Spatial-temporal Variations And Influencing Factors Of FCO₂ And PCO₂ From The Middle Of Yellow River

The oceanic carbon pool and terrestrial carbon pool are connected by rivers.Carbon dioxide(CO2)evasion from rivers to the atmosphere represents a substantial flux in the global carbon cycle.As China's second largest river,the Yellow River is the typical river containing high sediment concentrations in the world.In this study,two river cross-sections of the middle reach of the Yellow River,e.g.the Toudaoguai Gauging Station in Inner Mongolia and Tong Guan Gauging Station in Shaanxi Province,were chosen as the study sites.Emission of CO2(FCO2)and C02 partial pressure(pCO2)were measured using Li-7000 static chamber method and gradient method from 2013 to 2016 at the both sampling points.The spatial and temporal variations of FCO2 and pCO2 were analyzed.The relevant hydrological indexes,including water temperature,pH,discharge and current velocity as well as wind velocity were measured at the sampling points.The main conclusions of the paper are as follows:1.During the period of sampling,the seasonal change of pCO2 showed a similar trend at at the Toudaoguai and Tong Guan Gauging Stations.The average pC02 showed spatial difference at two sampling sites.The maximum pCO2 occurs in spring with thepCO2 as 2130 ?atm in Toudaoguai and 3368 ?atm in Tongguan.2.During the period of sampling,FCO2 in Toudaoguai and Tong Guan Gauging stations showed similar seasonal variation,but the FCO2 at the two sampling sites were different even in the same season.The highest value of FCO2 occurred in the autumn,with the value of 689 mmol m-2 yr-1 at Toudaoguai and 793 mmol m-2 yr-1 at Tong Guan,respectively.The second highest FCO2 occurred in summer,with the value of 456 mmol m-2 yr-1 and 609 mmol m-2 yr-1,respectively.The third highest FCO2 occurred in spring,with the value of 159 mmol m-2 yr-1 and 164 mmol m-2 yr-1,respectively.The minimum FCO2 occurred in the winter,with the value of 41 mmol m-2 yr-1 and 61 mmol m-2 yr-1,respectively.The average FCO2 at Tong Guan sampling was 376 mmol m-2 yr-1 which is higher than 283 mmol m-2 yr-1 at Tou Daoguai sampling sites.3.In the middle reach of Yellow River,the spatial-temporal changes of FCO2 and pCO2 were shown at Tong Guan and Tou Daoguai monitoring sites.The spatial-temporal changes also existed at the one river cross-section.We found different sampling points have obvious difference.Taken Tou Daoguai cross-section as an example,the FCO2 and pCO2 are at four sampling points obviously different from each other.S1 sampling point near the left bank has the minimum pCO2 of 949?atm and FC02 of 75 mmol m-2 yr-1.The pC02 and FCO2 at S2 sampling point were 1002 ?atm and 196 mmol m-2 yr-1,respectively.The pC02 and FC02 at S3 sampling point were 1080 ?atm and 347 mmol m-2 yr-1,respectively.S4 sampling point nearthe right bank has the maximum pCO2 and FCO2,1103 ?atm and 634 mmol m-2 yr-1,respectively.FCO2 gradually increase from left to right bank.This phenomenon is mainly influenced by flow velocity and water depth at different sampling points.The faster the low velocity is and the deeper the water depth is,the greater emission of CO2 is.4.We found the spatial-temporal changes of pCO2 at Tou Daoguai and Tong Guan monitoring sites were relevant to pH,discharge,and TSS.The analysis of factors influencing pC02 indicated that pCO2 was positively correlated with water temperature(Tou,R2=0.22,p<0.001;Tong,R2=0.3,p<0.001),and negatively correlated with discharge(Tou,R2=0.31,p<0.001;Tong,R2=0.42,p<0.001)and pH(Tou,R2=0.83,p<0.001;Tong,R2=0.98,p<0.001).Carbonate system,DO,salnity,chlorophyll and human activities are also the impact factors of pCO2.The analysis of factors influencing FCO2 indicated that FCO2 was positively correlated with water tempture(Tou,R2=0.41,p<0.001;Tong,R2=0.37,p<0.001)and flow velocity(Tou,R2=0.52,p<0.001),and uncorrelated with wind speed(Tou,R2=0.03,p>0.05).The aveage of pCO2 and FCO2 were 1826 patm and 136 mol m-2 yr-1,respectively in the middle of Yellow River.Compared with the other rivers in the world,the emission of C02 in the middle reach of the Yellow River is at a high level.This result proves that the Yellow River is a carbon source.The results of this study provides scientific basis for quantifying the magnitude of CO2 emission and spatial-temporal variations.At the same time,we also supply the reference how to set up pCO2 and FCO2 monitoring sites at river cross-section,especially for larges rivers.