| Carbon is a fundamental element and primary component for natural minerals,which not only plays a dominant role in the reproduction and growth of living organisms,but also affects the various biogeochemical processes and is generally accompanied by ecological,hydrological and climatic changes.River is regarded as the important form of inland waters,which links the lands and oceans,and thus exhibits the major role in carbon storage,transport,conversion and emissions.In aquatic environments,dissolved carbon and particulate carbon are main forms of river carbon.Particularly,dissolved carbon(including dissolved organic carbon,DOC and dissolved inorganic carbon,DIC)is largely affected by environmental variables,which exhibits the highly variable in rivers and thus characterizes the global and regional carbon cycling.Besides,worldwide rivers are supersaturated with carbon dioxide(CO2)due to the inputs of terrestrial source and anthropogenic activities,and thus have a high potential in CO2 emissions in globe.Meanwhile,the in-situ aquatic respiration contributes to riverine CO2 supersaturation,and converts the aquatic DOC to DIC species.In this context,the study of water-air CO2 flux and riverine DOC biodegradation becomes a frontier issue in current earth science.The Yangtze River represents as the largest river in China,which exhibits a wide distribution range and a large number of tributaries.Currently,it is urgent to investigate the ecohydrological impacts on dissolved carbon transport and riverine CO2 outgassing in the Yangtze River,especially for its upstream tributaries.However,previous observations were mainly concentrated on main stem of the Yangtze River,little was known about the dissolved carbon transport and riverine CO2 evasion in its upstream tributaries,and thus is necessary to supplement available information in the typical tributaries of the Yangtze River.Here I investigated 39 first-order tributaries and 27 second-order tributaries in the Three Gorges Reservoir(TGR)region(including the Daning River),as well as the Longchuan River in the headwater.The various topographic,climate and eco-environmental impacts on riverine dissolved organic transport and CO2 emissions were examined.This study is expected to improve the understandings of regional and global carbon transport and outgassing under global change.The main results are as follows:(1)The spatio-temporal variations of riverine dissolved organic were distinct when it compared the representative tributaries in the upper Yangtze River.For example,The DIC concentrations were higher in the dry season than that in the rainy season(p<0.001),and averaged 2875.3±844.0 ?mol/L in the TGR rivers.However,there were no significant seasonal differences in the Daning River and Longchuan River(p>0.05),and the average DIC concentrations were 2630.5 ± 518.2 ?mol/L and 3320.1±867.1 ?mol/L,respectively.Riverine HCO3-was considered as the dominant component(occupying 97%)of DIC for all rivers in this study,and followed by CO32-.On the other hand,the DOC concentrations ranged from 4.34 mg/L to 41.55 mg/L and averaged 12.10±6.20 mg/L in the TGR river,and the concentration frequency was mainly distributed in the interval of 5-10 mg/L.The DOC concentrations were low in the Daning River,and the mean value was 1.52 ± 1.65 mg/L.The seasonal variations of DOC were significant in the Longchuan River(p<0.05)and decreased as follows:initial-rainy season(5.75 ± 1.52 mg/L)>post-rainy season(4.93 ± 1.49 mg/L)>dry season(4.56 ± 1.67 mg/L),and the annual average DOC concentration was 5.06 ± 1.62 mg/L.(2)The UV-Vis Spectroscopy could explain the sources,compositions and molecular information of dissolved organic matter(DOM)of the representative tributaries in the upper Yangtze River.For instance,a254 values indicated the aromatic abundance of DOM,which followed a descending order of the TGR rivers(18.5±12.2 m-1)>Longchuan River(17.2±9.5 m-1)>Daning River(7.40 ± 10.1 m-1);a280 values represented the protein abundance of DOM,and were decreased as follows:TGR rivers(13.3±8.63 m-1)>Longchuan River(12.9± 8.55 m-1)>Daning River(4.79±3.86 m-1);a350 values explained the lignin abundance of DOM,and followed a descending order of Longchuan River(4.40±1.89 m-1)>TGR rivers(4.13 ± 3.89 m-1)>Daning River(1.47±0.93 m-1).E2:E3 values tracked changes in the relative size of DOM molecules,which were decreased as follows:TGR rivers(6.85 ± 4.89)>Daning River(6.46 ± 7.78)>Longchuan River(5.41±5.01);E4:E6 values exhibited the aromaticity of DOM,which showed the following descending order of TGR rivers(3.14±3.70)>Longchuan River(2.13 ±0.74)>Daning River(1.68±0.84);The spectral slope S275-295 and S350-400 revealed the molecular weight of DOM,which exhibited the similar tend to the E4:E6 values;The SR values were high in the Daning River(2.95±4.62),indicating the high plant or algae sources and low terrestrial origins,and followed by TGR rivers(1.82± 3.15)and Longchuan River(1.59±0.73).