Plant d<SUP>15</SUP>N correlates with the transpiration efficiency of nitrogen acquisition in tropical trees

Abstract

Based upon considerations of a theoretical model of 15N/14N fractionation during steady-state nitrate uptake from soil (Comstock 2001, Planta 214:220-234), we hypothesized that, for plants grown in a common soil environment, whole plant {delta}15N ({delta}P) should vary as a function of the transpiration efficiency of nitrogen acquisition (FN/v) and the difference between whole-plant {delta}15N and root {delta}15N ({delta}P-{delta}R). We tested these hypotheses with measurements of several tropical tree and liana species. Consistent with theoretical expectations, both FN/v and {delta}P-{delta}R were significant sources of variation in {delta}P, and the relationship between {delta}P and FN/v differed between non-N2-fixing and N2-fixing species. We interpret the correlation between {delta}P 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 {delta}15N of terrestrial plants, and have implications for understanding nitrogen cycling in ecosystems.