Riverine CO2 and CH4 concentrations and fluxes in the subtropical Pearl River system

Subtropical rivers and streams are identified as significant ecosystems of CO₂ and CH₄ emissions, yet their contribution to the global carbon cycle remains highly uncertain, partly due to field-based data paucity for the subtropics and spatial-temporal heterogeneity of CO₂ and CH₄ concentrations and fluxes. Here we examine the regional pattern of CO₂ and CH₄ concentrations and fluxes from headwater catchments (i.e., the Xijiuxi, Xiaojianghe, Liujiang, and Nanshanhe river catchments) and large river basins (i.e., the Xijiang, Beijiang, and Dongjiang river basins) in the subtropical Pearl River basin in south China. The river water CO₂ partial pressure (pCO₂) ranged from 208 to 3141 μatm and 433 to 4527 μatm during the high flow season and the low flow season, respectively. Positive relationships between CO₂ partial pressure (pCO₂) and dissolved oxygen (DO) and between pCO₂ and the stable carbon isotope of dissolved inorganic carbon (δ¹³C_DIC) demonstrated that aquatic photosynthesis and CO₂ exchange at the water-air interface play significant roles in controlling the magnitude of stream water pCO₂. The rivers were consistently oversaturated in CH₄, ranging from 14 to 11119 μatm during the high flow season and 43 to 9596 μatm during the low flow season. The mean CO₂ effluxes showed higher values in the high flow season (97 mmol m^-2 d^-1) and lower values in the low flow season (28 mmol m^-2 d^-1). The results also showed that CO₂ effluxes in the four headwater streams were much higher than those in the three large rivers during both seasons. This suggested that headwater streams are significant sources of CO₂ for the atmosphere. In comparison, the mean CH₄ fluxes were 6.3 mmol m^-2 d^-1 (high flow season) to 0.5 mmol m^-2 d^-1 (low flow season), and CH₄ concentrations and fluxes were higher in high flow season than in low flow season in headwater streams. Additionally, dissolved CH₄ concentrations in urban and agricultural rivers are higher than those in forest rivers. This study highlighted the significant role of CH₄ emissions from urban and agricultural river systems and CO₂ emissions from headwater streams in riverine carbon cycling.