Riverine carbon cycling in the East River (Dongjiang) basin, south China

Abstract

Riverine carbon cycling is a complex biogeochemical process characterized by strong spatial and temporal dynamics. Three interconnected facets of this cycle were explored in this study. The first aspect pertained to riverine partial pressure of CO¬2 (pCO2) and CO2 emissions (FCO2), as well as their controlling factors, from rivers spanning seven Strahler orders in the East River Basin (ERB) in south China. Spatial and temporal patterns of pCO2 were mainly affected by terrestrial carbon inputs and in-stream metabolism, both of which varied due to different land cover, catchment topography, and seasonality of precipitation and temperature. Standardized gas transfer velocity (k600) in small rivers were 8.29±11.29 m d−1 and 4.90±3.82 m d−1 for the wet season and dry season, respectively, which were nearly 70% higher than that of large rivers. In addition, a significant correlation was observed between k600 and flow velocity, but not wind speed regardless of river size. From the basin-scale perspective, the CO2 emission fluxes from the entire ERB were explored. The results suggest small rivers are major contributors to riverine CO2 emissions, despite their relatively low CO2 concentrations and small water surface areas. They contributed disproportionately to 74.4% of the total fluxes due to high gas transfer velocity (k) across the water-air interface. Anthropogenic land use changes have substantially enhanced CO2 emissions from river networks. Normalized areal riverine CO2 fluxes in the urban- and cropland-dominated middle and lower ERB (27.6 and 39.4 g C m-2 yr-1, respectively) were two and three times higher than the 9.1 g C m-2 yr-1 in the forest-dominated Upper ERB. Due to the larger water surface area and higher k caused by monsoon-induced precipitation, the East River acts as a stronger carbon source during the wet season, emitting 0.67 Tg C to the atmosphere, about twice that during the dry season (0.33 Tg C). The third part centered on chemical weathering rates and weathering-induced carbon sink. The results indicate that carbonate rocks could contribute disproportionately to the total chemical weathering in this silicate-dominated river basin. The carbonate-weathering-induced carbon sink fluxes are responsible for more than 50% of the total weathering fluxes, even though carbonate rocks account for about 3% of the total area. In addition, secondary silicate minerals emerged as key factors in influencing silicate weathering rates, showing a chemostatic behavior with runoff. Moreover, silicate-weathering-induced carbon sink is less than 1% of the terrestrial carbon sinks but equivalent to 10% of riverine CO2 emissions. Although exerting little impact on the terrestrial carbon sinks, it has important implications for understanding the riverine carbon cycle. Overall, the three interrelated investigations have greatly improved the understanding of the complexity of riverine carbon cycling. The findings underscore the importance of small rivers in CO2 emissions, the role of climate and human activities in basin-scale CO2 fluxes, and the intricate relationships between chemical weathering and weathering-induced carbon sink. This study contributes to the broader context of regional carbon balance analysis and aids in comprehending the intricate interplays of various environmental and human factors in driving riverine carbon cycling processes.