Nutrient acquisition strategies of tree species determine their competitive ability under different nutrient regimes. The process for absorbing soil nitrogen (N) can be estimated by the N stable isotope ratio of leaves (15N:14N, expressed as δ15N), which is determined by different N sources and isotope fractionation in the absorption process. In this study, we examined leaf δ15N of 23 tree species which grew in four forests of the subtropical, warm-temperate and cool temperate zones to understand the absorption process of tree species with different types of microbial symbiosis, i.e., ectomycorrhizal (ECM), arbuscular mycorrhizal (AM) and root nodule bacteria (RNB). In contrast to global patterns, where ECM plants averaged 1‰ lower than AM plants, leaf δ15N of tree species forming ECM symbioses averaged 1.7‰ higher than those forming AM symbioses. Plants with N-fixing symbioses were 0.9‰ higher than plants with ECM symbioses. However, genus-level variability within AM plants was greater than variability among other plant symbioses. The leaf δ15N of these species may be influenced more strongly by the N form acquired or the depth of N acquisition than by 15N fractionation by ECM fungi. The leaf δ15N of tree species associated with RNB was closer to zero than other species, but negative. We thus suggest that leaf δ15N of tree species with RNB was determined mainly by nitrogen fixation although it may reflect the isotope fractionation by RNB.
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