大气沉降氮中氨氮、硝氮%不同，不同植物对不同形态氮的利用不同。大量研究表明，氮沉降可促进外来植物入侵，但相关研究很少考虑氮形态的效应，对植物氮形态偏好机制的研究更少。项目假设，植物氮形态偏好与其对土壤主要形态氮的长期适应有关，植物可影响土壤微生物，进而影响土壤氮形态，反过来影响植物的氮偏好。为验证假设，项目以前期发现的偏好硝氮的外来入侵植物瘤突苍耳和偏好铵氮的本地植物苍耳为材料，测定根围土铵氮和硝氮含量及氨氧化菌和亚硝酸氧化菌的群落结构与功能，分析植物氮形态偏好、土壤主要氮形态和硝化细菌间的关系。为评估氮偏好机制的普适性，项目用15N技术研究15对入侵植物和本地植物对氮形态的偏好及其与根围土氮转化的关系；选择偏好铵氮和硝氮的入侵植物各3种，进一步研究氮形态偏好的机制。这对认识环境变化下外来植物入侵性的演化及机制，外来植物入侵对土壤氮循环和相关微生物的影响，以及植物的适应进化等均有重要意义。

The percentages of ammonium nitrogen (NH4+) and nitrate nitrogen (NO3-) are different in the nitrogen deposited from atmosphere, and the abilities to use different forms of nitrogen are different between plant species. Many studies found that nitrogen deposition facilitates invasion success of introduced plants. However, very few studies considered the effects of nitrogen forms on biological invasions, and nearly no studies considered the mechanisms underlying the preferences for nitrogen forms by exotic invasive plants. We hypothesize that nitrogen form preference by plants may be associated with their long term adaptation to the main nitrogen form in soils. Plants could alter nitrogen forms in soils by altering soil microbes, which in turn influences nitrogen preference by plants. In order to test the hypothesis, we plan to grow the invasive plant Xanthium strumarium and its native congeneric X. Sibiricum in pots, which prefer NO3- and NH4+ respectively based on our previous studies. We will measure NO3- and NH4+ contents in rhizosphere soils and community structures and functions of ammonia oxidizing bacteria (AOB) and nitrite oxidizing bacteria (NOB), and analyze the relationships among plant N form preference, N forms in soils, and those functional soil microbes. In order to evaluate the generality of the mechanisms underlying N form preference among different plants, we plan to determine NO3- and NH4+ contents in soils and N form preferences by 15 invasive plants and 15 co-occurring natives using 15N stable isotope technique, analyzing the differences in the relationships between N form preference and main soil N forms among plants. In addition, we plan to further explore the mechanisms underlying N form preference using three plants preferring NO3-and three plants preferring NH4+, in terms of N form preference, main soil N forms, AOB and NOB. The results are important for understanding the possible changes and related mechanisms in the impacts of exotic plants under climate change (N deposition) in the future, and the effects of exotic plant invasions on nitrogen cycling and related soil microbes, and adaptive evolution of invasive plants.