Fluvial CO2 and CH4 in a lowland agriculturally impacted river network: Importance of local and longitudinal controls

Despite streams and rivers play a critical role as conduits of terrestrially produced organic carbon to the atmosphere, fluvial CO2 and CH4 are seldom integrated into regional carbon budgets. High spatial variability hinders our ability to understand how local and longitudinal controls affect underlying processes of riverine CO2 and CH4 and challenge the prediction and upscaling across large areas. Here, we conducted a survey of fluvial CO2 and CH4 concentrations spanning multiple stream orders within an agriculturally impacted region, the North China Plain. We explored the spatial patterns of fluvial CO2 and CH4 concentrations, and then examined whether catchment and network properties and water chemical parameters can explain the variations in both carbon gases. Streams and rivers were systematically supersaturated with CO2 and CH4 with the mean concentrations being 111 and 0.63 μmol L-1, respectively. Spatial variability of both gases was regulated by network properties and catchment features. Fluvial CO2 and CH4 declined longitudinally and could be modeled as functions of stream order, dissolved oxygen, and water temperature. Both models explained about half of the variability and reflected longitudinal and local drivers simultaneously, albeit CO2 was more local-influenced and CH4 more longitudinal-influenced. Our empirical models in this work contribute to the upscaling and prediction of CO2 and CH4 emissions from streams and rivers and the understanding of proximal and remote controls on spatial patterns of both gases in agriculturally impacted regions.