The nitrogen cycle in the natural ecosystem is one of the most important processes in the global climate change and biogeochimestry cycles. The nitrogen isotope is becoming one of the primary methods to trace nitrogen cycle and response to climate change. This project intend to systematically study the elemental and bulk nitrogen of the soil under the different ecosystem and climate conditions during the nitrogen cycle in the ecosystem (plant-soil-water), especially to research the soil inorganic nitrogen (nitrate and ammonium) isotope variation. Meanwhile, this project intends to discuss the soil inorganic nitrogen isotope responding to the different ecosystem types and climate conditions. In this way, the soil and plant nitrogen isotope variation mechanism during biogeochemistry cycle processes may be realized, and thoroughly understood the potential relationship between soil and plant nitrogen isotope composition with the environmental humidity (or precipitation) and the temperature variation. The primary study contents in this project are the problems that international researchers do not well understand of nitrogen cycle and nitrogen geochemical in ecosystem. By this project, we will thoroughly understand the nitrogen isotope variation under different ecosystem in the semi-arid area. In addition, this research may be provide more theory mechanisms by using nitrogen isotope to trace nitrogen cycle during biogeochemistry processes.