CH₄ concentrations and fluxes in a subtropical metropolitan river network: Watershed urbanization impacts and...

Urbanization and greenhouse gas emissions are of great global concern, especially in developing countries such as China. However, little is known about the relationship between the two. In this study, we examined the influences of the urbanization of Chongqing Municipality, which covers an area of 5494km², in China, on the CH₄ emissions of in its metropolitan river network. The results from 84 sampling locations showed an overall mean CH₄ concentration of 0.69±1.37μmol·L^-1 and a CH₄ flux from the river network of 1.40±2.53mmolCH₄m^-2d^-1. The CH₄ concentrations and fluxes presented a clear seasonal pattern, with the highest value in the spring and the lowest in the summer. Such seasonal variations were probably co-regulated by the dilution effect, temperature and supply of fresh organic matter by algal blooms. Another important result was that the CH₄ concentrations and fluxes increased with the degree of urbanization or the proportion of urban land use, being approximately 3-13 times higher in urban and suburban areas than in rural ones. The total nitrogen, dissolved oxygen (O%) and possible sewage discharge, which could affect the in situ CH₄ production and exogenous CH₄ input respectively, were important factors that influenced the spatial patterns of CH₄ in human-dominated river networks, while the nitrogen (N) and phosphorus (P) could be good predictors of the CH₄ emissions in urban watersheds. Hydrologic drivers, including bottom sediment type, flow velocity and river width, were strongly correlated with the CH₄ concentrations and could also affect the spatial variance and predict the CH₄ hotspots in such metropolitan river networks. With increasing urbanization, we should pay more attention to the increasing greenhouse gas emissions associated with urbanization.