Urban Development Enhances Riverine Greenhouse Gas Emissions by Increasing Nutrient Levels in a Typical Metropolitan Region in South China

A Global concern for riverine greenhouse gas (GHGs) emissions associated with diverse landscapes is increasing, since anthropogenic activities such as land use change can strongly modify carbon emissions in adjacent rivers. In this study, the patterns and drivers of riverine CO and CH total fluxes (FCO and FCH) were investigated along a subtropical river basin (Pearl River Delta) that has undergone rapid urbanization and economic development in recent decades. Increasing CO and CH fluxes were observed from upstream to estuary, with obvious spatial clustering in the lower reaches of the Pearl River Delta. The results showed that urban reaches were large C gas emission hotpots, with mean fluxes of 283.14 ± 192.99 mmol m d for CO and 21.26 ± 12.75 mmol m d for CH4, which were more than 6 times higher than those in the forested reaches. Importantly, riverine CO and CH fluxes were positively correlated with the percentage of urban area surrounding the river segments. High riverine CO and CH emissions were primarily associated with carbon and nitrogen concentrations, for which nutrient inputs were dominant in the middle and lower reaches. The dissolved organic carbon (DOC) and total phosphorus (TP) were identified as the main control factors for CO production in urban rivers. The CH4 emissions were controlled by hydrology and sediment properties, with the dissolved oxygen (DO), sediment total nitrogen (STN) and sediment total organic carbon (SSOC) as crucial factors for the urban river reaches, respectively.