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

Abstract

<p>Rivers are important ecosystems for carbon emissions and play a crucial role in the global carbon cycle. However, CO₂ and CH₄ emissions from subtropical rivers are substantially under-represented in global-scale estimates. Here, we explored the regional patterns of riverine CO₂ and CH₄ dynamics in the Pearl River basin with a subtropical monsoon climate. We found that its CO₂ and diffusive CH₄ emissions showed a decreasing trend with increasing stream order. Seasonality in CO₂ and diffusive CH₄ emissions was primarily driven by variations in partial pressure of CO₂ (<em>p</em>CO₂) and CH₄ (<em>p</em>CH₄) and gas transfer velocities, which were strongly regulated by hydrology and climate. We further estimated the basin-wide CO₂ and diffusive CH₄ 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 CO₂ and CH₄, respectively, which were 1.3 and 2.5 times higher than the global mean riverine CO₂ and CH₄ fluxes, respectively. This suggests that the global significance of subtropical rivers is probably underestimated because their substantially higher CH₄ fluxes are unaccounted for. Furthermore, compared with measured <em>p</em>CO₂, the alkalinity-based <em>p</em>CO₂ 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 CO₂ and diffusive CH₄ emissions in the PRB through direct <em>p</em>CO₂ and <em>p</em>CH₄ measurements, and highlights the role of hydrologic and climatic factors in governing riverine carbon emissions.<br></p>