Magnitude of and Hydroclimatic Controls on CO2 and CH4 Emissions in the Subtropical Monsoon Pearl River Basin

Abstract Rivers are important ecosystems for carbon emissions and play a crucial role in the global carbon cycle. However, CO 2 and CH 4 emissions from subtropical rivers are substantially under‐represented in global‐scale estimates. Here, we explored the regional patterns of riverine CO 2 and CH 4 dynamics in the Pearl River basin with a subtropical monsoon climate. We found that its CO 2 and diffusive CH 4 emissions showed a decreasing trend with increasing stream order. Seasonality in CO 2 and diffusive CH 4 emissions was primarily driven by variations in partial pressure of CO 2 ( p CO 2 ) and CH 4 ( p CH 4 ) and gas transfer velocities, which were strongly regulated by hydrology and climate. We further estimated the basin‐wide CO 2 and diffusive CH 4 fluxes at 17.8 ± 7.4 Tg C yr −1 and 191.5 ± 139.9 Gg C yr −1 , respectively. When normalized to the water surface, the mean diffusive fluxes were 790.1 and 8.5 mmol m −2 d −1 for CO 2 and CH 4 , respectively, which were 1.3 and 2.5 times higher than the global mean riverine CO 2 and CH 4 fluxes, respectively. This suggests that the global significance of subtropical rivers is probably underestimated because their substantially higher CH 4 fluxes are unaccounted for. Furthermore, compared with measured p CO 2 , the alkalinity‐based p CO 2 could introduce significant errors by 20% at ∼30% of the sampling sites, underscoring the necessity of direct measurements to reduce uncertainty. This study provides the first estimate of basin‐wide CO 2 and diffusive CH 4 emissions in the PRB through direct p CO 2 and p CH 4 measurements, and highlights the role of hydrologic and climatic factors in governing riverine carbon emissions.