城市灌丛截获大气不同形态氮沉降的过程及其与城市化要素的耦合机制

Urbanization effect has been one of major issues of global change research. According to the results of water quality across the urban stream watershed in USA, urban greenspace is the potential nitrogen sink of atmosphere nitrogen deposition, but the nitrogen retention mechanism is poorly understood. With the rapidly increasing urbanization and its consequences of elevated atmosphere nitrogen deposition in China, the nitrogen retention ability and its interaction with urban atmospheric nitrogen forms and soil phosphorus enrichment need be studied especially in urban shrub greening, which is one of important mode of urban greenspace and has a increasing area due to the expand of city. We selected typical urban-suburban-rural gradients along the cities of Nanchang (the capital of Jiangxi province) and Xinyu (one of China ecological garden city in Jiangxi province) to study the spatial-temporal pattern of atmosphere nitrogen deposition using ion exchange resin column method, and to in situ study the nitrogen retention in aboveground, soils and plant tissues of shrub greenspace along the gradients with the experiments of simulating atmosphere deposition of various nitrogen forms including inorganic and organic nitrogen and two levels of soil phosphorus enrichment (2 and 5 g/m2/a). Our objectives are to explore the separating and interactive effects of urban environment, atmosphere deposition of various nitrogen forms, urban soil phosphorus enrichment on nitrogen retention and rhizospheric microbial process in shrub greenspace. Our expected results would be helpful to understand how influence of city development mode on urban effect, to explain the nitrogen retention potential and its belowground ecological mechanism in urban greenspace, and to show the interaction of nitrogen and phosphorus cycling in urban ecosystem. We also expect to push forward urban belowground ecology, to guide urban construction of ecological garden, and to provide supports for dealing with global change.

城市化效应是全球变化研究的重要内容；美国城市溪流水质监测结果表明，城市绿地是截获大气氮沉降的潜在氮汇，但其截获机制尚不清楚；城市环境,尤其是城市发展程度和土壤磷积累与城市灌丛截获不同形态氮沉降的耦合过程有待剖析。本项目以大都市南昌和中等城市新余为代表，采用离子交换树脂柱和15N脉冲示踪技术等方法，在沿城区、郊区、农村梯度监测大气氮沉降基本组成和时空格局的基础上，分别开展不同形态氮（无机和有机氮）沉降和不同水平磷积累（2和5 g/m2/a）对灌丛绿地截获氮过程影响的原位模拟试验，剖析城市环境条件、氮沉降形态、磷积累状况对灌丛绿地土壤和植物体氮截获、根际土壤微生物氮转化的影响及其交互作用。结果有助于理解城市发展状况如何影响城市化效应，解释城市绿地对大气氮沉降的截获潜能及地下生态机制，揭示城市生态系统氮磷循环的耦合效应，推动城市地下生态学的发展，指导城市生态园林的建设，并为应对全球变化提供参考。

城市化效应是全球变化研究的重要内容；城区大气氮沉降和土壤磷积累等城市化过程的重要特征，研究城市绿地如何适应和反馈城市化关键要素及其耦合作用对于理解城市生态系统的运行规律和发挥城市绿地削减氮磷的功能具有重要价值。本项目以我国南方代表性的省会城市南昌为主要研究对象，沿城区、郊区和农村梯度布设主要类型绿地的固定样地30块，定期收集表层土壤，监测不同形态氮和不同组分磷的含量水平，分析大气氮沉降和土壤磷积累的来源和转化及其潜在去向，结果表明，城区氮沉降以硝态氮为主，土壤磷积累为磷酸钙类为主，氮主要被绿地植物吸收，磷主要贮存于土壤；模拟开展磷积累和增温对城乡梯度土壤碳氮转化速率的影响，结果表明，磷积累对城区土壤氮矿化速率影响不显著，但显著提高土壤有机碳分解速率，增温将进一步加快氮硝化速率和提高有机碳分解速率；模拟开展土壤和凋落物对大气氮沉降的固持过程，结果表明，土壤固持15N低于凋落物，外源非氮养分和碳源输入可提高土壤固持15N的量，而对凋落物固持15N的量取决于模拟试验时间长短，推断土壤磷积累及碳源供应有利于提高绿地固持和吸附大气氮沉降。综上所述，城市绿地具有截获大气不同形态氮沉降的能力，且城市化要素如土壤磷积累有利于增强其截获能力，而增温会降低其截获能力，城市化要素的叠加互作效应还有待深入研究。