Daily CO2 partial pressure and CO2 outgassing in the upper Yangtze River basin: a case study of the Longchuan River, China

Rivers have been under sampled to investigate carbon degassing, especially in the tropical and sub-tropical regions. An unprecedented high-temporal-resolution (daily) sampling during July 2008–August 2009 was conducted in the Longchuan River of the upper Yangtze basin, a subtropical monsoon river in China, to reveal the daily-to-seasonal dynamics of the partial pressure of CO₂ (pCO₂) and CO₂ degassing flux from the river using Henry’s constant and CO₂SYS. The pCO₂ levels ranged from 230 to 8300 μatm with an average of 1230 μatm and obvious daily and seasonal variations. More than 92% samples were supersaturated with CO₂ in contrast to the atmospheric equilibrium (380 μatm). pCO₂ values in the river water in the wet season were relatively low, except in the flooding event in November, due to a dilution effect by heavy rainfall. In contrast, the pCO₂ levels in the dry season were much higher, due to lower pH resulted from anthropogenic activities. Net CO₂ degassing and pCO₂ were strongly correlated with dissolved nitrogen, but weakly with water temperature, dissolved inorganic carbon and water discharge, and uncorrelated with particulate nutrients and biogenic elements. The estimated water-to-air CO₂ degassing flux in the Longchuan River was about 27 mol/m²/yr, with the upper limit of 50 mol/m²/yr. Our study also indicated that among the carbon remobilized from land to water, around 7% (2800 t C/yr) of the total carbon was emitted to the atmosphere, 42% (17,000 t C/yr) deposited in the river-reservoirs system and 51% (21,000 t C/yr) exported further downstream. High spatial and temporal resolution of estimates of CO₂ emission from the world large rivers is required due to extremely heterogeneous catchment characteristics and anthropogenic activities in space and time.