The nitrogen source utilization by Fagus crenata distributed on soils with different forms of inorganic nitrogen in a cool-temperate deciduous forest in central Japan was determined by measuring foliar delta(15)N. Two soil habitat types along a slope were delineated based on nitrogen transformation patterns, i.e., soils with high net nitrification rates and with no or low net nitrification, respectively. Despite differences in soil types, the study species, F. crenata, was distributed along the entire slope. The foliar delta(15)N value of F. crenata from the lower slope area was significantly lower than that from the upper slope. Given the finding of a previous study that the delta(15)N of NO3- was lower than that of NH4+, our results indicate that reliance on NO3- as a nitrogen source was greater in the lower slope area than in the upper slope area. Differences in the values of foliar delta(15)N were about 1 parts per thousand, which is far less than the 10 parts per thousand delta(15)N value of soil inorganic N reported in the previous study. This discrepancy might suggest that the study species utilized NO3- even in the upper site where net nitrification had not been detected. Measurements of nitrate reductase activity, an index of NO3- uptake, also supported this interpretation. Nitrate reduction occurred in leaves and roots at both the lower and the upper sites. Thus, the study species may be able to use NO3- even in soils with no net nitrification, a factor that could allow the distribution of F. crenata along the entire length of the slope.