Tight coupling between leaf δ¹³C and N content along leaf ageing in the N₂‐fixing legume tree black locust (Robinia pseudoacacia L.)

N 2 ‐fixing legumes can strongly affect ecosystem functions by supplying nitrogen (N) and improving the carbon‐fixing capacity of vegetation. Still, the question of how their leaf‐level N status and carbon metabolism are coordinated along leaf ageing remains unexplored. Leaf tissue carbon isotopic composition (δ 13 C) provides a useful indicator of time‐integrated intrinsic water use efficiency (WUEi). Here, we quantified the seasonal changes of leaf δ 13 C, N content on a mass and area basis (N mass , N area , respectively), Δ 18 O (leaf 18 O enrichment above source water, a proxy of time‐integrated stomatal conductance) and morphological traits in an emblematic N 2 ‐fixing legume tree, the black locust ( Robinia pseudoacacia L.), at a subtropical site in Southwest China. We also measured xylem, soil and rainwater isotopes (δ 18 O, δ 2 H) to characterize tree water uptake patterns. Xylem water isotopic data reveal that black locust primarily used shallow soil water in this humid habitat. Black locust exhibited a decreasing δ 13 C along leaf ageing, which was largely driven by decreasing leaf N mass , despite roughly constant N area . In contrast, the decreasing δ 13 C along leaf ageing was largely uncoupled from parallel increases in Δ 18 O and leaf thickness. Leaf N content is used as a proxy of leaf photosynthetic capacity; thus, it plays a key role in determining the seasonality in δ 13 C, whereas the roles of stomatal conductance and leaf morphology are minor. Black locust leaves can effectively adjust to changing environmental conditions along leaf ageing through LMA increases and moderate stomatal conductance reduction while maintaining constant N area to optimize photosynthesis and carbon assimilation, despite declining leaf N mass and δ 13 C.