| Atmospheric nitrogen deposition is increasing due to environmental changes such as global warming and air pollution.High nitrogen input changes nitrogen state and cycling process in forest ecosystem.The xylem is the main channel through which trees transport water,nutrients and metabolites from their roots to leaves.Studying the changes of nitrogen compositions and concentrations in xylem sap is the key infor-mation to analyze the nitrogen utilization process of forest trees,which can improve the diagnostic evaluation of forest physiological growth status and how to respond to envi-ronmental changes.Based on this,this study selected the Pinus tabuliformis Carr.trees as the research object in the beijing-tianjin-hebei region,discussed the collection method of the trunk xylem sap of adult trees under in situ conditions,and analyzed the diurnal changes of nitrogen compositions and stable nitrogen isotopes in the trunk xy-lem sap and its axial transport characteristics.The results show:（1）There was no significant difference in the concentration of chemical composi-tions in the xylem sap of Pinus tabuliformis Carr.trees collected by centrifugation method and water displacement method.The concentration of potassium ions（potas-sium ions are related to cell permeability）was used as the test index to verify that the concentration of potassium ions in xylem sap polluted by centrifugation method was negligible.It is proved that the centrifugation method is feasible to extract xylem sap,which provides a very practical method for studying nitrogen transport and distribution in the trunk of adult trees under situ conditions.（2）At the end of the growing season,the nitrogen components in the trunk xylem sap were mainly DON,followed by NO3-and NH4+.The trunk xylem sap of juvenile trees carries more inorganic nitrogen（mainly NO3-）than adult trees.In addition,the composition of nitrogen in the trunk xylem sap and its 15N showed obvious diurnal changes,and TN concentration reached its maximum value at noon;DON concentra-tion peaked in the afternoon;NO3-concentration is at its minimum in the afternoon.This may be due to the xylem sap nitrogen dilution and concentration effect caused by transpiration and the coordination among tissues and organs of trees to meet their own nitrogen needs.（3）There were significant changes in the trunk xylem sap nitrogen concentration and 15N axial transport of Pinus tabuliformis Carr.trees.The xylem sap 15N decreases and then increases during transport from the root to the canopy,and was positively correlated with the phloem sap 15N,indicating that the xylem sap in the trunk had a close exchange with the phloem sap during the xylem sap transport.The relationship between xylem sap and other tissues（soil,roots and leaves）in nitrogen concentration and stable isotope composition was also discussed to provide useful information on nitrogen transport and utilization in forest trees. |
