Using robust, pairwise comparisons and a global dataset, we show that nitrogen concentration per unit leaf mass for nitrogen-fixing plants (N(2)FP; mainly legumes plus some actinorhizal species) in nonagricultural ecosystems is universally greater (43–100%) than that for other plants (OP). This difference is maintained across Koppen climate zones and growth forms and strongest in the wet tropics and within deciduous angiosperms. N(2)FP mostly show a similar advantage over OP in nitrogen per leaf area (N(area)), even in arid climates, despite diazotrophy being sensitive to drought. We also show that, for most N(2)FP, carbon fixation by photosynthesis (A(sat)) and stomatal conductance (g(s)) are not related to N(area)—in distinct challenge to current theories that place the leaf nitrogen–A(sat) relationship at the center of explanations of plant fitness and competitive ability. Among N(2)FP, only forbs displayed an N(area)–g(s) relationship similar to that for OP, whereas intrinsic water use efficiency (WUE(i); A(sat)/g(s)) was positively related to N(area) for woody N(2)FP. Enhanced foliar nitrogen (relative to OP) contributes strongly to other evolutionarily advantageous attributes of legumes, such as seed nitrogen and herbivore defense. These alternate explanations of clear differences in leaf N between N(2)FP and OP have significant implications (e.g., for global models of carbon fluxes based on relationships between leaf N and A(sat)). Combined, greater WUE and leaf nitrogen—in a variety of forms—enhance fitness and survival of genomes of N(2)FP, particularly in arid and semiarid climates
