Large spatiotemporal shifts of CO2 partial pressure and CO2 degassing in a monsoonal headwater stream

Abstract Carbon dioxide (CO2) emissions from rivers are a vital part of the global carbon budget. However, data from subtropical areas, especially in the headwater streams, are scare. Spatiotemporal dynamics and drives of partial pressure of CO2 (pCO2) and water-air CO2 fluxes (FCO2) in a monsoonal headwater stream Jinshui River of the Yangtze were unraveled. Our findings suggested that natural process (i.e., hydrology, CO2 outgassing and lithology) and human activities were recognized as significant players in regulating the variability of riverine pCO2. The mean pCO2 was significantly higher in the dry season (1562 ± 975 µatm) than the wet season (834 ± 639 µatm), seasonal trends of pCO2 were controlled by in situ biogenic activities and rainfall events. The slight fluctuations of pCO2 from upstream to downstream along main stem implied the mixed influences of distinct water environments and anthropogenic disturbance. Correlation analysis showed that environmental factors, i.e., temperature, pH, TN and DOC were relevant to pronounced spatial and seasonal variability of pCO2. We highlighted that high water-air CO2 flux was estimated at 343 ± 413 mmol/m2/d (dry: 542 ± 477 mmol/m2/d vs wet: 192 ± 278 mmol/m2/d) in the monsoonal headwater stream, and the watershed-scale carbon budget demonstrated carbon loss via atmospheric exchange was 1.2 times the riverine dissolved carbon export. Our results would fill a large gap in the headwater stream of the Yangtze, and help to accurately estimate the regional to global CO2 outgassing from rivers to the atmosphere.