Deciphering large-scale spatial pattern and modulators of dissolved greenhouse gases (CO2, CH4, and N2O) along the Yangtze River, China

The Yangtze River, the third largest river around the globe, has been heavily engineered with a series of hydroelectric dams. Meanwhile, it receives elevated organic matter and nutrient loads from its densely populated catchment, subsequently altering dissolved greenhouse gas (GHG) concentrations along the river. However, the large-scale longitudinal patterns and drivers of GHG concentrations in the Yangtze River remain poorly understood. Using longitudinal sampling design in a 2400 km section, we report dissolved carbon dioxide, methane, and nitrous oxide concentrations along the Yangtze River at 145 sites. We observe significant spatial clustering with higher carbon dioxide and nitrous oxide concentrations in the middle reach of the Yangtze River. The results of nonlinear regression reveal that riverine GHGs are high when wetland coverage is high and dissolved oxygen is low. Wetlands and oxygen, not the Three Gorges Dam and tributaries, are the primary correlates of spatial variations of CO2 and CH4 concentrations, respectively. N2O is surprisingly well predicted by CO2, implying their common drivers or sources. We strongly recommend that wetland contribution to GHG budgets and its sensitivity to environmental change be considered when estimating riverine GHGs in the Yangtze River. In light of our study, future control of GHG emissions from large rivers may largely depend on how external inputs and internal metabolism are regulated by decreasing nutrient loading.