中文摘要：

为了解森林生态系统对持续氮增长和快速氮循环的响应模式及反馈机制,选择3种林龄杨树人工林作为试验样地,设置N_0（0 g/（m~2·a））、N1（5 g/（m2·a））、N2（10 g/（m2·a））、N_3（15 g/（m2·a））、N_4（30 g/（m2·a））共5个不同浓度进行模拟氮沉降实验,探讨3种林龄杨树人工林土壤有效氮含量及对模拟氮沉降的响应。结果表明：1幼龄林、中龄林和过熟林的铵态氮占总有效氮含量的比例分别为1850%~2881%、2314%~3452%和32.60%~49.92%;2随着外源氮浓度的不断增加,3种林龄土壤硝态氮含量都呈显著的增加趋势,且高氮处理对有效氮的影响高于低氮处理,而铵态氮只在幼龄林和中龄林中高氮处理（N_3和N_4）之间差异显著;3幼龄林土壤硝态氮含量对不同浓度的氮沉降响应比中龄林和过熟林更为敏感,而铵态氮在3种林龄之间无显著规律;43种林龄土壤表层（0~10 cm）的铵态氮、硝态氮含量对氮沉降响应更加敏感。

英文摘要：

In order to understand the response model and feedback mechanisms of the forest ecosystem to continued increasing of nitrogen and rapid nitrogen cycle, simulating nitrogen deposition by using five treatments including N0（0g/（m2·a））, N_1（5 g/（m2·a））, N2（10 g/（m2·a））, N_3（15 g/（m2·a））, N_4（30 g/（m2·a）） in the field, we conducted a field-based experiment to examine the soil available nitrogen（N） contents and its response to nitrogen depositioninPopulusplantation with different stand ages（6, 10 and 16 years old）in the coastal plain of Dongtai, Jiangsu Province.The results showed that： 1 In the young, middle-aged and over mature plantation, ammonium nitrogen（NH＋4-N）contents accounted for 18.50%-28.81%, 23.14%-34.52% and 32.60%-49.92% of total available nitrogen, respectively.2 Soil nitrate nitrogen（NO-3-N） contents in three plantations presented a significant increasing trend with the increasing of exogenous nitrogen, moreover, the impact of NO3--N in the high nitrogen treatment was higher than that in the low nitrogen treatment. The NH＋4-N contents had significant difference in high nitrogen（N3and N4） only between young and middle age plantation. 3 The responses of the soil NO-_3-N contents in young plantation were more sensitive to nitrogen deposition than that in middle age and over mature plantations, while NH＋4-N had no significant regularity among different forest ages.4 The responses of soil NH＋4-N and NO-_3-N contents in the soil surface layer（0-10 cm）to nitrogendeposition were more sensitive than that in deeper layers.