Analysis of the ecological relationships and hierarchical structure of Beijing's nitrogen metabolic system

The global nitrogen cycle changes under the influence of human activities. This is mainly due to the uncoordinated relationships among social, economic, and natural actors. This problem is particularly prominent at an urban scale. In this paper, we used ecological network analysis to study Beijing's nitrogen metabolism from 1995 to 2015. We determined the symbiotic status and benefit changes within the system, identified the major types of ecological relationships, plotted the flow of benefits among nodes of the city's metabolism, and analyzed the changes in the hierarchical structure of the nitrogen ecological network. We found that the mutualism index (M) fluctuated between 0.7 and 0.9, and was dominated by exploitation/control relationships (more than 42% of the total). Although the network as a whole showed a net positive benefit, with values of the synergism index (S) between 7 and 11, there were large differences in the benefits received by each node (with S values ranging from −2.56 to 2.04). As a result of the city's high biomass and industrial nitrogen fixation, combined with relatively low industrial nitrogen consumption, the hierarchical structure of Beijing's nitrogen metabolism evolved from a pyramid to a barbell shape (i.e., became less balanced). By identifying the key nodes that affected the hierarchical structure, we were able to locate the nodes with the key benefit flows and the types of relationships responsible for the negative benefits for certain nodes. The results will guide the development of policies for optimal adjustment of the nitrogen flows through Beijing's metabolism.