Abstract: Leaf stable carbon isotope composition (δ¹³C) is a pivotal proxy for evaluating plant intrinsic water-use efficiency (iWUE). While existing studies have extensively explored the relationship between leaf δ¹³C and environmental factors, the spatiotemporal distribution of leaf δ¹³C and its coupling with nutrient stoichiometry in topographically complex valley habitats remain poorly understood. In this study, we investigated the Wangjiapo Valley—a typical U-shaped glacial valley in Lushan, Jiangxi Province—along an elevational gradient from 280 m to 980 m. Leaf samples (211 in total, representing 94 dominant plant species) were collected to determine δ¹³C values and carbon (C) and nitrogen (N) contents using an elemental analyzer coupled with an isotope ratio mass spectrometer (EA-IRMS).Our results indicate that: (1) Leaf δ¹³C values ranged from −35.47‰ to −25.61‰ (mean: −30.72‰), consistent with typical C₃ plant characteristics. Mean leaf δ¹³C was significantly higher in spring (−29.98‰) than in autumn (−31.38‰), reflecting higher iWUE in spring. Spatially, leaf δ¹³C increased significantly with elevation, with a higher altitudinal lapse rate in autumn (3.17‰/km) compared to spring (1.66‰/km), suggesting that leaf δ¹³C is more sensitive to elevational changes in autumn. Furthermore, leaf δ¹³C was negatively correlated with mean annual temperature and partial pressure of CO₂, but positively correlated with mean annual precipitation; Notably, the correlations between leaf δ¹³C and environmental factors were stronger in autumn than in spring. (2) Leaf C and N contents exhibited marked seasonal variation, with higher mean values in spring (495.90 mg/g and 33.61 mg/g, respectively) compared to autumn (443.52 mg/g and 26.26 mg/g). Conversely, the mean leaf C/N ratio was lower in spring (17.56) than in autumn (19.11). When integrated across seasons, leaf δ¹³C showed a weak positive correlation with leaf C content (R² = 0.10, P ＜ 0.01), but was decoupled from leaf N content and the C/N ratio.We conclude that the cooling trend and reduced partial pressure of CO₂ at higher elevations are the primary drivers of leaf carbon isotope fractionation in this valley. The “decoupling” between leaf δ¹³C and nitrogen status suggests that under microclimatic stress in mountain valleys, plant carbon-water metabolism is primarily regulated by external physical environmental factors rather than internal nitrogen nutrient availability. This study provides new insights into plant water-use strategies and seasonal physiological responses within complex mountain topographies.