随着我国城市化的快速发展，高强度的人为活动对城市系统氮物质流的干扰日益强烈，这不仅导致了自然资源的浪费，同时也造成物质流动过程中向自然环境输出大量的污染物，从而引发了一系列的生态环境问题。然而，城市尺度氮物质流在人为来源、通量、归趋和时空分布格局的变化尚未摸清，对于其生态环境影响的空间差异化评估也相对缺乏，难以为城市氮素可持续利用及负面生态环境效应防控提供有效的决策依据。

本研究以协调人类社会经济活动和活性氮输入之间的关系、降低活性氮负面生态环境影响、促进城市可持续发展为目标，构建城市氮物质流及其生态环境影响模拟评估模型，全面刻画氮物质流过程并系统评估其富营养化空间影响潜力，设置氮输入减量策略及2050年社会经济情景实现减量潜力分析，为城市防控氮负面生态环境影响、促进氮素可持续利用提供科学支持和决策依据。本研究以广州市为案例区主要从以下四方面开展研究：

（1）基于物质流和不确定性分析理论，构建城市系统氮物质流模拟模型, 为城市氮物质流模拟分析框架体系的搭建与运算提供方法论。城市系统受人类活动影响巨大，其活性氮在来源、大小、归趋和时空分布等方面均与大尺度的物质流过程有着巨大差异，相对于全球、全国和流域尺度，城市尺度氮物质流模拟方法体系研究较少。因此，本研究基于物质流分析和不确定性分析等理论基础，构建城市系统氮物质流模拟模型，模型涵盖了生产、消费、处理和自然环境4个过程群组，农田、畜禽养殖、森林、城市绿地、水产养殖、工业、人类、污水处理、垃圾处理、地表水、地下水和大气等12个子系统，实现了在不确定条件下全面刻画城市氮物质流过程。

（2）基于城市氮物质流模拟模型，综合分析人为活动下的城市氮物质流时空格局分布及变化特征，为认识城市氮物质流过程提供典型科学案例与研究样本。结果显示，在1995-2015年期间，广州市活性氮流被人为地强化以维持人类日益增长的生产和消费需求，同时环境中的活性氮损失持续加强。在人为氮源输入方面，不仅表现为输入强度的持续人为强化，并且表现为输入结构的严重外源依赖。在分布格局方面，人为扰动极大地改变了城市环境中的活性氮分布，在大气中表现出大量的活性氮富集，而陆地系统保留活性氮的能力有所下降。在氮流归趋方面，工业活性氮在人类子系统中迅速积累，这可能作为已有学者报道的全球未知氮汇的一种解释。在环境损失方面，不仅体现为污染负荷的增加，还表现出排放来源结构的改变，城市系统中密集人口的大量消费过程极大地增加了环境中的活性氮排放，特别是对地表水的活性氮排放。

（3）基于城市氮物质流模拟结果，构建氮在地表水空间差异化（1km*1km）的富营养化影响潜力评估模型，实现了宏观氮物质流模拟和物种损失终点效应的因果量化连接，评估了广州市氮对地表水富营养化生态环境影响的空间分布。模型结合了归趋模拟与效应模拟，包含了完整的生态环境影响机制因果链，延伸到终点层面，量化为物种损失，模型考虑了地表水通过平流、反硝化、人为使用和滞留等作用对氮素的去除作用，并结合广州当地物种情况实现了在中国典型城市的本土化应用。地表水富营养化影响的空间分布结果显示，2015年广州市富营养化影响潜力空间差异显著（0-325,933 PAF m³ yr）。最高区域出现在广州市区中心(越秀区、海珠区、天河区以及荔湾区)（54,802-325,933 PAF m³ yr），位于珠江口河网区的番禺、南沙等地富营养化影响潜力次之（6,371-54,802 PAF m³ yr）。城镇用地富营养化影响潜力显著高于全市总平均富营养化潜力，平均值高达15,830.84 PAF m³ yr，人口和GDP分布与对应空间上富营养化影响潜力有着显著的正相关性。地表水富营养化影响潜力的空间差异和贡献分布评估结果可为广州制定针对性和空间化的环境效应防控政策提供科学支持和决策依据。

（4）综合考量人为活性氮输入及其环境影响，构建氮输入减量策略及2050年社会经济情景，实现减量潜力的分析，为城市氮素可持续利用提供决策支持。基于社会经济驱动因素分析结果构建氮输入减量策略，将其纳入通过共享社会经济路径（shared socio-economic pathways, SSPs）构建的2050年广州市社会经济发展情景，通过参考情景与减量情景的对比实现减量潜力的分析。结果表明，在SSP2中度发展路径下，输入城市系统的人为活性氮将从2015年的295.65 Gg上升到2050年的381.37 Gg，在没有控制的情况下，SSP2中度发展路径是不可接受的。而总体减量策略将使2050年人为活性氮输入降至175.61 Gg，该值是2015年活性氮输入水平的60%。城市活性氮未来可持续利用的理论潜力是存在的，要通过减少化石燃料燃烧、提高工业合成氨产品的循环利用率、优化控制人均食物以及材料产品需求等措施，有效降低活性氮环境负荷，缓解活性氮污染。

The nitrogen (N) cycle is one of most important biogeochemical cycle. With the continuous acceleration of urbanization, high-intensity human activities not only reshape the nitrogen substance flow process, but also affect environment and human health through reactive nitrogen (Nr) loss during production and consumption. However, in urban regions, the variations in sources, magnitude and spatiotemporal patterns of Nr flows still remain unclear, and the site-specific assessment of concomitant eco-environmental impact potential is relatively lacking, which is difficult to provide effective decision basis for sustainable use of urban nitrogen and prevention of negative eco-environmental effects.

