Urbanization and weather dynamics co-dominated the spatial-temporal variation in pCO and CO fluxes in small montanic rivers draining diverse landscapes.

Rivers have been widely reported as important CO emitters to the atmosphere. Rapid urbanization has a profound impact on the carbon biogeochemical cycle of rivers, leading to enhanced riverine CO evasions. However, it is still unclear whether the spatial-temporal patterns of CO emissions in the rivers draining diverse landscapes dominated by urbanization were stable, especially in mountainous areas. This study carried out a two-year investigation of water environmental hydrochemistry in three small mountainous rivers draining urban, suburban and rural landscapes in southwestern China, and CO partial pressure (pCO) and fluxes (fCO) in surface water were measured using headspace equilibrium method and classical thin boundary layer model. The average pCO and fCO in the highly urbanized river were of 4783.6 μatm and 700.0 mmol m d, conspicuously higher than those in the rural river (1525.9 μatm and 123.2 mmol m d), and the suburban river presented a moderate level (3114.2 μatm and 261.2 mmol m d). It provided even clearer evidence that watershed urbanization could remarkably enhance riverine CO emissions. More importantly, the three rivers presented different longitudinal variations in pCO, implying diversified spatial patterns of riverine CO emissions as a result of urbanization. The urban land can explain 49.6-69.1% of the total spatial variation in pCO at the reach scale, indicating that urban land distribution indirectly dominated the longitudinal pattern of riverine pCO and fCO. pCO and fCO in the three rivers showed similar temporal variability with higher warm-rainy seasons and lower dry seasons, which are significantly controlled by weather dynamics, including monthly temperature and precipitation, but seem to be impervious to watershed urbanization. High temperature-stimulated microorganisms metabolism and riched-CO runoff input lead much higher pCO in warm-rainy seasons. However, it showed more sensitivity of pCO to monthly weather dynamics in urbanized rivers than that in rural rivers, and warm-rainy seasons showed hot moments of CO evasion for urban rivers. TOC, DOC, TN, pH and DO were the main controls on pCO in the urban and suburban rivers, while only pH and DO were connected with pCO in the rural rivers. This indicated differential controls and regulatory processes of pCO in the rivers draining diverse landscapes. Furthermore, it suggested that pCO calculated by the pH-total alkalinity method would obviously overestimate pCO in urban polluted rivers due to the inevitable influence of non-carbonate alkalinity, and thus, a relatively conservative headspace method should be recommended. We highlighted that urbanization and weather dynamics co-dominated the multiformity and uncertainty in spatial-temporal patterns of riverine CO evasions, which should be considered when modeling CO dynamics in urbanized rivers.