Abstract. CO2 efflux at the water–air interface is an essential component of the riverine carbon cycle. However, the lack of spatially resolved CO2 emission measurements prohibits reliable estimation of the global riverine CO2 emissions. By deploying floating chambers, seasonal changes in river water CO2 partial pressure (pCO2) and CO2 emissions from the Dong River in south China were investigated. Spatial and temporal patterns of pCO2 were mainly affected by terrestrial carbon inputs (i.e., organic and inorganic carbon) and in-stream metabolism, both of which varied due to different land cover, catchment topography, and seasonality of precipitation and temperature. Temperature-normalized gas transfer velocity (k600) in small rivers was 8.29 ± 11.29 and 4.90 ± 3.82 m d−1 for the wet season and dry season, respectively, which was nearly 70 % higher than that of large rivers (3.90 ± 5.55 m d−1 during the wet season and 2.25 ± 1.61 m d−1 during the dry season). A significant correlation was observed between k600 and flow velocity but not wind speed regardless of river size. Most of the surveyed rivers were a net CO2 source while exhibiting substantial seasonal variations. The mean CO2 flux was 300.1 and 264.2 mmol m−2 d−1 during the wet season for large and small rivers, respectively, 2-fold larger than that during the dry season. However, no significant difference in CO2 flux was observed between small and large rivers. The absence of commonly observed higher CO2 fluxes in small rivers could be associated with the depletion effect caused by abundant and consistent precipitation in this subtropical monsoon catchment.

River networks are key members in the regional and global carbon (C) cycle (Battin et al., 2009;Cole et al., 2007;Drake et al., 2018). Large quantities of C stabilized by the terrestrial ecosystems are transported from the land to rivers through runoff and groundwater, (Regnier et al., 2022), and this land-river C transport could offset terrestrial C gain and diminish the C sequestration capacity of terrestrial ecosystems (Chi et al., 2020;Duvert et al., 2020;Lauerwald et al., 2020). Emission in the form of carbon dioxide (CO 2 ) from water to the atmosphere is a primary pathway of those land-derived riverine C. Annual CO 2 emissions from rivers could reach up to 2 Pg C (S. Liu et al., 2022), outpacing the C transported by rivers from the land to the ocean or those buried within the river networks (Regnier et al., 2022). However, there is still great uncertainty in estimations of riverine CO 2

It has been well recognized that global terrestrial ecosystems play a vital role in the global carbon (C) cycle. The photosynthesis of the terrestrial biosphere can annually remove approximately one-fifth of the carbon dioxide (CO 2 ) present in the atmosphere, thereby constituting a large and persistent net C sink (Battin et al., 2009;Keenan & Williams, 2018). However, there remains a high degree of uncertainty associated with the strength of the terrestrial C sink because previous studies on terrestrial ecosystems as C sinks are primarily based on the

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Terrestrial ecosystems as a significant carbon (C) sink are a key player modulating atmospheric carbon dioxide (CO 2 ) and Earth's climate. Accurately quantifying the magnitude of the land C sink is therefore crucial for improving global C accounting and developing effective climate change mitigation strategies. Accruing evidence suggests that terrestrial ecosystems are not isolated from other earth systems with all the absorbed C being permanently stored on land (Butman et al., 2016;Ciais et al., 2020). In contrast, strong interactions between terrestrial ecosystems and fluvial networks indicate that a portion of the C sequestered on land by vegetation can be transported to the ocean through inland waters, the land-to-ocean aquatic continuum