中文摘要：

在全球大气氮沉降日益加剧的背景下，研究川西亚高山地区主要阔叶树种红桦（Betual albo-sinensis）幼苗在不同水、氮供应下的形态变化和生理机制，以了解该区主要植被的生长特性，并可为森林生态系统的恢复重建提供一定的科学依据. 采用两因素随机区组试验设计，设置5个水分梯度，即40%（W1）、50%（W2）、60%（W3）、80%（W4）、100%（W5）的土壤田间持水量（FC）和3个施氮梯度，即模拟氮沉降施加N 0（N0）、20（N1）、40（N2）g m-2 a-1的硝酸铵. 结果显示：土壤水分和氮素及二者交互作用对生长和生理特征均具有显著（P 〈 0.001）影响. 表现为：1）幼苗生长（株高和基径）、叶片碳同化特征（最大净光合速率、叶绿素含量和叶片氮含量）及根系生理特征[呼吸速率、NH4＋和NO3-净吸收速率及根系硝酸还原酶（NR）活性]均随土壤水分增加而增大，且在土壤水分为W4和W5时达到最大. 根冠比的变化呈相反趋势. 2）施氮在土壤水分不足（W1和W2）时除对基径、NO3-净吸收速率和NR活性无影响外，显著抑制了红桦株高及叶片和根系生理特征. 而在水分充足（W4和W5）时，除N2在土壤水分W4时对叶片生理特征和根系呼吸速率无影响外，对红桦的生长和根系生理特征有显著的促进作用，且N1效果更显著. 在W3时两种施氮浓度效果不同：N1对红桦的生长和相关生理特性具有促进作用而N2呈抑制作用（除对根系NH4＋和NO3-净吸收速率及NR活性无影响外）. 根冠比变化呈相反趋势. 3）根系离子（NH4＋和NO3-）净吸收速率与根系呼吸速率和叶片氮含量呈极显著正相关. 本研究表明，土壤水分和氮素供应对植物的影响存在显著的耦合效应，当土壤水分不足时施氮不利于红桦幼苗生长，而适量的施氮在水分充足时有利于生长. 对于供试土壤以及本试验设置的水、氮梯度而言，土?

英文摘要：

Soil water and nitrogen availabilities have significant effects on plant morphological and physiological characteristics, especially on the root physiological functions. The subalpine coniferous forest of western Sichuan, China, is sensitive to climate changes, including precipitation and atmospheric nitrogen deposition. In this research we exposed seedlings to five water regimes （field water holding capacity （FC） as 40% （W1）, 50% （W2）, 60% （W3）, 80% （W4） and 100% （W5）） and three nitrogen （N） regimes （with NH4NO3 content as 0 g N m-2 a-1 （N0）, 20 g N m-2 a-1 （N1） and 40 g N m-2 a-1 （N2） ）, and determined how N-fertilization affected the growth, carbon assimilation and root physiological activity of the dominant broadleaf Betual albo-sinensis seedling under different soil water conditions. The results showed the growth and physiological characteristics of Betual albo-sinensis significantly affected by water, N-fertilization and their interaction. 1） Plant growth （plant height and basal diameter）, leaf carbon assimilation characteristics （maximum net photosynthetic rate, chlorophyll and nitrogen content） and root physiological characteristics （respiration rate, NH4＋ and NO3- net influxes and nitrate reductase （NR） activity） increased with the increase of soil water content, with the highest values at W4 and W5, respectively. However the response of root/shoot ratio was opposite. 2） Except basal diameter, NO3- net influx and NR activity, nitrogen applications had a significant negative effect on plant growth, leaf carbon assimilation and root physiological characteristics when soil moisture was insufficient （W1 and W2）. Whereas nitrogen applications under the superior soil water conditions （W4 and W5） gave a remarkable promotion on growth and root physiological characteristics, except that N2 had no effect on leaf carbon assimilation characteristics or root respiration rate under the soil water condition of W4. Nitrogen applications at 60%