium preferred nitrate.(3)the values of 7 growth traits of X.strumarium under nitrate to ammonium ratio of 9 : 1 were significantly higher than those of X.sibiricum.the values at high nitrogen level were significantly higher than those at low nitrogen level.the biomass and stem diameter of X.strumarium were significantly higher than those of X.sibiricum under nitrate to ammonium ratio of 1 : 9,No matter the nitrate to ammonium ratio of 9 : 1 or 1 : 9,enhance nitrogen level obviously increasing the competitive advantage of X.sibiricum,only this advantage is more obvious in nitrate to ammonium ratio of 9 : 1.Which proves that high nitrate to ammonium ratio and high nitrogen level are more favorable to the invasion of X.strumarium.Above all,X.strumarium and X.sibiricum can be used in both forms of nitrogen,the X.strumarium more prefer to nitrate,X.sibiricum more prefer to ammonium,which may be related to the different absorption mechanisms of nitrogen in X.strumarium and X.sibiricum.the X.strumarium may contain more nitrate transport protein,X.sibiricum may contain more ammonium transport protein.The high of nitrate reductase activity of X.strumarium can increase the leaf nitrogen content and specific leaf area,make further to increase photosynthetic rate of X.strumarium and accelerate growth.