Plant delta 15N correlates with the transpiration efficiency of nitrogen acquisition in tropical trees

Based upon considerations of a theoretical model of 15N/14N fractionation during steady-state nitrate uptake from soil, we hypothesized that, for plants grown in a common soil environment, whole-plant ?15N(?P) should vary as a function of the transpiration efficiency of nitrogen acquisition (FN/v) and the difference between ?Pand root ?15N (?P- ?R). We tested these hypotheses with measurements of several tropical tree and liana species. Consistent with theoretical expectations, both FN/v and ?P-?R were significant sources of variation in ?P, and the relationship between ?Pand FN/v differed between non-N2-fixing and N2-fixing species. We interpret the correlation between ?Pand FN/v as resulting from variation in mineral nitrogen efflux-to-influx ratios across plasma membranes of root cells. These results provide a simple explanation of variation in ?15N of terrestrial plants and have implications for understanding nitrogen cycling in ecosystems. � 2009 American Society of Plant Biologists.