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Understanding intra-plant variations in <i>δ</i>¹⁵N is essential for fully utilizing the potential of <i>δ</i>¹⁵N as an integrator of the terrestrial nitrogen (N) cycle and as an indicator of the relative limitation of N and phosphorous (P) on plant growth. Studying such variations can also yield insights into N metabolism by plant as a whole or by specific organs. However, few researchers have systematically evaluated intra-plant variations in <i>δ</i>¹⁵N and their relationships with organ nutrient contents. We excavated whole plant architectures of <i>Nitraria tangutorum</i> Bobrov, a C₃ species of vital regional ecological importance, in two deserts in northwestern China. We systematically and simultaneously measured N isotope ratios and N and P contents of different parts of the excavated plants. We found that intra-plant variations in <i>δ</i>¹⁵N of <i>N. tangutorum</i> were positively correlated with corresponding organ N and P contents. However, it was the N × P interaction, not N and P individually or their linear combination, that was the strongest predictor of intra-plant <i>δ</i>¹⁵N. Additionally, we showed that root <i>δ</i>¹⁵N increased with depth into soil, a pattern similar to profiles of soil <i>δ</i>¹⁵N reported by previous studies in different ecosystems. We hypothesized that the strong positive intra-plant <i>δ</i>¹⁵N–N and P relationships are caused by three processes acting in conjunction: (1) N and P content-driven fractionating exchanges of ammonia between leaves and the atmosphere (volatilization) during photorespiration, (2) resorption and remobilization of N and P from senescing leaves, and (3) mixture of the re-translocated foliar N and P with existing pools in stems and roots. To test our hypothesis, future studies should investigate plant N volatilization and associated isotope fractionation and intra-plant variations in <i>δ</i>¹⁵N in different species across ecosystems and climates.