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

Abstract

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 (δ13CDIC) 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.