森林植物树干液流氮素化学和稳定同位素组成研究

Stem is the major conduit for the transport of nitrogen among tissues of higher plants. Examinations on chemical composition and stable isotope of nitrogen compounds of xylem sap can provide key information for elucidating plant nitrogen utilization strategies and their responses to environmental changes. Therefore, this proposal, by combining methods and theories of stable isotope, forest ecology and plant physiology, will be conducted in coniferous plantation and deciduous broadleaved forest in Beijing-Tianjin-Hebei region, North China. The chemical composition and isotopic composition of main nitrogen compounds (e.g., nitrate, ammonium and dissolved organic nitrogen) of xylem sap, in addition to their characteristics in different stem positions, forest types, diurnal and seasonal variations, will be analyzed in the dominant tree species. Meanwhile, this proposal will also analyze the sap flow density, concentrations of main anions and cations, stand structures (e.g., leaf area index, sapwood area and diameter at breast height) and environmental factors (e.g., soil nitrogen concentration, solar radiation and vapor pressure deficit), to examine the correlations between them and the chemical composition and isotopic composition of xylem sap and elucidate the flux characteristics of xylem transporting nitrogen in coupling with tree transpiration. This proposal focuses on in situ nitrogen biogeochemical behaviors of xylem sap for elucidating plant root nitrogen utilization strategies in forest ecosystems, which can deepen the application of nitrogen stable isotope for plant physiological ecology, and also provide scientific evidences for accurately evaluating forest ecological functions and their responses to environmental nitrogen pollution.

树干是植物根-叶片之间的氮素运输通道。研究树干木质部液流氮素化学和稳定同位素组成是解译植物氮利用策略及其如何响应环境变化的关键信息。本项目结合稳定同位素、森林生态学和植物生理学等方法原理，选取京津冀地区落叶阔叶林和人工针叶林，分析主要树种木质部液流主要氮组分（硝酸根、铵根和溶解有机氮）浓度和同位素组成，以及它们随树干位点、树种、昼夜和月际变化的特征。同时，分析木质部液流流速、主要阴阳离子浓度、林分结构（叶面积指数、边材面积、胸高直径等）和环境因子（土壤氮浓度、光照强度、饱和水汽压差等）及其与液流氮素浓度和同位素组成的相关关系，详细探讨木质部液流氮素浓度和同位素组成变化的生理生态学机制，揭示植物根系氮还原和同化的变化过程。本项研究聚焦建立树干木质部液流氮素浓度和同位素新指标的方法原理，用于解译森林植物原位的氮素利用策略，为评价森林生态功能及其如何响应环境氮污染提供科学依据。

树干是植物根-叶片之间的氮素运输通道。研究树干木质部液流氮素地球化学行为特征是解译植物氮利用策略及其响应环境变化的关键信息。然而，这方面研究主要基于模式植物（如蓖麻）或树木幼苗的室内培养实验，关于森林生态系统植物氮运输过程机制亟待进一步阐明。本项目选取华北温带森林典型树种（天然落叶阔叶林和人工针叶林）为研究对象，详细分析了木质部液流氮素化学组成、浓度和稳定同位素的变化特征及其影响因素，得到以下主要结论：1）对比分析了基于离心法和水置换法树干木质部液流化学组分浓度的差异性，并以钾离子浓度（钾离子与细胞渗透性有关）为检验指标，验证了离心法污染木质部液流的程度可忽略，为原位条件下研究成年树树干氮运输和分配过程提供了一个非常实用的方法。2）溶解有机氮是华北人工油松林树干木质部液流氮素的主要成分，其中谷氨酰胺是溶解有机氮的主要组分，其次是精氨酸和天冬氨酸；木质部液流不同氮素组分浓度具有不同的昼夜节律现象，这可能与树木蒸腾引起木质部液流氮素稀释-浓缩效应以及树木自身氮需求有关。3）在树干木质部氮素运输过程中，树干底部和树枝木质部液流氮素同位素组成较高，树干中部较低，且木质部液流与韧皮部液流总氮浓度或氮素同位素组成的显著性相关，这说木质部液流氮素长距离运输过程中会发生储存和代谢行为，并与韧皮部液流交换密切。本项目建立的原位、无扰动下基于离心法提取树干木质部液流的方法体系，使原位解译森林植物氮运输和分配策略成为现实，能够提高对森林生理生长状态及其如何响应环境变化的诊断评估，对植被、大气和陆地水文系统之间的氮动态研究带来新的见解。