Plant δ15N Correlates with the Transpiration Efficiency of Nitrogen Acquisition in Tropical Trees

Based upon considerations of a theoretical model of N-15/N-14 fractionation during steady-state nitrate uptake from soil, we hypothesized that, for plants grown in a common soil environment, whole-plant delta N-15 (delta(P)) should vary as a function of the transpiration efficiency of nitrogen acquisition (F-N/nu) and the difference between delta(P) and root delta N-15 (delta(P) - delta(R)). We tested these hypotheses with measurements of several tropical tree and liana species. Consistent with theoretical expectations, both F-N/v and delta(P) - delta(R) were significant sources of variation in delta(P), and the relationship between delta(P) and F-N/v differed between non-N-2-fixing and N-2-fixing species. We interpret the correlation between delta(P) and F-N/nu 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 delta N-15 of terrestrial plants and have implications for understanding nitrogen cycling in ecosystems.