(3)Dissolved carbon transport was distinct when it compared the representative tributaries in the upper Yangtze River.Particularly,TGR river exhibited the high transport potential.The annual discharge of DIC and DOC in 36 first-order rivers were 2.62 Tg C/y and 1.01 Tg C/y,respectively.Meanwhile,I estimated that annual discharge of riverine dissolved carbon(DIC and DOC)in the Daning River were 0.081 and 0.004 Tg C/y,respectively.The Longchuan River exhibited the low runoff(7.76 × 108 m3/y),and I reported that the river delivered 0.031 Tg C/y of DIC flux and 0.004 Tg C/y of DOC flux into the Jinshajiang River.(4)The partial pressure of aqueous CO2(pCO2)in most investigated rivers in this study were higher than the atmospheric CO2 equilibrium(410 ?atm),and thus have a high potential of CO2 emissions.The pCO2 exhibited pronounced spatial and seasonal variations in the TGR rivers(p<0.05),and the pCO2 in the dry season(1572.9 ± 1059.6 ?atm)was higher than that in the rainy season(1276.3 ± 1166.0 ?atm).The riverine pCO2 levels were mainly concentrated in the interval of 500-2000 Natm.The pCO2 in the Daning River showed significant mothly shifts(p<0.001)but no seasonal differences(p>0.05),which ranged from 483.2 ± 293.9 ?atm in May to 2183.3 ± 1309.2 ?atm in Aug.The intensive agricultural practices and serious soil erosion affected the aquatic environments in the Longchuan River,and the spatio-temporal variations of pCO2 was statistically insignificant(p>0.05).The riverine pCO2 levels in the representative tributaries of the upper Yangtze River were decreased as follows:Longchuan River(1647.7 ± 1540.2 patm)>TGR rivers(1424.6 ±1119.8 patm)>Daning River(1198.2 ±1122.9 ?atm).(5)The water-air CO2 areal fluxes in rivers were affected by riverine pCO2 and gas transfer velocity(k).Particularly,current velocity is taken as the fatal factor in riverine k value calibrations due to its dominant role in water turbulence.The k values varied within 15.1-80.5 cm/h in this study,which were higher than the world average(8-15 cm/h).There were no significant differences by Mann-Whitney U test(p>0.05)when using different k values and mean k values for CO2 emission rate calculations,and the uncertainties were within 10%.The CO2 emission rates decreased in the following order:Longchuan River(442.5-513.5 mmol/m2/d)>Daning River(329.8 ± 470.2 mmol/m2/d)>TGR rivers(113.6?338.0 mmol/m2/d).The annual CO2 fluxes were high on the TGR rivers(1.01 Tg C/y),and followed by the Daning River(0.004 Tg C/y)and Longchuan River(0.004 Tg C/y).(6)The DOC biodegradation(%BDOC)exhibited the significant seasonal differences in the Longchuan River(p<0.001),and the%BDOC was higher in the dry season than that in the rainy season.The average%BDOC varied from 33.5%to 62.5%in the Longchuan River.The elevated temperature could enhance the riverine%BDOC,and the Q10 values in the rainy season and dry season were 2.12 ± 1.65 and 1.70±0.53,respectively.The DOM optical parameters exhibited the pronounced variations after DOC incubation.The results suggested that BDOC would be preferentially biodegraded in natural waters,and the large molecular DOMs were incompletely degraded and accumulated.Meanwhile,the partial aromatic compounds could be also degraded when it extremely lacked the BDOC,which could decrease the SR values in aquatic DOM.(7)The abnormal high TDN:TDP ratio suggested the phosphorus(P)limitation,and indicated that P availability was a dominant driver on DOC biodegradability in the Longchuan River.Riverine%BDOC exhibited the co-variations with DOM optical parameters,which illustrated that DOM could be the reliable indicators for DOC biodegradability.On the other hand,the significant relationships between%BDOC and DIC,DOC:DIC andpCO2 highlighted the microbial controls on the DOC biomineralization.In summary,dissolved carbon transport and riverine CO2 emissions in the representative tributaries of the upper Yangtze River were explored,the main results indicated the nonnegligible potential of DOC and DIC transport and CO2 emissions in the investigated rivers.This study supplemented the available information about regional carbon emissions,and was expected to improve the understandings of regional and global carbon cycling. |
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