Changes of urban nitrogen metabolism in the Beijing megacity of China, 2000–2016.

Abstract Rapid growth in metropolitan areas is associated with high nitrogen (N) flows and subsequent environmental and human health consequences. Many studies on the contemporary aspects of urban N metabolism have conducted in recent years, but comprehensive analysis from life cycle perspective is limited. In this study, a detailed quantitative framework for a coupled human-natural N flow model, comprising a full cycle analysis based on the substance flow analysis approach to cover and integrate all specific N flows and stocks associated with N production, consumption and emission, was developed to study the temporal changing patterns of N metabolism in Beijing megacity during 2000–2016. The results show that total N inputs continuously increased from 413.3 to 529.5 Gg N during the study period, primarily attributing to fossil fuel combustion (53%), fertilizer/feed import (19%), and food import (15%). Agriculture subsystem contains the largest N internal flows, and a decreasing trend is exhibited by a widening gap between local production and household consumption, reflecting Beijing's increasing dependence on the external environment. Moreover, N outputs (394.9 Gg in 2016) contribute to upstream air emissions, landfills accumulation and downstream wastewater discharges. Furthermore, driving force analysis demonstrates that population growth has the largest positive effect on N inputs, and a decoupling of N input with GDP growth is identified. Overall, N flows exhibit an inefficient and unsustainable trend, and possible options for optimizing more sustainable situations while simultaneously minimizing negative consequences are discussed. This study provides decision-makers with an integrated view of N management at the city scale. Graphical abstract Unlabelled Image Highlights • An analytical framework for coupled human-natural N flow model is established. • The whole N production-consumption-emission processes were identified. • Population and economic growth are key factors driving the increasing N inputs. • The disparity between local production and household consumption widens. • A considerable N emission to air and accumulated in landfills were observed. [ABSTRACT FROM AUTHOR]