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

Abstract

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 15Wuding 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 20 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 CO2 emissions 25 from water surface were (3.7±0.5)×10 10 g C year 1 . Radiocarbon analysis revealed that the degassed CO2 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 30 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.

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

“…Preferential outgassing of 12 CO 2 may also have contributed to the more depleted δ 13 C values in the emitted CO 2 than that of the C 4 plants. Aquatic algae with their δ 13 C value ranging from −40 to −26 ‰ (Alin et al, 2008) is likely another contributor, as suggested by the 2-fold higher Chl a contents in summer and autumn than in spring at some sites (Ran et al, 2017). Deeply incised stream channels provide favorable stagnant water, albeit highly site specific, for algae growth during non-flooding periods.…”

“…While detailed information has been provided in Ran et al (2017), a brief description is provided here. Three sampling campaigns were conducted in the Wuding River catchment in 2015: before the wet season (March-April; denoted as spring), during the wet season (July-August; sum- Figure 1.…”

“…The major objectives are to (1) explore the spatial and temporal variability in riverine carbon export, (2) trace the sources and age of the emitted CO 2 using carbon isotope techniques, and (3) evaluate the riverine carbon cycle in relation to the catchment's terrestrial ecosystem production. This study is built upon our earlier work of Ran et al (2017) in which we analyzed environmental controls and dam impacts on riverine CO 2 emissions. These results will provide insights into riverine carbon studies for rivers in arid to semiarid climates and improve the accuracy of extrapolation from watershed-based carbon studies to global-scale estimates.…”

Integrating Aquatic and Terrestrial Carbon Fluxes to Assess the Net Landscape Carbon Balance of a Highly Erodible Semiarid Catchment

“…Fluvial C is primarily of terrestrial origin and its transport in stream networks is closely linked to watershed hydrology. Heavy storm events typically promote the flushing of C‐rich soils leading to high DOC and POC fluxes, whereas the predominance of groundwater sources during baseflow contributes to high DIC flux in streamflow (Ran et al., 2018; Roebuck et al., 2018; Yoon & Raymond, 2012). The decrease in DOC and DIC concentrations in stream and river waters with increasing discharge (Figure 3a) can be attributed to the dilution effect, as widely observed in other river systems (e.g., Duvert et al., 2020; Roebuck et al., 2018; Reiman & Xu, 2019).…”

Soil Organic Carbon Redistribution and Delivery by Soil Erosion in a Small Catchment of the Yellow River Basin

“…Bouchez et al (2010) indicated that OC petro was oxidized during transport in the Amazon River, at a rate of 0.25 Mt·C·yr −1 to the atmosphere. Ran et al (2018) measured the radiocarbon ages of degassed CO 2 during transit in the Wuding River basin and revealed that a significant fraction of old carbon was released to the atmosphere during fluvial transport (Figure 9). On geological timescales, the oxidation of OC petro during fluvial transport is a significant flux for the atmospheric CO 2 budget (Bouchez et al, 2010; Galy et al, 2008).…”