Managing urban development could halve nitrogen pollution in China

The Haber-Bosch process revolutionized food production, supporting population growth, but reactive nitrogen (Nr) losses have caused widespread environmental damage, including air and water pollution, soil acidification, biodiversity loss, and global warming. The UN aims to halve global nitrogen waste to achieve SDGs, a goal directly linked to several SDGs, including poverty reduction, food security, clean water, responsible consumption, climate action, and life on land. China, a major contributor to global Nr pollution, faces significant air and water quality challenges due to Nr wastage from agriculture, industry, and human activities, leading to millions of premature deaths annually. Reducing nitrogen waste is challenging due to the multitude of sources and forms of Nr, especially non-point source pollution from agriculture. While advanced technologies exist, socioeconomic barriers often hinder their full implementation. This study investigates the potential synergy of Nr abatement through well-managed urbanization in China, focusing on the biophysical changes in land intensification, agricultural practices, and pollution treatment capacity, while acknowledging that the social and economic benefits are beyond the scope of this specific investigation. China's rapid urbanization, with millions migrating annually from rural to urban areas, significantly impacts the spatial distribution of resource use and pollution. This research aims to estimate the potential for halving nitrogen pollution in China through well-managed urbanization by analyzing changes in population, land use, agriculture, Nr pollution from various sources and forms, and the resulting impacts on air and water quality, culminating in a cost-benefit analysis.

Existing literature highlights the negative consequences of reactive nitrogen pollution and the need for mitigation strategies. Studies show the impacts of air pollutants from rural households, the agronomic effectiveness of nitrification inhibitors, and the socioeconomic barriers to nitrogen management. Research also indicates that well-managed urbanization can benefit large-scale crop production by increasing cropland area and farm size, facilitating modern agricultural practices, and potentially improving crop-livestock integration. However, the effect of urbanization on nitrogen use and losses remains poorly understood. Previous research has explored the potential of reducing nitrogen pollution through various methods, including advanced technologies and policy interventions, but a comprehensive assessment of the role of managed urbanization as a mitigation strategy, particularly in the context of China’s unique socio-economic landscape, was lacking until this study.

This study estimated the potential to halve nitrogen pollution in China through well-managed urbanization using a four-stage analysis across all of China's >2800 counties. **Stage 1:** Assessed changes in population, land use, and agriculture associated with urbanization using data from the High Resolution Remote Sensing Monitoring of Chinese Land Cover 2018 (HRRSM-CLC2018), digital elevation models (DEM), county-level census data, and provincial-level statistics from the National Bureau of Statistics of China. Projections of future urban population were based on the United Nations’ World Urbanization Prospects. **Stage 2:** Quantified how these changes reduce Nr pollution from different sources and forms using spatial analysis and linear optimization techniques within ArcGIS. The potential for large-scale farming was predicted and mapped, assuming connectivity as long as not divided by other land uses. Livestock relocation was simulated based on the distribution of large-scale farming, considering factors such as manure carrying capacity and crop demand. The influence of farm size on fertilizer use and NH3 emissions followed previous research findings. **Stage 3:** Estimated the reductions in wasteful Nr losses on air and water quality. The CHANS model was used to analyze the impact of urbanization on the national nitrogen budget. The WRF-Chem model estimated changes in surface concentrations of PM2.5 based on NH3 and NOx emission reductions. A water network-based framework (WNF) estimated changes in Nr output to the sea based on Nr loss to surface water. **Stage 4:** Examined the feasibility and policy implications, including a cost-benefit analysis. The costs of waste treatment facilities, agricultural management optimization, and industrial upgrading were estimated using data from various sources including actual investment projects and government reports. The benefits of reduced Nr pollution were estimated in terms of human health, ecosystem health, climate change mitigation, and economic returns (fertilizer savings and yield increases).

Well-managed urbanization in China has the potential to roughly halve national reactive nitrogen (Nr) losses by 2050. This would involve a 44% reduction in NH3 emissions, a 30% reduction in NOx emissions, and a 33% reduction in N2O emissions. Nr loss to water bodies would decline by 53%. These reductions are primarily driven by the increased adoption of large-scale farming, which improves nitrogen use efficiency (NUE) and enhances manure recycling. Rural-urban migration contributes to reductions in rural sewage and agricultural non-point source pollution. The estimated cost to achieve this reduction is approximately US$61 billion annually, while the benefits are estimated at US$245 billion annually, suggesting a high benefit-cost ratio. The largest reductions in Nr losses are projected in the North China Plain and Sichuan Basin due to increased large-scale farming, livestock relocation, and improved sewage treatment. Reductions in NH3 and NOx emissions are predicted to decrease national average surface concentrations of PM2.5 by 9%, leading to substantial improvements in air quality and public health. Urbanization also significantly reduces Nr loading to water bodies, with a 49% decrease in Nr export to the ocean. While a few counties in major metropolitan areas might experience increased Nr losses, over 96% of counties are projected to see reductions.

The findings demonstrate the significant potential for well-managed urbanization to contribute to substantial reductions in reactive nitrogen pollution in China. The high benefit-cost ratio suggests that investing in measures to manage urbanization effectively is a cost-effective strategy for environmental improvement and sustainable development. The results highlight the importance of considering the interconnectedness of urban development and environmental sustainability, showcasing how strategic planning and investment can create positive synergies between economic growth and environmental protection. These findings are particularly relevant for other rapidly urbanizing countries facing similar challenges of nitrogen pollution and sustainable development. The study’s focus on biophysical changes, while omitting detailed social and economic aspects, allows for a clearer evaluation of the environmental impacts of urbanization. Future research should integrate social and economic factors to provide a more complete picture.

This study demonstrates the significant potential of well-managed urbanization to dramatically reduce nitrogen pollution in China, offering a cost-effective pathway toward achieving multiple Sustainable Development Goals. The substantial benefits associated with reduced air and water pollution, coupled with the economic advantages of large-scale farming, emphasize the need for integrated urban planning and environmental management strategies. Further research should investigate the complexities of dietary shifts and their impact on nitrogen cycling within the context of urbanization. Moreover, exploring the detailed toxic effects of NOx compounds beyond PM2.5, and enhancing the validation of water quality models are important directions for future research.

The study acknowledges several limitations. Firstly, it primarily focuses on biophysical changes related to well-managed urbanization, omitting a detailed analysis of social and economic impacts. Secondly, the health impacts assessment relies primarily on PM2.5-related diseases, neglecting other health consequences of NOx exposure. Thirdly, uncertainties exist in national-scale water quality modeling due to data limitations; however, the estimates are considered reasonable when compared to the results of other existing models. Finally, the study assumes a consistent dietary structure and production model over time, which might not fully reflect the complexity of these dynamics in a rapidly changing environment.