Special Issue Editorial: Urban and Regional Nitrogen Cycle and Risk Management

Disturbance of urban and regional nitrogen cycles due to urbanization have resulted in the greenhouse effect, acid rain, eutrophication, and reductions in biodiversity. In light of this, the ‘nitrogen cascade’ effect induced by nitrogen cycle disruption has been recognized as the third most important global environmental problem after biodiversity loss and global warming. Possible risk managements to reduce reactive nitrogen being released into the environment include proper nitrogen management within the production and consumption cycles of essential resources, which could be supported by anthropogenic approaches (e.g., environmental pollution monitoring, environmentally friendly technology and residents’ behavior) and natural-based approaches, including nitrogen retention within ecosystems. More importantly, from the perspective of synergizing the reduction in pollution and carbon emissions, the risk of nitrogen pollution is often accompanied by excessive emissions of greenhouse gases and atmospheric particles in the early stage. Progress in new areas of research will depend on relevant risk management. The reports presented in the Special Issue, ‘Urban and Regional Nitrogen Cycle and Risk Management’, are the result of collaborative work between researchers in an effort to reduce atmospheric emissions and mitigate the nitrogen risks, including both experimental and monitoring studies and mathematical/numerical modeling studies on the urban and regional scale. The publications of the issue (12 articles) cover the subjects of nitrogen and carbon coupling (5), ecological effects of nitrogen deposition (4), urban nitrogen flow analysis (1), and environmental monitoring and modeling (2). A brief overview of the main findings and conclusions of articles in the Special Issue will be presented below.

Wu and Shen [1] used bibliometric analysis and found that there are gaps between low-carbon policy and public awareness/behavior in the research hotspots of “Carbon Emission Reduction”, which hinder research progress in synergizing the reduction of nitrogen (N) pollution and carbon emissions addressed by risk managements. Urbanization is a significant indicator of city progress, which may drive the growth of carbon emissions accompanied by N release, especially in urban agglomeration. Gao et al. [2] examined the spatial and temporal variations of carbon emissions in the Pearl River Delta (PRD) urban agglomeration in China, which is suffering from water N pollution. They found that total carbon emissions in the PRD region have been increasing over 2009–2019 with hotspots mainly distributed in Guangzhou, Shenzhen, and Dongguan cities. Liu et al. [3] comprehensively accounted for the greenhouse gas budgets of Beijing and Shenzhen cities from 2005 to 2020 and revealed that the energy activity sector was the greatest contributor to GHG emissions in this period, accounting for 82.5% and 76.0% of the total GHG emissions in Beijing and Shenzhen, respectively. The carbon sink provided by these two urban ecosystems could absorb only small parts of their emissions, and the neutralization rates of sinks ranged from 1.7% to 2.3% in Beijing and from 0.3% to 1.5% in Shenzhen. This study found that household size had opposite effects on the two cities’ emissions, i.e., a 1% increase in household size would increase GHG emissions by 0.487% in Shenzhen but reduce them by 2.083% in Beijing. Song et al. [4] also found that household size was the main diver influencing personal carbon emissions in the Sanjiangyuan region of the Qinghai–Tibet Plateau by interviewing more than 1000 herder households of 15 counties. The more people living in the household, the lower the per-capita carbon emissions. However, the effect size of potential carbon reductions was weakened when the number of family members rose to over three. They proposed that grazing prohibitions and low-carbon dietary shifts would contribute to low-carbon herder livelihoods, which also may contribute to lower N herder lifestyles with less fertilizer-N and livestock-N being released. Lama et al. [5] evaluated the conservation status and effectiveness of national parks, nature reserves, forest parks, geo-parks, and scenic spots on carbon sequestration within the Loess Plateau in China throughout 2000–2020. They found that all existing protected area types have good representation and conservation effectiveness on carbon sequestration. Nature reserves, where the natural N cycle is maintained, are the most representative form of carbon sequestration but are the least effective in protecting carbon sequestration. They proposed that implementing restoration measures in low carbon sequestration areas within grassland and wild plant nature reserves will help to achieve the goal of carbon neutrality early, which also can maximize the protection of the natural nitrogen cycles in these places from human interference.

