Legumes are different: Leaf nitrogen, photosynthesis, and water use efficiency

Significance Leaf traits are used to drive models of global carbon fluxes and understand plant evolution. Many syntheses have highlighted relationships between plant leaf nitrogen and photosynthesis as evidence of a strong evolutionary drive to “intercept light and capture CO2.” Different from previous studies, we compiled a global dataset constrained to sites and studies where nitrogen-fixing plants (N2FP) and nonfixing species [other plants (OP)] could be directly compared. We show that photosynthesis is not related to leaf nitrogen for N2FP, irrespective of climate or growth form. N2FP have clear advantages in water use efficiency over OP. These findings contribute to a more complete explanation of global distributions of N2FP and can help improve models of global carbon and nitrogen cycles. Using robust, pairwise comparisons and a global dataset, we show that nitrogen concentration per unit leaf mass for nitrogen-fixing plants (N2FP; 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. N2FP mostly show a similar advantage over OP in nitrogen per leaf area (Narea), even in arid climates, despite diazotrophy being sensitive to drought. We also show that, for most N2FP, carbon fixation by photosynthesis (Asat) and stomatal conductance (gs) are not related to Narea—in distinct challenge to current theories that place the leaf nitrogen–Asat relationship at the center of explanations of plant fitness and competitive ability. Among N2FP, only forbs displayed an Narea–gs relationship similar to that for OP, whereas intrinsic water use efficiency (WUEi; Asat/gs) was positively related to Narea for woody N2FP. 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 N2FP and OP have significant implications (e.g., for global models of carbon fluxes based on relationships between leaf N and Asat). Combined, greater WUE and leaf nitrogen—in a variety of forms—enhance fitness and survival of genomes of N2FP, particularly in arid and semiarid climates.

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