Long-term spatial and temporal variation of COsub2/sub partial pressure in the Yellow River, China

摘要

Carbon transport in river systems is an importantcomponent of the global carbon cycle. Most rivers of the world act asatmospheric CO₂ sources due to high riverine CO₂ partial pressure(pCO₂). By determining the pCO₂ from alkalinity and pH, weinvestigated its spatial and temporal variation in the Yellow Riverwatershed using historical water chemistry records (1950s–1984) and recentsampling along the mainstem (2011–2012). Except the headwater region wherethe pCO₂ was lower than the atmospheric equilibrium (i.e. 380 μatm),river waters in the remaining watershed were supersaturated withCO₂. The average pCO₂ for the watershed was estimated at2810 ± 1985 μatm, which is 7-fold the atmospheric equilibrium. Asa result of severe soil erosion and dry climate, waters from the LoessPlateau in the middle reaches had higher pCO₂ than that from the upperand lower reaches. From a seasonal perspective, the pCO₂ varied fromabout 200 μatm to > 30 000 μatm with higher pCO₂usually occurring in the dry season and lower pCO₂ in the wet season (at73% of the sampling sites), suggesting the dilution effect of water.While the pCO₂ responded exponentially to total suspended solids (TSS)export when the TSS concentration was less than 100 kg m?3, itdecreased slightly and remained stable if the TSS concentration exceeded100 kg m?3. This stable pCO₂ is largely due to gully erosion thatmobilizes subsoils characterized by low organic carbon for decomposition. Inaddition, human activities have changed the pCO₂ dynamics. Particularly,flow regulation by dams can diversely affect the temporal changes ofpCO₂, depending on the physiochemical properties of the regulated watersand adopted operation scheme. Given the high pCO₂ in the Yellow Riverwaters, large potential for CO₂ evasion is expected and warrantsfurther investigation.