| The terrestrial water system is an active area of the global carbon cycle.As an important part of inland waters,reservoirs have high carbon storage efficiency.Under the combined action of dam interception and reservoir storage,the habitat and trophic structure of the water body in the basin have been changed,and the organic carbon(OC)cycle mechanism of the river system has become more complicated.Identifying the source of OC in cascade reservoirs and estimating the contribution rate of OC from allochthonous and autochthonous sources in the reservoir can provide basic data for the calculation of carbon flux in the process of river cascade damming and impoundment,which is of practical significance for clarifying the true and effective "carbon sink".This paper takes four important reservoirs in the cascade development of the Pearl River Basin:Chaishitan Reservoir(CST),Longtan Reservoir(LT),Yantan Reservoir(YT)and Dahua Reservoir(DH)as the research objects.Analyzed the sediment cores samples,using lignin as a biomarker,combined with TOC and stable carbon isotope(?13C)indicators,from the horizontal watershed dimension and the vertical time dimension,the lignin content in the sediments,the lignin parent vegetation and lignin degradation parameters in space and time series have been investigated in detail.A three-end-member mixing model is used to identify the allochthonous and autochthonous sources of OC in the reservoir sediments selected in the study,which provides basic data for the study of the carbon cycle in the Pearl River Basin.The main conclusions of this study are as follows:1)Judging from the lignin content of the four selected reservoirs,the average value of the absolute lignin concentration ?8 of the sample is 2.58 mg/10 g,and the average value of the lignin concentration A8 in the total organic carbon(TOC)is 1.63 mg/100 mg OC,with the increase of the deposition time,the lignin content in the sediments has a significant increase trend,which is due to the increase in the water level after the water storage,which reduces the land area,the original vegetation is submerged and decomposed,and the lignin is released in sediments.The values of ?8 and ?8 of each reservoir have obvious spatial heterogeneity.2)The average value of the ratio of the C series monomer to the V series monomer(C/V)of the vegetation source parameters in the sediments of the cascade reservoirs in the Pearl River Basin is 0.31,and the average value of the ratio of the S series monomer to the V series monomer(S/V)is 0.86,indicating that it is angiosperm herb tissue,mixed with a small amount of gymnosperms and woody tissue.Among them,the herb tissue in CST sediments contributes more.After the construction of the reservoir,there is no significant change in the source of vegetation in the reservoir.It can be seen that the impact of river dams on the source of dominant plant populations in the reservoir area is small.3)The value for judging whether the lignin is derived from terrestrial plants in the cascade reservoirs of the Pearl River Basin.The ratio of hydroxyacetophenone to P series monomers(Pon/P)ranges from 0.05 to 0.38,with an average value of 0.16,which is a low,indicating that the p-hydroxyphenols in the lignin monomer are not only a source of land-derived vascular plants,that is,the proportion of fresh landderived vascular plants in the terrestrial organic matter(OM)of sediments is small,and the increase is small as the reservoir stores water.4)The average value of the lignin side chain oxidative degradation parameter(Ad/Al)v of in the Pearl River Basin is 0.47,the average value of(Ad/Al)s is 0.52,and the demethylation degradation parameter P/(V+S)is 0.28,indicating that the sediment is mainly oxidized and demethylated,and the demethylation effect is weak.Among them,CST has the highest level of degradation of sediments,which is mainly affected by runoff.5)The results of the three-end-member mixing model show that the source of OC in the sediment cores of the cascade reservoirs in the Pearl River Basin shows the OC source of sediments in the Pearl River Basin is mainly land-based OC.Therefore,reservoirs play a role as a "carbon source" in the global carbon cycle.The source of OC showed obvious characteristics of changes before and after water storage.Before water storage,the OC in the sediment is mainly contributed by soil sources,followed by freshwater plankton,and higher vascular plants have the least contribution.After water storage,rivers become lakes,and changes in flow velocity and water quality lead to problems such as sedimentation and eutrophication.The thermal stratification effect of the water body caused by the reservoir water makes the species and quantity of plankton increase from the bottom to the surface of the reservoir.Microbial activities intensify,leading to an increase in primary productivity and an increase in the contribution rate of endogenous OC.From the perspective of the watershed,the terrestrial OC source changes in the upstream and downstream reservoirs show obvious spatial heterogeneity,and the 1 terrestrial OC in the downstream reservoirs gradually decreases,showing a more obvious cascade dam interception effect.In addition,factors such as the regulation type of the reservoir,the replenishment of the reservoir interval,natural events(such as rainfall),and human factors will all have an impact on the changes in the source of terrestrial OC in the reservoir. |
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