| Outgassing of CO2 across water-air interface in river systems has been given increasing attention. Prior studies have been mainly focused on the estimation of CO2 outgassing fluxes and the underlying influencing factors of riverine pCO2. As a representative turbid river in the world, the Yellow River plays an important role in China’s carbon budget assessments and in the global biogeochemical cycle of carbon.Based on two hydrological stations, namely Toudaoguai in Inner Mongolia and Tongguan in Shaaxi, in the middle mainstem channel of the Yellow River during the period July 2013 and July 2014, water and sediment samples were collected and analyzed.By applying different methods of headspace gas-chromatography and Li-7000, the pCO2 in surface river water was measured, and CO2 outgassing fluxes were further estimated. A comparative analysis of the two stations shows the contribution of the Loess Plateau to riverine carbon export by analyzing the spatial-temporal changes of the riverine pCO2 and the CO2 outgassing fluxes. Furthermore, the underlying influencing mechanisms controlling CO2 emission on the Loess Plateau were investigated. The results will shed light on future studies on carbon dynamics and CO2 outgassing in the Yellow River network. Three major conclusions can be drawn as follows:1, The pCO2 presented strong spatial and temporal changes between the Toudaoguai and Tongguan stations. The average pCO2 at Toudaoguai was 2962 uatm and at Tongguan station was 5079 uatm. The average pCO2 in the wet season at Toudaoguai station was higher than that in the dry season. The values were 3419 and 2686 respectively. The average value in the dry season at Tongguan station was higher than that in the wet season. The average value was 5158 μatm and 4430 μatm, respectively. In addition, alkalinity (Alk) also showed significant spatial variations. It showed stronger seasonal differences at Tongguan station than that at Toudaoguai station.2, The riverine pCO2 at Toudaoguai station increased with water discharge (Q) and total suspended sediment (TSS) in the wet season. The higher pCO2 in the wet season is largely because of the strong chemical weathering of carbonates on the Loess Plateau soils. Based on the year-long dataset at the two stations, there is no significant correlation between pCO2 and Q and TSS. This indicates the complexity of pCO2 changes and its controlling factors in a watershed system. Furthermore, the pCO2 at both stations was higher in the wet season than that in the dry season. This reveals the combined results of high temperature and high flood runoff in the wet season, suggesting the impact of hydrological regime of the Loess Plateau. The generally higher pCO2 at Tongguan station than at Toudaoguai station is because the Yellow River flows through the Loess Plateau, from which huge amounts of terrestrial carbon is exported into the river network. Thus, decomposition of terrestrial carbon would result in a higher pCO2 value at Tongguan station.3, The CO2 outgassing fluxes in the middle reaches of the Yellow River were estimated 125.6 mol/m2/yr, similar to the tropical Amazon River. The CO2 outgassing at Toudaoguai station showedhigher fluxes in summer and autumn than in the other two seasons. Among the controlling factors of CO2 emission, the CO2 effluxes present a positive correlation with water temperature T (℃) and flow velocity, while a negative correlation with pH. No significant relationship between CO2 outgassing and wind velocity was observed. cO2二氧化碳(Carbon Dioxide) DIC (mg/L)溶解无机碳(Dissolved Inorganic Carbon) DOC (mg/L)溶解有机碳(Dissolved Organic Carbon) pCO2 (μatm)水体二氧化碳分压(Partial Pressure of CO2) Q (m3/s)径流量(Water Discharge) Alk (μmol/L)碱度(Alkalinity) TSS (mg/L)颗'总悬浮固体(Total Suspended Sediment/Solids) Flux (mmol/m2/d) CO2逸出通量(CO2 Outgassing Fluxes)... |
PDF Full Text Request