Differences In Preferential Uptake Of Nitrate And Ammonium Nitrogen Between Seven Pairs Of Invasive And Native Plants

Exotic plant invasion has become one of the major problems threatening economic development and ecological environment.Studies on the mechanisms underlying exotic plant invasions can provide theoretical bases for controlling invasive plants.Nitrogen is an essential nutrient for plant growth and development,and also affects invasion success of exotic plants.Many studies have determined the effects of soil nitrogen levels on invasiveness on exotic plants.However,few studies determined the differences in preference for different nitrogen forms between invasive and native plant species.Thus,we still do not know clearly the effects of plant strategies for using different nitrogen forms on invasion success of exotic plants.To address these problems,we compared seven invasive plant species with their co-occurring phylogenetically related invasive plants using pot experiment in a common garden.At the same growth stage,we compared the invasive and native plants in terms of characteristics of soil nitrogen forms,plant uptakes and preferences for different nitrogen forms,dominant nitrogen sources for plants,and the activities of nitrogen assimilation enzymes.We want to know whether the invasive plants are different in these parameters with the natives,and the effects of the characteristics of soil nitrogen forms and the activities of the enzymes on plant utilization of different nitrogen forms.The main conclusions are as follows:?1?The dominant nitrogen form in the soil was ammonium nitrogen for all the 14 studied species.The average of soil nitrate concentration was 2.60 ?g g^-1;The average of soil ammonium concentration was 11.73 ?g g^-1;The average of ammonium/nitrate ratio was 0.25.?2?The dominant nitrogen source was ammonium nitrogen for eight species,and nitrate nitrogen for three species.For the other three species,however,ammonium and nitrate nitrogen were utilized similarly.For the 14 species during the experimental treatment,the average amount of nitrate nitrogen absorbed was 74.99 ?g,and average absorption rate of nitrate nitrogen was 94.25 ?g g^-1 h^-1,The average amount of ammonium nitrogen absorbed was 135.34 ?g,and average absorption rate of ammonium nitrogen was 81.03 ?g g^-1h^-1,The mean of nitrate-N/ammonium-N uptake ratio was 1.65;the mean of the ratio of nitrate-N to ammonium-N uptake rates was 1.51.?3?Nitrate nitrogen was preferentially uptaken by all studied species except the native plant Xanthium sibiricum,who preferred for ammonium nitrogen.Based on the amount of different forms of nitrogen absorbed,?an index,positively correlatd with preferential extent?,with the mean of 0.23.Based on the absorption rates of different forms of nitrogen,the average extent of preference for nitrate nitrogen was 0.25.?4?For the 14 species,the mean of leaf nitrogen concentration was of 2.07%;the mean of root nitrogen concentration was 1.72%;the mean of total plant nitrogen concentration 1.34%.The average of ratio of leaf nitrogen concentration to root nitrogen concentration was 1.29.?5?For the 14 species,the mean of root nitrate reductase activity was 2.14 ?mol h^-1 g^-1;the mean of leaf nitrate reductase activity was 1.83 ?mol h^-1 g^-1,the mean of root glutamine synthetase activity was 7.03 ?mol h^-1 g^-1;the mean of leaf glutamine synthetase activity was 10.88 ?mol h^-1 g^-1.?6?The overall differences were not significant between the seven invasive and the seven native plants in all measured parameters except leaf nitrate reductase activity,in which the invasive plants were significantly lower than the natives.?7?For the 14 studied species,root nitrate reductase activities were significantly correlated with the amounts or rates of nitrate nitrogen absorption,the extent of nitrate nitrogen preference,and contribution percentages to plant nitrogen assimilation,respectively.The characteristics of soil nitrogen forms also influenced plant absorption of different nitrogen forms,dominant nitrogen source,and nitrogen form preference.With the increase of the content or percentage in total inorganic nitrogen of a given form of inorganic nitrogen in soil,plant absorption of this nitrogen form,its contribution percentage to plant nitrogen assimilation,and the extent of preference for it increased linearly;and vice versa.Plant nitrogen form preference may be associated with the long-term adaptation to soil nitrogen forms.Nitrogen form preference may help exotic plants to invade habitats with nitrogen forms for which they prefer.