Dynamics of CO2 partial pressure and CO2 outgassing in the lower reaches of the Xijiang River, a subtropical monsoon river in China

Abstract The partial pressure of carbon dioxide ( p CO 2 ) in surface water was surveyed monthly at 6 sampling sites along the entire length of the lower reaches of the Xijiang River, a subtropical monsoon river in China, and at the mouths of its major tributaries, over a whole hydrological year from April 2005 to March 2006, to reveal the seasonal and spatial dynamics of p CO 2 . Intensive sampling and measurements were also conducted at Wuzhou gauge station in June and July to investigate the impact of floodwater on p CO 2 and to further explore the relationship between river discharge and p CO 2 . The p CO 2 levels were well above atmospheric equilibrium (380 μatm) during the entire survey period with obvious seasonal and spatial variations, ranging from 600 μatm to 7200 μatm for the mainstream and from 700 to 11000 μatm for tributaries, respectively. The pattern of p CO 2 seasonal variation across 6 sites was almost consistent with each other with little difference. The p CO 2 levels in the dry season were relatively low, with relatively slight temporal and spatial fluctuations that were predominantly controlled by in situ biogenic activities. While the p CO 2 in the wet season greatly varied with river discharge, both annual maximum and minimum p CO 2 levels occurring in this period. The much higher p CO 2 in the early wet season were mainly induced by increasing baseflow and interflow that flushed significant soil CO 2 into the streams, whereas the lower p CO 2 observed after floods from July to September, some even lower than p CO 2 levels in the dry season, potentially resulted from in situ plankton blooms. The annual minima p CO 2 levels occurring in this period were caused by the dilution effect of floodwater. There was no obvious downstream trend in p CO 2 variation during the whole survey period, probably a consequence of disturbance from tributaries or spatially distinct channel characteristics and water environments. Based on measurements, we estimate that the water-to-air CO 2 flux in the lower reaches of the Xijiang River is about 8.3–15.6 Mg C ha − 1 y − 1 . The role of the Xijiang River as a net source of atmospheric CO 2 is undoubted.