The interaction between nitrogen and phosphorous is a strong predictor of intra-plant variation in nitrogen isotope composition in a desert species

Abstract. Understanding intra-plant variations in δ¹⁵N, which can be large, is essential for fully utilizing the role of δ¹⁵N as an integrator of the terrestrial nitrogen cycle. Studying such variations can yield insights into nitrogen metabolisms by the plant as a whole or by specific plant organs. However, systematical evaluation of intra-plant variations in δ¹⁵N and their relationship with organ nutrient contents is rare. We excavated whole plant architectures of Nitraria tangutorum Bobrov, a C₃ species that has an exceptional capability of fixing sands and building sand dunes, in two deserts in northwestern China. We systematically and simultaneously measured nitrogen isotope ratios and nitrogen and phosphorous contents of different parts of the excavated plants. We found that intra-plant variations in δ¹⁵N of N. tangutorum were positively correlated with corresponding organ nitrogen (N) and phosphorous (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 δ¹⁵N. We hypothesized that this strong positive intra-plant δ¹⁵N–N/P relationship was caused by fractionating gaseous N losses (volatilization) from plants and that the volatilization depended on the interaction between organ N and P contents. We also showed that root δ¹⁵N increased with depth into soil. This pattern was similar to profiles of soil δ¹⁵N reported by previous studies in different ecosystems although the exact relationship between root and soil profiles in δ¹⁵N was not clear. Studies of intra-plant variations in δ¹⁵N in different species, ecosystems, and climates and measurements of plant nitrogen volatilization and associated isotope fractionation are needed in order to determine the exact mechanisms responsible for the significant patterns first reported in this study.