Two-year dynamics of foliage labelling in 8-year-old Pinus pinaster trees with N-15, Mg-26 and Ca-42-simulation of Ca transport in xylem using an upscaling approach

Introduction Atmospheric deposition is an important input of major nutrients into forest ecosystems. The long-term goal of this work was to apply stable isotope methodology to assess atmospheric nutrient deposition in forest systems. Materials and methods A labelling experiment of foliage with stable isotopes of primary and secondary macro nutrients (N-15, Mg-26 and Ca-42 injected into the stem sapwood) was carried on standing trees to monitor interactions between canopy and precipitations. N-15 rapidly reached the foliage; however, Mg and Ca were not detected in foliage until more than a year after injection. Results and discussion The delay in mobilization of Mg and Ca prevented us from accurately modelling deposition contributions of these two elements. Nonetheless, an upscaling approach based on published results on Ca transport in shoots xylem was used to simulate our results. These simulations of Ca transport at the tree scale were consistent with our experimental data. Conclusion This consistency suggested that mechanisms of nutrient transport are the same at the different scales. Nitrogen was rapidly transported in the xylem to foliage, probably mainly by mass flow. Conversely, transport of Mg and particularly Ca was considerably delayed, probably due to successive cation exchanges along the xylem vessels.