中文摘要：

氮沉降已经成为全球变化背景下的热点问题，并呈现逐渐加重趋势，了解森林生态系统对这种持续氮增长和快速氮循环的响应模式及反馈机制，对于维护森林生态系统健康具有重要的理论意义。本研究选择不同林龄杨树人工林作为试验样地，设置N0（0gN·m^-2·a^-1）、N1（5gN·m^-2·a^-1）、N2（10gN·m^-2·a^-1）、N3（15gN·m^-2·a^-1）、N4（30gN·m^-2·a^-1）5个不同浓度，进行氮沉降野外模拟实验，探讨不同林龄杨树人工林细根生物量的垂直分布及对模拟氮沉降的响应。结果表明：（1）70％-80％细根生物量分配在0-20cm土层，呈现表层富集特征；外源氮增加后，幼龄林（4年生）中，0—10cm土层细根生物量所占比例有所增加，而中龄林（8年生）和成熟林（15年生）则不同程度的减少；（2）细根生物量主要分布在0～0．5和0．5～1．0mm径级，其中0—0．5mm径级细根约占总细根（〈2．0mm）生物量的50％，外源氮输入增加极小径级（0～0．5mm）的根系生物量，特别是幼龄林；（3）30～40cm土层中，成熟林0～0．5mm细根生物量分配量远大于幼龄林和中龄林，表明随着林龄的增加，小直径细根有向下分配趋势；（4）林龄、土层、径级以及施氮浓度4个因素的综合效应能够解释细根生物量66．3％的变异，其中林龄、土层、径级3个因素各自对细根生物量的影响极显著（P〈0．01），分别能解释细根生物量17．6％、16．1％、10．4％的变异，而增氮处理仅能解释细根生物量0．24％的变异，影响效应不显著（P〉0．05）。

英文摘要：

Nitrogen deposition is a hot topic issue of global change. Understanding the response patterns and feedback mechanisms of forest ecosystems associated with this continued, increased nitrogen deposition and rapid nitrogen cycle is vital for the maintenance of forest ecosystem health and productivity. Increasing nitrogen deposition may alter the distribution structure of fine roots, such as vertical distribution and diameter size distribution formed in the long-term evolution process. We conducted an experiment to examine the allocation of fine root biomass and its re- sponse to nitrogen deposition in poplar plantations with different stand ages in the coastal plain of Dongtai, Jiangsu. The randomized block design was used with five nitrogen deposition concentra- tion treatments, i.e. N0（0gN·m^-2·a^-1）、N1（5gN·m^-2·a^-1）、N2（10gN·m^-2·a^-1）、N3（15gN·m^-2·a^-1）、N4（30gN·m^-2·a^-1） in the field. The results showed that： （1） The fine root biomass was obviously enriched in the surface soil, with 70% -80% of fine root bio- mass allocated to the 0-20 cm soil layer. With the increase of exogenous nitrogen, the proportion of fine root biomass at the 0-10 cm soil layer increased in the young plantation （4 years old） ,while reduced in the mid-aged and mature plantations （8 and 15 years old, respectively） to some extent. （2） Fine root biomass was mainly distributed in the diameter class of 0-0.5 and 0.5 - i.O ram, and about 50% of the fine root biomass （〈2.0 ram） was allocated in the diameter class of 0-0.5 mm. Exogenous nitrogen increased the proportion of very fine root biomass （0- 0.5 ram） , especially in the young plantation. （3） In the 30-40 cm soil layer, fine root biomass was allocated with a greater proportion in 0-0.5 mm diameter class in the mature plantation than in the young and mid-aged plantations, which showed that small fine roots had a decreasing trend with the increase age. （4） Four factors （ plantation age, soil layer, diameter class, a