Sources Of Inorganic And Organic Carbon And Karst Carbon Cycle In Lijiang River Basin

The karst carbon sink produced by carbonate mineral weathering coupled with aquatic photosynthesis can be up to one third of unbalanced global carbon budget.The magnitudes,variations and mechanisms responsible for the carbon sink produced by carbonate mineral weathering coupled with aquatic photosynthesis which termed coupled carbonate weathering(CCW)are still unclear.Bicarbonate in karst catchment was mainly sourced from the weathering of carbonates.With the participation of aquatic photosynthetic organisms,bicarbonate was converted into organic carbon,forming a stable and long-time carbon sink,and affected climate on a long time scale.River was an important place for carbon transformation and an important channel for carbon transport.The riverine inorganic carbon and organic carbon can reflect the weathering of carbonates and aquatic photosynthesis in karst watershed,but the sources and variation characteristics are complex.Based on the CCW model,this paper focused on a typical karst catchment,analysed the sources of inorganic and organic carbon and their spatiotemporal variation,revealed the variation of karst carbon cycle,evaluated the karst carbon sink.The analytical results will help to improve evaluation of karst carbon sink flux transported by river,and make a new scientific contribution to solve the major scientific problem of“stability of karst carbon sink”and the quantitative calculation of karst carbon sink.The Lijiang River basin,a typical karst catchment,was selected for research for this paper.In different geological backgrounds and different land uses,the dissolution test of standard test pieces was carried out to obtain the dissolution rate of test pieces and the physical and chemical properties of soil:water content,electrical conductivity,p H,total organic carbon(TOC),total nitrogen(TN).In this study,we addressed seasonal variations in water chemistry at nine sampling locations,deil monitoring under four different weather conditions at site YS,a long-time regular monitoting and sampling at sites LQ,DR,GL and YS.The water chemistry parameters such as p H,DO,special conductivity,chlorophyll and HCO₃^-were tested in situ.The anion,cation,DOC,POC,?¹³C_DIC,?¹³C_POC,?¹³C_DOC and biomarkers of water samples have been tested in the laboratory.The results showed that:(1)The dissolution of carbonates was mainly controlled by the rainfall and hydrological in Lijiang River basin.There was a positive correlation between the dissolution of the samples and the precipitation or the accumulative rainfall time.While the time and intensity of the reaction between the samples and the rainfall decreased when the samples were screened by vegetation,and the dissolution decreased significantly.The average dissolution amount was higher in the region with high rainfall.Due to the inhomogeneity of soil and soil water in karst watershed,the dissolution amount was significantly different even between two sites very close together,which were mainly controlled by the hydrological process.The acid rain rate in Lijiang River basin is relatively high,and the sink reduction generated by acid rain directly reacting with carbonate rocks can reach 2.1%of the dissolution rate.If the p H value of acid rain drops,the proportion will increase sharply.(2)As a karst basin,the dissolved inorganic carbon(DIC)in Lijiang River basin was mainly HCO₃^-and mainly derived from carbonate weathering.There were three sources of HCO₃^-:weathering of silicates,carbonate weathering by carbonic acid and carbonate weathering by sulfuric and/or nitric acids.The contributions of carbonate weathering by carbonic acid and carbonate weathering by sulfuric and/or nitric acids varied with the inputs of sulfuric acid and nitric acid.The contribution of carbonate weathering by sulfuric and/or nitric acids in Lijiang River basin was 10%?30%,which is a considerable amount and cannot be ignored.(3)The water chemistry was significantly influenced by aquatic photosynthesis in Lijiang River basin.The riverine organic carbon was created chiefly by the transformation of bicarbonate to organic carbon by aquatic phototrophs during the non-flood period,which can be up to half of river organic carbon.In the flood,the proportion of allochthonous organic carbon increases significantly with the increase of flow,but POC was mainly from soil particles,and DOC was mainly from human pollutants.(4)The carbon sink produced by coupled carbonate weathering in the Li River basin was calculated to be 14.41 t C·km^-2·yr^-1,comprised of the sink attributable to carbonate weathering(12.17 t C·km^-2·yr^-1)and sink due to the“biological carbon pump”(S_BCP)(2.24 t C·km^-2·yr^-1).The S_BCP thus accounted for approximately 15.54%of the total carbon sink,indicating that the proportion of riverine TOC sourced by the transformation from bicarbonate to organic carbon by aquatic phototrophs may be high and must be considered in the estimation of carbonate weathering-related carbon sinks elsewhere.