Dynamic controls on riverine pCO2 and CO2 outgassing in the Dry-hot Valley Region of Southwest China

Abstract

Inland waters frequently exhibit carbon dioxide (CO2) supersaturation and potentially serve as CO2 emitters to the atmosphere. However, study on water–air interface CO2 exchange from small rivers remains limited, which causes large uncertainty of local and global carbon budgets. Based on seasonal field measurements, carbonate equilibria (CO2SYS program) and empirical models of gas transfer velocity (k value), we investigated the CO2 partial pressure (pCO2), CO2 emission and its controlling factors in the Longchuan River, a subtropical river in the Dry-hot Valley Region of Southwest China. Over 88% of samples were found to be oversaturated with atmospheric CO2, and the mean pCO2 ranged from 1497 ± 1415 μatm (dry period) to 1734 ± 1648 μatm (post-wet period). With the calibrated k values of 43.4–80.5 cm/h, the CO2 emission rates were estimate at 443–514 mmol/m2/day. We highlighted that long-term gully erosion enhanced the buffering capacity of carbonates in the aquatic environment and resulted in seasonal differences in riverine pCO2. Intensive agricultural practices could alter the pCO2 and further stimulate CO2 emissions. There were positive correlations of pCO2-elevation and pCO2-DOC in the initial-wet period but not in the dry period. This study suggests that extreme soil erosion and anthropogenic activities can regulate environmental variables and thus greatly affect the riverine CO2 outgassing. Our findings provide key insights into the understandings of dynamic controls on riverine pCO2 and CO2 outgassing under global change.