不同林龄杨树人工林土壤有效氮对模拟氮沉降的初期响应

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N₀(0 g/(m²·a))、N₁(5 g/(m²·a))、N₂(10 g/(m²·a))、N₃(15 g/(m²·a))、N₄(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18.50%～28.81%、23.14%～34.52%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(N₃和N₄)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。

The short-term responses of soil available nitrogen to simulated nitrogen deposition in Populus plantations at different ages

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(0 g/(m²·a)), N₁(5 g/(m²·a)), N₂(10 g/(m²·a)), N₃(15 g/(m²·a)), N₄(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 deposition in Populus plantation 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⁺_<sub>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(N₃ and 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 nitrogen deposition were more sensitive than that in deeper layers.