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

Based upon considerations of a theoretical model of N/N fractionation during steady-state nitrate uptake from soil, we hypothesized that, for plants grown in a common soil environment, whole-plant dN (dP) should vary as a function of the transpiration efficiency of nitrogen acquisition (FN/v) and the difference between dP and root d N (dP 2 dR). We tested these hypotheses with measurements of several tropical tree and liana species. Consistent with theoretical expectations, both FN/v and dP 2 dR were significant sources of variation in dP, and the relationship between dP and FN/v differed between non-N2-fixing and N2-fixing species. We interpret the correlation between dP and 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 dN of terrestrial plants and have implications for understanding nitrogen cycling in ecosystems.