Flux,Variation And Control Mechanism Of CO₂ Exchange Across Water-Air Interface Of Typical Section In Lijiang River,SW China

Rivers play an important role in global carbon cycle by directly linking terrestrial ecosystems and the oceans.Rivers are not a simply passive pipe transporting terrestrial carbon,but have various physically and biogeochemically active in processing massive quantities of carbon along river course,such as sediment,burial,evasion,in-situ production,and organic carbon mineralization.Recent research indicates that the global CO₂ evasion rates is up to 1.8 PgC/yr from streams and rivers,which brings great pressure to the imprecise"residual land sink".Therefore,CO₂ evasion across water-air interface in river has become one of the hot topics in the biogeochemical processes of rivers in recent years.As the largest carbon reservoir in the word?10⁷ PgC?,carbonate rock actively participates in modern global carbon cycle,and its importance to the global carbon cycle is self-evident.Carbonate rock consumes soil/atmosphere CO₂ through the karst processes,thus reducing the carbon source to the atmosphere and increase carbon sink.However,the carbon sequestration effect in karst is questioned due to its stability,which is considered that the soil/atmosphere CO₂ consumed by carbonate weathering could return to the atmosphere by CO₂ evasion,so the carbon sink is not fixed.Therefore,the CO₂ exchange process across water-air interface in karst water is also the key process to evaluate the stability of karst carbon sequestration.China is a karst big country with the karst area accounting for one-third of the total continental area.Therefore,it is great significant for understanding the carbon exchange process deeply and evaluating the karst carbon sink quantitatively by coupling the regional characteristics of our country and the CO₂ exchange process.In this paper,two typical transections were selected in the Lijiang Basin,Guilin,China.The upstream transection?Damianxu,DM?is located in junction area between the karst area and non-karst area.The downstream transection?Puyi,PY?is located in the area of carbonate rock.The monitoring and sampling works on CO₂ exchange at different timescales including seasonal,diunal,and rainfall event has been carried out from August 2016 to November 2017.The atmospheric environment parameters?temperature,pressure,atmospheric CO₂ concentration?,hydrodynamic conditions?flow rate,depth?,and water chemistry parameters(water temperature,pH,dissolved oxygen,electrical conductivity,HCO₃^-?Ca²⁺)were tested in situ.Two common approaches,floating chamber?FC?and thin boundary layer models?TBL?,were used to measure the CO₂ exchange flux.In addition,the anion,cation,DOC,POC,?¹³C_DIC,?¹³C_POC,?¹⁴C_DIC and?¹⁴C_POC of water samples have been tested in the laboratory.The results are listed below.?1?The CO₂ exchange across water-air interface in the two transections?LJTS?showed significant seasonal variations,which was mainly controlled by the changes of rainfall,discharge,and water flow velocity,and it is also affected by the biological effects.In the dry season due to relatively stable flow velocity,less rainfall and less soil microbial activity,less input of CO₂ in the catchment resulted in realatively stable of CO₂ exchange across water-air interface.In addition,the slow flow-velocity and relatively high water temperature in subtropical winter provided an ideal environment for aqueous biogenic activities,which lead to a slight fluctuation in CO₂ exchange.In contrast,in the rainy season,the increasing rainfall and soil microbial activity will result in more input of high concentration soil CO₂,and thus lead to the increase of CO₂exchange.Simultaneously,the significantly increase of discharge due to heavy rainfall will cause dilution effect,which lead to significant fluctuations in CO₂ exchange.?2?The CO₂ exchange across the water-air interface in LJTS showed significant variations at short timescales in the whole year.When there is no rainfall during the year,CO₂ exchange flux showed obvious diurnal variation of increase in the day and decrease in the night basically,which mainly controlled by metabolism of sub-aquatic community.During the rainfall,CO₂ exchange is mainly controlled by rainfall intensity and water level change.When the water level changes very little,CO₂ exchange intensity was changed,but it showed still diurnal variation,which was still controlled by metabolism of sub-aquatic community.However,when the water level changes drastically,CO₂ exchange was controlled by the changes in water level and affected by different factors.When the water level located in the rising or falling stage,the CO₂exchange was controlled by the change of water level.When the water level tended to be stable during rainfall,it was influenced by the metabolism of sub-aquatic community and showed still regular diurnal changes.When the water level changed slowly,it was influenced jointly by the change of water level and the metabolism of sub-aquatic community.?3?The?¹³C and?¹⁴C values of DIC,DOC,and POC in LJTS were calculated by the endmember method.The results indicated that the DIC was controlled by the atmospheric CO₂ at DM and the contribution rates were all more than 57%,which made little different between pCO₂ of the water and of the atmosphere and showed significant diurnal variations in both CO₂ evasion and uptake in both winter and summer seasons.The release of CO₂ in summer was mainly derived from carbonate rocks at PY and the contribution rate was 44.3%.The DIC was controlled by the atmospheric CO₂ at PY in winter and the contribution rate was 61.3%.The pCO₂ of the water were significantly greater than atmosphere in summer,but significantly less than atmosphere in winter,which displayed CO₂ evasion in all summer and CO₂ uptake in all winter.?4?The strength and direction of CO₂ exchange across the water-air interface in LJTS showed significant spatiotemporal variations.In the period of water level stability,the strength of CO₂ exchange took the diurnal variation trend which controlled by the metabolism of sub-aquatic community.But in the period of heavy rainfall and water level change dramaticlly,the diurnal variation trend of CO₂ exchange strength will be weakened or disturbed by changes in water level.With seasonal changes,the changes of CO₂ exchange direction were controlled by the source of the river DIC?released CO₂?.The CO₂ exchange showed significant both CO₂ evasion and uptake when the DIC mainly derived from atmospheric CO₂.In contrast,The CO₂ exchange displayed significant CO₂ evasion when the released CO₂ derived from carbonate rocks mainly.Therefore,controlling by the prerequisite of changes in river water level?discharge?,the spatiotemporal variations of CO₂ exchange intensity and direction were jointly controlled by the internal process of metabolism of sub-aquatic community and the sources of DIC.?5?The CO₂ exchange flux and the karst carbon sink flux in the main stream of the Lijiang River?from DM to PY,LJR?were calculated.Overall,the LJR showed CO₂evasion with the flux of 9352.34 tC/yr,which derived from carbonate was 4190.58tC/yr.In addition,the LJR showed CO₂ uptake in winter with the flux of 393.12 tC/yr.Carbon sink flux derived from carbonate weathering by carbonic acid in LJR was44861.82 tC/y.Specifically,a flux of 40671.24 tC/yr of karst carbon flux was formed after deducting the CO₂ evasion flux,The CO₂ evasion flux derived from carbonate rock accounts for only 9.34%of the net carbonate weathering carbon flux,indicating not all of CO₂ are returned to the atmosphere as CO₂ evasion,but rather form a very significant flux of karst carbon sink,about 0.04 TgC/yr.