1316 / 2024-09-21 10:38:07

Increased CH4 and CO2 Production and Diffusive Flux in Rivers with High Urbanization

urban rivers, human activities, greenhouse gases, gas fluxes, climate change

Rapid urbanization considerably influences carbon biogeochemical cycle of river systems, contributing to higher CH₄ and CO₂ diffusive fluxes via water-air interface from rivers to the atmosphere. Our knowledge is, however, limited on the magnitude and extent of urbanization effects on CH₄ and CO₂ fluxes from inland waters (river, lakes and reservoir). Field samplings and measurements were conducted in 27 river segments of two 4^th-order and three 3^rd-order tributary rivers in China’s mountainous Three Gorges Reservoir (TGR) area during 2018-2019 to underscore the impacts of urbanization intensity on riverine dissolved gas concentrations and gases diffusive fluxes. pCO₂ level was significantly higher in the river segments with increased proportion of urban land. Both pCO₂ level and CO₂ flux rate of the higher urban intensity River Taohua showing the highest urban land coverage (3872 μatm and 574 mmol m^-2 d^-1) were significantly higher than those of the Nan River (1737 μatm and 218 mmol m^-2 d^-1) and Puli River (1218 μatm and 130 mmol m^-2 d^-1) that drain less urbanized land areas. We found that pCO₂ was positively correlated with the concentrations of chlorophyll-a (Chl-a), nutrients (i.e., TDN and TDP), dissolved organic carbon (DOC) and colony-forming units (CFU), and was negatively correlated with pH and dissolved oxygen (DO). pH and DOC loading were the better parameters for predicting pCO₂ in the river draining more urbanized land area, and pH and Chl-a were the better parameters for predicting pCO₂ in the river draining less urbanized area. dCH₄ concentration (3546 ± 6770 nmol L^-1) in the river segments draining higher urban area (20% ≤ urban land proportion ≤ 46%) was 5-6 times higher than those (615 ± 627 nmol L^-1 and 764 ± 708 nmol L^-1) in the river segments draining less urban area (0.1% ≤ urban land proportion < 2% and 2 ≤ urban land proportion < 20%). Total nitrogen (TN) showed the most powerful prediction capacity when compared to other water parameters. Nutrient elements could predict dCH₄ well in rivers draining higher urban areas (urban ≥ 2%), which also reflected the lateral input of pollutants (e.g., TN, ammonia nitrogen, total phosphorus and dissolved oxygen). River bottom sediment fraction contributed to trapping organic matter and nutrients as well as to oxic and anoxic conditions, thereby determining reach-scale spatial patterns of dCH₄ concentration. Being rapid urbanization a common feature, we suggest that riverine GHGs emissions will increase into the future under urban development.