Nitrogen (N) dynamics in terrestrial ecosystems are quite complex because of the high spatial and temporal variabilities together with its high turnover due to the high biological N demand by plants and microbes. To elucidate the N dynamics in intact terrestrial ecosystems, the use of the natural abundance of stable N and oxygen (O) isotopes has been carried out in many ways. The recent methodological progress (Sigman et al. 2001; Casciotti et al. 2002) now can allow us to measure not only N but also O isotope ratios with samples with quite tiny amounts (e.g. 20nmol-N). We applied this analytical method to elucidate the fate of nitrate. Among many fates of nitrate in terrestrial ecosystems, plant nitrate uptake is quite difficult to evaluate. In this talk, I will introduce our latest results on the uptake of the soil nitrate by tundra plants where soil net nitrification rates are quite low with N and O isotopes.