Aiming at coordinating the relationship between human socio-economic activities and Nr input, reducing the negative eco-environmental impact, and promoting urban sustainable development, I propose a comprehensive urban nitrogen flow analysis and eco-environmental impact potential assessment model. Based on the theoretical framework of the model, this thesis comprehensively maps the urban nitrogen substance trajectory, systematically evaluates concomitant spatial eutrophication potential and quantitatively estimates the potential of reducing Nr input and mitigating Nr pollution under socio-economic scenario in 2050. The findings could provide scientific support and decision-making basis for urban Nr pollution reduction and Nr resource sustainable utilization. Taking Guangzhou city as the case, this study contains the following four aspects:

Based on the theory of substance flow analysis and uncertainty analysis, the simulation model of nitrogen substance flow in urban system is constructed, which provides a methodology for the construction and operation of urban nitrogen substance flow simulation analysis. Compared with large scales, in urban regions, the N cycle is greatly mediated by complex interactions between human and natural factors, resulting in variations in sources, magnitude and spatiotemporal patterns. However, a detailed and holistic approach to mapping urban N flow trajectory is still lacking. According to substance flow analysis and uncertainty analysis theories, I constructed the N flow analysis model based on the N fate as modified by human activity, which divided the system into four process groups, including production, consumption, treatment and environment, four process groups were further divided into 12 subsystems, including farmland, urban green, livestock, forest, aquaculture, industry, human, sewage disposal, garbage treatment, atmosphere, surface water, and groundwater subsystems. The model achieves a comprehensive description of urban nitrogen substance flow process under uncertain conditions.

Based on urban nitrogen substance flow simulation model, the spatial and temporal distribution and variation characteristics of urban nitrogen substance flow under artificial activities are analysed synthetically, which provides typical scientific cases and research samples for understanding the process of urban nitrogen substance flow. The results showed that during the period of 1995-2015, Nr flows in Guangzhou city have been artificially intensified to sustain the increasing demands for production and consumption with continuous Nr loss in the environment. Nr input into the urban system is manifested in not only artificial intensification but also the change of the input structure. An important characteristic of Nr input in Guangzhou was heavy reliance on external supplies, especially agricultural products. Compared with global and national studies, the anthropogenic perturbations greatly changed the Nr distribution pattern in the urban environment, showing substantial Nr enrichment in the atmosphere. In contrast, the Nr accumulation in the terrestrial system was much lower than the national level, indicating a reduced capacity to retain Nr input. In industrial Nr fate, HBNF (Haber-Bosch N fixation) tended to be used to generate synthetic ammonia products for human consumption rather than fertilizers for agricultural use in the urban system, thus resulting in the accumulation of synthetic ammonia products in human settlements, which could contribute to explain the unknown Nr pool worldwide cited by Schlesinger. The dependence on external food input resulted in relatively low Nr emissions from agricultural production. In contrast to production, consumption by the dense population in the urban system greatly increased Nr emissions to the environment, especially to the surface water.

Based on urban nitrogen substance flow simulation results, the concomitant eutrophication potential assessment model with a spatial resolution of 1km*1km is constructed, which builds the cause-effect chains linking the macro nitrogen substance flow and species loss endpoint impact and realizes the spatial distribution of eutrophication potential of N to surface water in Guangzhou. Combing fate factor and effect factor, the model indicates eco-environmental impact mechanism and characterizes the endpoint effect level by species loss. The nitrogen removal processes by surface water through advection, denitrification, human use and retention are taken into account. According to the aquatic species in Guangzhou, the localization application in a typical metropolis in China is realized. The results showed that the spatial difference of eutrophication potential in Guangzhou in 2015 was significant (0-325,933 PAF m³ yr). Highest eutrophication potential value appeared in the center of urban region (Yuexiu district, Haizhu district, Tianhe district and Liwan district) (54,802-325,933 PAF m³ yr), followed by the eutrophication potential in Panyu and Nansha district (6,371-54,802 PAF m³ yr). The eutrophication potential on urban built-up area (15,830.84 PAF m³ yr) was significantly higher than the total average level of the whole city. The population and GDP distribution had a significant positive correlation with the corresponding spatial eutrophication potential. The assessment results of spatial difference and contribution distribution of surface water eutrophication potential can provide scientific support and decision basis for Guangzhou to formulate targeted and spatial environmental effect prevention and control policy.

Considering the anthropogenic Nr input and its environmental impact, the pathways of Nr input reduction and the socio-economic scenario of 2050 are constructed to analyze the potential of reducing Nr input and mitigating Nr pollution, which can provide decision-makers with a novel perspective and rational strategies for urban N management. Based on the results of anthropogenic driving factor analysis, the pathways of Nr input reduction were constructed, which were included in the socio-economic development scenarios of Guangzhou in 2050 by shared socio-economic pathways (SSPs). I quantitatively estimated the Nr inputs required to satisfy a given future demand for urban systems and assessed the potential to mitigate Nr pollution. Under the reference scenario, Nr inputs to the urban system would rise from 295.65 Gg in 2015 to 381.37 Gg in 2050 without control, which is 112–138% of the 2015 value. Under the mitigation scenario, the anthropogenic Nr inputs in Guangzhou could possibly decrease to 52–64% of the 2015 value in 2050, which suggests that the biophysical potential for a sustainable future for urban N use exists. Combining mitigation pathways including reducing Nr emissions from fossil fuel combustion, increasing recycling rate of industrial N, controlling per capita demand for food and materials and improving NUE (nitrogen use efficiency) of farmland, substantial reduction in Nr (relative to SSP2 level) and mitigation of Nr pollution can be achieved. Although this study focuses on the Guangzhou as an example, the outcomes of this study could be valuable for sustainable development of N cycle for megacity systems.