Carbon Burial In A Typical Groundwater-fed Reservoir In Karst Area

Recent studies show that the terrestrial aquatic systems are playing an important role in the global carbon cycle through regulating and controlling carbon cycle in a catchment,which may be a significant part of the“missing sink”.Karst reservoir is a part of terrestrial aquatic systems,which changes the regional carbon cycle processes through coupling of karst geological system and terrestrial aquatic system under the control of karst background.However,at some extent,carbon cycle in a karst reservoir has been long ignored.Therefore,it is important for global or regional carbon cycle to study the carbon cycle characteristics,influencing factors and fluxes in a karst reservoir for analyzing the carbon budget and the effect of carbon source/sink about the karst reservoir in different scales.In this study,we collected settling particulate matter,surface sediment and sediment cores in a typical karst reservoir supplied by a large of groundwater?Dalongdong reservoir,Shanglin County,Guangxi Province?of southwest karst area for analyzing inorganic/organic carbon content in different processes?such as deposition process,decomposition and preservation,burial process?.The organic carbon sources in these sediments were studied through two models.We also discussed the influencing factors on organic carbon variations in different processes.Thes studies estimated the carbon flux in water-air interface and water-sediment interface using carbon budget methods.and also preliminarily evaluated the possible carbon sink effect in the Dalongdong reservoir.Results indicated that the carbon storage capacity in karst aquatic environment played an important role in regional carbon sink and even global carbon sink.Therefore,more attention on studies of karst carbon sink should be paid in the future.The results show that:?1?Inorganic carbon produced by karstification was discharged into the Dalongdong reservoir consistenting with groundwater flow and was transformed to organic carbon through algal assimilation in the form of biological carbon pump.These settling particulate organic carbon?SPOC?,including in autochthonous organic carbon and allochthonous organic carbon,was partially deposited into the reservoir bottom and then became the surface sediment.Then,a part of surface sediment was returned to the water column due to sediment mineralization in the form of CO₂ or CH₄,and the others was buried as more stable organic carbon.?2?The main sources of organic carbon of settling particulate matter and surface sediment were algae and soil.The surface sediment and settling particulate matter from algae accounting for45.82%and 31.57%,respectively.The algae deposition efficiency was 54.25%.In the deposition,decomposition and preservation processes in the reservoir,the spatial distribution of inorganic and organic carbon in the surface sediment was impacted by local geological environment features,water source,hydrochemical parameters,the thermal stratification effect,water level fluctuation and water flow velocity change.Algae consumed inorganic carbon in karst water through assimilation processes,which also produced co-precipitation with inorganic carbon.These processes promoted to the settlement of organic matter from algae,and slowed down their mineralization velocity,which finally improved carbon storage.?3?Environmental evolution of Dalongdong reservoir has been mainly affected by human activities since 1950s,such as Green for Grain,Rocky desertification control,land-use changes and reservoir maintenance.By comparing the carbon stable isotope ratios of the sediment core in other lakes and reservoirs,this study found that the lighter inorganic carbon isotope composition in karst groundwater made the lighter inorganic carbon isotope value in sediment core,indicating that the modes of water sources possibly affected the carbon stable isotope ratios in the sediments.In the sediment cores,the main sources of organic carbon was algae,accounting for 60.84%,followed by soil?22.93%?,domestic sewage?14.56%?and terrestrial plants?1.67%?.?4?The organic carbon accumulation rate?OCAR?and inorganic carbon accumulation rate?ICAR?in the Dalongdong reservoir increased logarithmically with increasing time.The mean of OCAR and ICAR was 116.42 gC.m^-2.a^-1 and 185.99 gC.m^-2.a^-1,respectively,which showed a special spatial distribution chanracteristics:the middle reaches>the upper reaches>the downstream.The deposition fluxes of organic carbon and inorganic carbon were 126.85 gC.m^-2.a^-1and 275.14 gC.m^-2.a^-1,respectively,and the buried fluxes of organic carbon and inorganic carbon were 107.55 gC.m^-2.a^-1 and 136.86 gC.m^-2.a^-1,respectively.Based on the carbon budget of different processes about carbon cycle,the carbon input amount was about 10478.65 t C.a^-1,which the main carbon input way was inorganic carbon of the ground water recharge.The carbon output amount was 11565.63 t C.a^-1,which the mainly output amount was inorganic carbon of 9566.66 t C.a^-1.The main output path was water drainage of hydropower station.The organic carbon output was 1998.97 t C.a^-1,and the main output path was organic carbon burial processes,which is an important carbon sink process.