中文摘要：

氮循环是陆地生态系统重要的物质循环过程之一。快速城市化改变了生态系统的氮输入和输出,从而影响了城市生态系统的氮循环规律,但目前有关研究还很少。选取北京城区和郊区的油松林、侧柏林和毛白杨林三种森林植被类型,比较研究了不同季节（春季、夏季、秋季）的土壤无机氮库和净潜在矿化和硝化速率的城郊差异。结果表明,（1）城区与郊区硝态氮、氨态氮含量均有明显的季节动态,夏季显著高于春季和秋季。（2）3个季节城区硝态氮含量均显著高于郊区（春季28.41 mg kg-1和19.66 mg kg-1;夏季99.35 mg kg-1和59.51 mg kg-1;秋季9.61 mg kg-1和5.63 mg kg-1）。（3）城区净潜在矿化速率也高于郊区。（4）夏、秋两季,城区和郊区的土壤硝态氮含量不同树种间存在差异,夏季城区的净潜在矿化和硝化也呈现出树种间的差异（P〈0.05）。（5）影响城区与郊区无机氮库的环境因子也存在差异,城区土壤无机氮库的变化主要与土壤有机碳和容重相关,而郊区主要与土壤水分相关。因此,城市化能够对森林土壤的氮循环产生一定影响,但其影响程度因研究指标、季节和树种而有所差异。

英文摘要：

Nitrogen cycle is an important nutrient cycle in terrestrial ecosystem. Urbanization significantly affects the nitrogen input and output in urban ecosystem which will affect the nitrogen cycle in urban ecosystem, however related researches on the characteristics of urban soil nitrogen cycle and process are not sufficient. In this study, we chose three forest types, specifically Pinus tabulaeformis, Platycladus orientalis and Populus tomentosa, in the urban and suburban regions of Beijing, to compare the urban-suburban and seasonal differences of inorganic nitrogen pools and the characteristics of potential nitrogen cycle. Our results indicated that soil nitrate nitrogen and ammonia nitrogen had obvious seasonal fluctuations; and the inorganic nitrogen concentration in summer was significantly higher than those in spring and autumn. In addition, as to all seasons, the concentrations of nitrate nitrogen in urban areas were considerably higher than those in suburban areas for all three seasons （spring 28.41 mg kg-1/19.66 mg kg-1; summer 99.35 mg kg-1/59.51 mg kg-1; autumn 9.61 mg kg-1/ 5.63 mg kg-1）, and the rates of net potential mineralization tended to increase towards the urban center. Moreover, in summer and autumn, the concentrations of nitrate nitrogen for different forest types displayed significant differences. Further regression analysis showed that the soil factors influencing the inorganic nitrogen concentrations and nitrogen cycle in urban and suburban regions were obviously different. Specifically, inorganic nitrogen concentrations in urban center were primarily influenced by TOC and bulk density; whereas those in suburban were affected by soil water content. This research indicated that urbanization significantly increased the soil nitrate accumulation and accelerated the rate of its mineralization in urban regions. It also revealed the variances of the impacts among different urban tree species for this response and regulation capacity.