Sedimentary Organic Carbon Burial Effect Of River Cascade Damming And The Influencing Mechanism Of Organic Carbon Burial

Artificial reservoirs play an important role in intercepting organic carbon,which affects the carbon cycle process of inland waters.However,limited by the methods of field observations,studies on the potential carbon sink of sediment in reservoirs are not accurate enough.Study of sedimentary organic carbon burial in reservoirs is significant for understanding the effect of reservoirs on regional and global carbon cycles.In this study,we investigated sedimentary carbon burial in seven reservoirs along the Wujiang River,Southwest China,by using a sub-bottom profiler with physicochemical data of sediment cores.This study analyzed the three-dimensional distribution of sediment in reservoirs,sedimentary carbon burial flux,sedimentary carbon burial rate,endogenous organic carbon burial flux and terrestrial organic carbon burial flux,and discussed the influencing factors of sedimentary organic carbon burial rate in reservoirs.It provided a promotional case study for the estimates of organic carbon burial in regional and global reservoirs.The main conclusions in this study were as follows(1)Underwater topography resulted in a highly spatial heterogeneity of sediment deposition.Thick sediment was accumulated in the deeper regions,and few sediments were deposited in the shallow areas.The sediment of canyon reservoirs is mainly deposited along the original river channel of the reservoir region,with the feature of large sediment thickness.(2)The organic carbon of sediment in reservoirs along the Wujiang River ranged from 0.89%to 7.86%.The total nitrogen and total phosphorus of sediment in these reservoirs varied from 0.12%to 0.84%,and from 0.35 mg/g to 3.68 mg/g,respectively.The C/N molar ratio of sediment ranged from 4.05 to 26.47,and the carbon isotope values of sediment varied from-31.0‰ to-15.3‰.A two-end-member mixing model coupled with carbon isotope values of sediment indicated that the autochthonous organic carbon in these reservoirs ranged from 0.7%to 97.7%of the total organic carbon in the sediment,with an average of 56.7%.The autochthonous organic matter generated in the reservoir was associated with nutrient input and long enough hydraulic retention time.(3)The average TOC in the surface sediment of the upstream reservoirs is higher than that of the downstream reservoir.After the input of terrigenous organic matter in the basin is intercepted by cascade damming,the flux of terrigenous organic carbon in the downstream reservoir decreases gradually,showing obvious cascade effect.The larger morphology ratio of the reservoir,the higher content of endogenous organic matter in the sediment of the reservoir.The reservoir morphology ratio is an important factor affecting the content of endogenous organic matter.(4)The mean sedimentary organic carbon burial rate in the Hongjiadu Reservoir,Dongfeng Reservoir,Hongfenghu Reservoir,Suofengying Reservoir,Wujiangdu Reservoir,Silin Reservoir and Pengshui Reservoir was 908.1 g C m-2 yr-1,2258.3 g C m-2 yr-1,236.3 g C m-2 yr-1,854.2 g C m-2 yr-1,579.2 g C m-2 yr-1,1426.5 g C m-2 yr-1 and 1700.9 g C m-2 yr-1,respectively.The sedimentary organic carbon burial rate was high in the reservoir with low dynamic ratio and low in the reservoir with high dynamic ratio,suggesting that the organic carbon burial rate in the sediment is controlled by reservoir dynamic ratio.(5)The sedimentary endogenous organic carbon burial flux in the Hongjiadu Reservoir,Dongfeng Reservoir,Hongfenghu Reservoir,Suofengying Reservoir,Wujiangdu Reservoir,Silin Reservoir and Pengshui Reservoir was 1.6×104 t C yr-1,1.4×104 t C yr-1,3.7×103 t C yr-1,1.0×103 t C yr-1,6.4×103 t C yr-1,1.3×104 t C yr-1 and 1.3×104 t C yr-1,respectively.The sedimentary autochthonous organic carbon burial flux in the Hongjiadu Reservoir,Dongfeng Reservoir,Hongfenghu Reservoir,Suofengying Reservoir,Wujiangdu Reservoir,Silin Reservoir and Pengshui Reservoir is equal to 5.3 times,14 times,4.1 times,1.4 times,2.2 times,3.2 times and 2.3 times of the carbon emission from each reservoir surface to the atmosphere as CO2-C,respectively.The ratio of the organic carbon burial flux to the CO2 emission flux from the water-air interface is usually large in reservoirs with long hydraulic retention time,which indicates that the reservoirs with long hydraulic retention time can form an effective "carbon sink".(6)The sedimentary organic carbon burial rate of cascade reservoirs along the Wujiang River was closely correlated with the runoff(p=0.044),recharge coefficient(p=0.077),burial efficiency(p=0.003),hydraulic load(p=0.084)and morphometric ratio(p=0.063)of the reservoir.There is no relationship between the sedimentary organic carbon burial rate in cascade reservoirs and reservoir area(p=0.387).The reservoir morphometric feature can provide extrapolation basis for the estimates of organic carbon burial in regional and global reservoirs.