Riverine carbon export in the arid to semiarid Wuding River catchment on the Chinese Loess Plateau

Riverine export of terrestrially-derived carbon represent a key component of the global carbon cycle. In this study we quantify the redistribution of riverine carbon within the Wuding catchment on the Chinese Loess Plateau. Export of dissolved organic and inorganic carbon (DOC and DIC) exhibited pronounced spatial and temporal variability. While the DOC concentration was spatially comparable within the catchment, it was generally higher in spring and summer than in autumn, especially in the loess subcatchment. This reflects the enhanced organic matter inputs from agricultural tillage in spring and from terrestrial ecosystems in summer. DIC concentration in the loess subcatchment is significantly higher than that in the sandy subcatchment, due largely to dissolution of carbonates that are abundant in loess. In addition, content of particulate organic carbon (POC) shown strong seasonal variability with low values in the wet season owing to input of subsurface soils by gully erosion. The downstream carbon flux was (7±1.9)×10 10  g C year −1 and dominated by DIC and POC. Total CO 2 emissions from water surface were (3.7±0.5)×10 10  g C year −1 . Radiocarbon analysis revealed that the degassed CO 2 was 810–1890 years old, indicating the release of old carbon previously stored in soil horizons. Riverine carbon export in the Wuding catchment has been greatly modified by check dams. Our estimate shows that carbon burial through sediment storage was (7.8±4.1)×10 10  g C year −1 , representing 42% of the total riverine carbon export from terrestrial ecosystems on an annual basis ((18.5±4.5)×10 10  g C year −1 ). Moreover, the riverine carbon export accounted for 16 % of the catchment NEP. It appears that the magnitude of carbon sink of terrestrial ecosystems in this arid-semiarid catchment has been significantly offset by riverine carbon export.