中文摘要：

N沉降对不同森林生态系统的影响是当今全球变化生态学研究的一个热点问题。山地湿性常绿阔叶林是我国西部高海拔地区重要的森林植被类型之一。该文以云南哀牢山中山湿性常绿阔叶林为对象,研究了其林下优势树种多花山矾（Symplocos ramosissima）和黄心树（Machilus gamblei）幼苗不同器官中C、N、P含量和生态化学计量特征及其对N沉降增加的响应。结果表明：两种幼苗C、N、P含量的差异均达到了显著性水平（p〈0.05）,多花山矾的C含量较低,N和P含量较高。N处理对植物幼苗元素含量及其比值影响极显著（p〈0.01）,且与物种和器官之间存在显著的交互作用。N处理提高了幼苗体内N含量,导致不同器官N：P值有不同程度的增加。随N处理水平的升高,多花山矾幼苗P含量下降,黄心树幼苗P含量整体升高,幼苗间P含量差异减小。在一定范围内,植物幼苗N含量与土壤无机N含量之间存在极显著的相关性（p〈0.01）。不同器官之间相比,植物幼苗根和茎的N内稳性比叶片更高,即植物叶片对N沉降的响应更为敏感。

英文摘要：

Aims Montane moist evergreen broad-leaved forest is an important vegetation type in the high altitude areas of western China. In this study, total carbon（C）, nitrogen（N）, phosphorus（P） contents and stoichiometry in roots, stems, and leaves of two dominant seedlings, Symplocos ramosissima and Machilus gamblei, and their responses to different levels of N addition were investigated in the montane moist evergreen broad-leaved forest in Ailao Mountains, Yunnan. Methods A simulation experiment with four N addition levels T0（0 kg N·hm–2·a–1）, T21（3 kg N·hm–·a–1）, T2（6 kg N·hm–2·a–1） and T3（12 kg N·hm–2·a–1） was carried out in the montane moist evergreen broad-leaved forest in Ailao Mountains. Total C, N and P concentrations in different organs of the two dominant seedlings and soil inorganic N concentration in each treatment were measured after one year＇s in situ experiment. Important findings The C, N and P concentrations of the two seedlings were significantly different（p 0.05）. Machilus gamblei had lower C concentration, but higher N and P concentrations compared with S. ramosissima. N addition had significant effects（p 0.01） on C, N and P concentrations and their stoichiometry. Significant interactions were detected among N treatments, species and plant organs. N addition increased N concentrations in all organs of the two seedlings, leading to higher ratio of N：P. P concentration of S. ramosissima decreasedsignificantly（p 0.05） under N addition, while that of M. gamblei increased under medium（T2） and high（T3） N addition treatments. Within a certain range, there was a significant correlation between the N concentrations of seedlings and soil inorganic N concentrations（p 0.01）. Comparisons of homeostasis index among different organs indicated that the N stoichiometry in roots and stems was more stable than that in leaves under N addition.