In terms of modeling for synergizing the reduction in atmospheric pollution and carbon emissions, Yang et al. [6] develop a combination prediction model for atmospheric water vapor, which is an essential source of predicting global climate change, combined with the Zenith Tropospheric Delay (ZTD) data from 13 global navigation satellite system (GNSS) service stations in the United States. This regional/single station ZTD combination prediction model, based on the machine learning algorithms of radial basis function (RBF) neural networks, assisted by the K-means cluster algorithm (K-RBF) and long short-term memory of real-time parameter updating (R-LSTM), was proposed for online modeling to serve the response and feedback of the carbon, nitrogen, and water cycle to climate change. In addition, high resolution simulation of the concentration of atmospheric pollutants in urban areas can help to develop air pollution control policies. Wu et al. [7] takes the Wenhua Road block in Shenyang city, China, as the research object, and analyzes the spatial distribution characteristics of local climate zones (LCZ) and particulate matter (PM) in the block based on the ArcGIS platform. Their findings show that the spatial distribution characteristics of PM1, PM2.5, and PM10 under the same pollution level are relatively similar, while the spatial heterogeneity of the distribution of the same particulate matter under different pollution levels is higher. The built-up LCZ always has a larger average concentration of PM than that of the natural LCZ and building height and building density are the main factors causing the difference. It also provides theoretical and practical references for the simulation of nitrogen oxide concentration growth at an urban block scale, although there are few studies focusing on this research field in recent years.

Acid deposition is an important component of atmospheric pollution, N deposition has become a major ecological problem that endangers ecosystems and residents in cities. In the case study of Shenzhen city, China, Shu et al. [8] has drawn high-resolution spatial distribution maps of N retention in the city’s ecological space, on the basis of a large number of soil sampling across the city, and they found that precipitation factors have the greatest impact on the spatial differentiation characteristics of N retention services provided by soil in three main types of land use (forested land, industrial land, and street town residential land). To examine the effects of atmospheric N deposition containing different N components on the functional differences between invasive plants and native plants, Li et al. [9] conducted a study over a four-month period using a pot-competitive co-culture experiment to elucidate the effects of artificially simulated N deposition containing different N components, which was found to facilitate the growth performance of native monocultural P. laciniata, particularly in terms of the sunlight capture capacity and leaf photosynthetic area. Invasive Bidens pilosa exhibited a more pronounced competitive advantage than P. laciniata under artificially simulated N deposition containing different N components. Furthermore, Li et al. [10] conducted controlled experiments in a greenhouse to evaluate the functional differences and growth performance between the invasive plant Amaranthus spinosus L. and the native plant A. tricolor L. in mono- and mixed culture when exposed to an acid deposition with different sulfur–nitrogen ratios. They found that the lower pH acid deposition had imposed a greater reduction in the growth performance of both Amaranthus species than the higher pH acid deposition. Amaranthus spinosus was more competitive than A. tricolor, especially when exposed to acid deposition. From the perspective of microbiology, the soil N-fixing bacterial (NFB) community may facilitate the successful establishment and invasion of exotic non-nitrogen (N)-fixing plants. Invasive plants can negatively affect the NFB community by releasing N during litter decomposition, especially where N input from atmospheric N deposition is high. Li et al. [11] conducted an indoor litterbag experiment to quantitatively compare the effects of the invasive Rhus typhina L. and native Koelreuteria paniculata Laxm. trees on the litter mass loss and the NFB. They found that the litter mass loss of the two trees was mainly associated with the taxonomic richness of NFB. The form of N was not significantly associated with the litter mass loss in either species, the mixing effect intensity of the litter co-decomposition of the two species, and NFB alpha diversity. Litter mass loss of R. typhina was significantly higher than that of K. paniculata under urea. In view of the above research, although we all know that N-related acid deposition is harmful to plant growth, the negative impacts of which on the invasive plants are much smaller than those on native plants, this indicates that regional N pollution has a substantial negative effect on urban and regional biodiversity.

From the perspective of systemic metabolism for N risk assessment, Li et al. [12] used the material flow analysis method to estimate anthropogenic nitrogen emissions in Xiamen city, China, and found that the quantity of reactive N generated by human activities increased 3.5 times from 1995 to 2018. Specifically,, the total reactive N entering the water environment showed a general increase with fluctuations, resulting in N overload in the nearby sea with a threefold augmentation compared with surface waters and groundwater. Population and per capita GDP were major factors contributing to water N pollution, demonstrating that there is an urgent need for sustainable nitrogen management in coastal cities.

In summary, the 12 papers include in this Special Issue cover serval developments and applications related to the urban and regional N risk managements on multiple scales (indoor laboratory, city block, city, region), which highlight the potential benefits of using model simulation, spatial analysis, and controlled experiments in the research of N cycling, involving, for example, the characterization of nitrogen and carbon coupling in the atmosphere, and risk analyses and corresponding policies for synergizing the reduction in pollution and carbon emissions.