Anthropogenic land use substantially increases riverine CO emissions.

Carbon dioxide (CO) emissions from inland waters to the atmosphere are a pivotal component of the global carbon budget. Anthropogenic land use can influence riverine CO emissions, but empirical data exploring cause-effect relationships remain limited. Here, we investigated CO partial pressures (pCO) and degassing in a monsoonal river (Yue River) within the Han River draining to the Yangtze in China. Almost 90% of river samples were supersaturated in CO with a mean ± standard deviation of 1474 ± 1614 µatm, leading to emissions of 557 - 971 mmol/m/day from river water to the atmosphere. Annual CO emissions were 1.6 - 2.8 times greater than the longitudinal exports of riverine dissolved inorganic and organic carbon. pCO was positively correlated to anthropogenic land use (urban and farmland), and negatively correlated to forest cover. pCO also had significant and positive relationships with total dissolved nitrogen and total dissolved phosphorus. Stepwise multiple regression models were developed to predict pCO. Farmland and urban land released nutrients and organic matter to the river system, driving riverine pCO enrichment due to enhanced respiration in these heterotrophic rivers. Overall, we show the crucial role of land use driving riverine pCO, which should be considered in future large-scale estimates of CO emissions from streams. Land use change can thus modify the carbon balance of urban-river systems by enhancing river emissions, and reforestation helps carbon neutral in rivers.