Transformation of dissolved inorganic carbon (DIC) into particulate organic carbon (POC) in the lower Xijiang River, SE China: an isotopic approach

Abstract

The sources and dynamics of riverine carbon have been discussed extensively, but knowledge about the transformation from DIC into organic carbon (OC) is still poorly understood. In this study, we conducted a comprehensive investigation on the riverine carbon, stable carbon isotopic components and C/N ratios for different seasons, including an extreme flood event, in the lower Xijiang and its three tributaries. Detailed analyses are also performed for soil samples across the study region. Downstream increase in δ<sup>13</sup>C<sub>DIC</sub> and downstream decrease in both δ<sup>13</sup>C<sub>POC</sub> and C/N have been observed for all the tributaries. Meanwhile, positive shift of δ<sup>13</sup>C<sub>DIC</sub> and negative shift of δ<sup>13</sup>C<sub>POC</sub> are also observed from summer to winter. These observations likely indicate that the isotopic compositions of both DIC and POC are significantly affected by in-river primary production that converts DIC into organic matter through photosynthesis. It is estimated that the percentage contribution of the riverine aquatic primary production to the riverine POC in the Xijiang and three tributaries of Guijiang, Hejiang and Luoding is respectively 7.1%, 43.2%, 36.4% and 9.9% in rainy season, and 35.6%, 47.3%, 50.3% and 40.1% in dry season. Based on the stoichiometry involved in chemical weathering of the bedrocks, the transformation of the carbonate-sourced DIC to POC is further quantified to be 3.4–20.5% in rainy season, and 12.3–22.1% in dry season. This may suggest an important sink of atmospheric CO<sub>2</sub> in river systems that was largely ignored previously

Sources and Dynamics of Inorganic Carbon within the Upper Reaches of the Xi River Basin, Southwest China

“…Li et al [ 28 ] used carbon isotopic composition and major ion data from river and spring waters to confirm that sulfuric acid acted as an agent of carbonate weathering in the Beipan River and highlighted its role in combination with atmospheric CO 2 on controlling carbonate weathering rates. Although several articles have reported on seasonal variations in pCO 2 as well as the DIC contents and isotopic compositions in the Xi River [ 18 , 28 – 31 ], spatial variations in inorganic carbon isotopes and the dynamics of pCO 2 (including CO 2 outgassing) are not well known within the upper reaches of Xi Basin. Headwater basins usually emit more CO 2 because of higher CO 2 partial pressures, water turbulence and wind/flow velocity [ 32 ].…”

Response of epikarst hydrochemical changes to soil CO2 and weather conditions at Chenqi, Puding, SW China

“…Soil CO 2 formed by root respiration and microbial decomposition diffuses to the atmosphere as an upward flux called soil respiration while part of it dissolves in soil water as corrosive H 2 CO 3 which could further react with carbonates and finally infiltrates into the groundwater (Brook et al, 1983;Jassal et al, 2004;Liu et al, 2007). HCO À 3 as one of the reaction products could be utilized by the photosynthesis of aquatic organisms and fixed as organic matter as a carbon sink when the groundwater is exposed to surface Sun et al, 2011).…”

Understanding the potential sources and environmental impacts of dissolved and suspended organic carbon in the diversified Ramganga River, Ganges Basin, India

“…Through temporary mechanism, for instance primary production and respiration, microbial consumption, organic and inorganic carbon in the river can transform to each other (Sun et al, 2011). The upper reach of the Ramganga River and its tributaries flowing in Himalayas shows low concentrations of DOC and DIC as compared to middle and lower reaches and the value of turbidity is also low in this stretch (Fig.…”

“…High concentrations of suspended organic and inorganic matters are a relentless component of numerous substantial streams (Smith et al, 1987). As similar to the dissolved carbon, huge RG2 RG3 RG4 RG5 RG6 RG7 RG8 RG9 RG10 RG11 RG12 RG13 RG14 RG15 RG16 T1 T2 T3 T4 T5 T6 T7 T8 T9 T10 Elevation ( RG2 RG3 RG4 RG5 RG6 RG7 RG8 RG9 RG10 RG11 RG12 RG13 RG14 RG15 RG16 T1 T2 T3 T4 T5 T6 T7 T8 T9 spatial variation has also been shown in the concentration of SOC, whereas SIC does not show the similar pattern in the Ramganga basin (Fig.…”

“…On the other hand, a value of DIC decreases in the upper and increases in the middle and lower reaches of the Ramganga River and its tributaries. According to Sun et al (2011), for most rivers, bicarbonate is the dominant form of DIC, primarily from chemical weathering of silicates and carbonates in the drainage basin. Soil erosion is the essentially derived source for DOC and SOC in the river which shows higher turbidity values, however, commitment of aquatic biomass may possibly enhance considerably in low turbid rivers (Ludwig et al, 1996).…”