High Riverine CO₂ Outgassing affected by Land Cover Types in the Yellow River Source Region

Abstract. Rivers connect the land and the oceans, acting as both active pipes and containers transporting carbon and other substances from terrestrial ecosystems to aquatic ecosystems. Meanwhile, rivers can release huge amounts of CO₂ to the atmosphere. However, estimates of global riverine CO₂ emissions remain greatly uncertain owing to the absence of a comprehensive spatially and temporally CO₂ emissions measurement, especially in river source regions. In this study, riverine partial pressure of CO₂ (pCO₂) and CO₂ efflux (FCO₂) in the Yellow River source region under different landcover types, including glaciers, permafrost, wetlands, and grasslands, were investigated in April, June, August, and October 2016. The relevant chemical parameters and environmental parameters, including pH, dissolved oxygen (DO), and dissolved organic carbon (DOC), were analyzed to explore the main control factors of riverine pCO₂ and FCO₂. The results showed that the rivers in the Yellow River source region were a net CO₂ source, with the pCO₂ ranging from 181 to 2441 μatm and the FCO₂ from −221 to 6892 g C m⁻² yr⁻¹. Both the pCO₂ and FCO₂ showed strong spatial and temporal variations. The average FCO₂ in August was higher than that in other months, with the lowest in October. In alpine climates, low temperature conditions played a crucial role in limiting biological activity and reducing CO₂ emissions. The lowest FCO₂ values (−221 g C m⁻² yr⁻¹) were observed in the glacier and permafrost regions. By integrating seasonal changes of water surface area, the total CO₂ efflux was estimated at 0.37 ± 0.49 Tg C yr⁻¹, which is significantly higher than previous studies. Although it is still a small proportion of CO₂ emissions compared with the whole Yellow River Basin, but there is a huge carbon emissions potential. Since the permafrost in the source region of the Yellow River is rich in large amounts of ice and organic carbon, the continuously increasing temperature due to global warming will accelerate not only the mobilization of organic carbon in permafrost, but also the degradation of organic carbon by soil microorganisms. As a consequence, huge amounts of CO₂ release from soils and rivers is anticipated.