Permafrost is one of the most important carbon reservoirs in the world.The degradation of permafrost caused by climate warming leads to the rapid release of soil organic carbon.Alpine region is a typical frozen soil area,which is extremely sensitive to climate change.In the past few decades,alpine region has experienced rapid warming and permafrost melting.The melting of frozen soil in alpine mountain areas changes the conversion relationship between surface water and groundwater and the water flow path,and affects the output of dissolved organic carbon(DOC)and dissolved inorganic carbon(DIC)in rivers.However,the role of seasonal freezing and thawing of frozen soil and frozen soil degradation on the output of dissolved organic carbon in rivers is still unclear.Based on the above background,this study intends to study the effect of frozen soil change on the output of dissolved organic carbon in alpine mountains.This study can provide a mechanism explanation for the characteristics of organic carbon output in Alpine mountainous areas,and provide a reference for the construction of hydrological models of alpine areas.In this study,the western branch basin of the upper reaches of the Heihe River and its 33 sub-basins in the northeast of the Qinghai-Tibet Plateau were used as the research objects.Four samples were collected from the western main stream,tributaries,groundwater,precipitation and glacial meltwater of the upper reaches of the Heihe River.The effects of seasonal freeze-thaw cycles on river-groundwater interactions in permafrost were studied by isotope and water chemical tracing techniques.Parallel factor analysis(PARAFAC)was used to identify Dissolved Organic Matter(DOM)components and their optical properties.By analyzing the spatial variation characteristics of DOC,DIC concentration and DOM components in the water of the West Branch of the upper reaches of Heihe River and their seasonal variation rules,the influence of the change of water-groundwater interaction under the seasonal freeze-thaw of frozen soil on the output of dissolved organic carbon in the river was revealed.In addition,the effects of water flow paths and runoff water sources on the output of dissolved organic carbon in rivers during the degradation of frozen soil were identified by using spatiotemporal equivalent methods and stable isotope and water chemistry tracing techniques,and the effects of changes in frozen soil hydrological processes caused by frozen soil degradation on river dissolved organic carbon output were revealed.This study can not only provide a mechanism explanation for the organic carbon output characteristics in alpine mountainous areas,but also provide a reference for the construction of hydrological models in alpine regions.Through this study,the following main understandings was obtained:1.Characteristics of hydrochemical and isotopic changes and their indications for hydrological processes in Alpine permafrost regions(1)In permafrost regions,the concentrations of TDS,Na+,Cl-,Ca2+,Mg2+and SO42-in the mainstream river water are relatively high,which may reflect the influence of freezing fractionation.It is found that in the initial stage of freezing period(October),the river source in the permafrost area is mainly supplied by the artesian supra-permafrost groundwater in the active layer,rather than the sub-permafrost groundwater.In the alluvial proluvial plain area in the lower half of the permafrost area,it is changed into river water to supply groundwater,and the main river water is cut off.At the initial stage of thawing period(April),the ion concentration of rivers in permafrost area is lower and the deuterium oxygen isotope is more negative,indicating that the source river water was mainly recharged by rapid lateral runoff formed by precipitation and winter snow meltwater,and the runoff path was shallow.In the thawed period(July),the ion concentration is the lowest,and the river water is mainly fed by slope runoff formed by precipitation and glacial meltwater.(2)In the seasonally frozen soil area,the concentration of main ions and TDS in the river water is the lowest and stable all the year round.The hydrochemical type is HCO3-Ca·Mg,and the deuterium oxygen isotope is stable.The contribution rate of precipitation and glacier melt water to the river water in this river section is high,indicating that the surface runoff from permafrost areas,precipitation and mountain glacial meltwater on both sides of the river valley infiltrate and recharge groundwater in the Quaternary aquifer in the seasonally frozen soil area,and groundwater replenishes the surface rivers in the seasonal permafrost area at low altitudes in the form of springs,and the main stream is reflowed.The pore aquifer in the riparian zone plays an important role in regulating the river Hydrochemistry in the alpine region.The Quaternary loose sediment distribution area is the area where groundwater and surface water interact frequently in the basin.(3)In the seasonally frozen rock area,the concentration of main ions and TDS in river water increases,and the hydrochemical types change to Ca·Mg-HCO3·SO4 and Ca·Mg-SO4·HCO3,indicating that the water flow path is shallow.Because most of the basin is covered by thin weathered residues and bedrock,the runoff infiltration is limited,mainly by slope flow and/or shallow underground flow.2.Spatial and temporal variation characteristics of dissolved organic carbon output from rivers in Alpine mountainous areas and the influence of frozen soil freezing and thawing on itBy analyzing the characteristics and seasonal changes of DOC,DIC concentration and DOM components along the flow of the West Branch of the upper reaches of Heihe River,the influence of river-groundwater interaction and water flow path on the output law of dissolved organic carbon in the river is discussed.It is found that the frequent interaction between groundwater and surface water affects the migration of DOC.(1)At the source of rivers in permafrost areas,in the early stage of freezing,the water in the permafrost layer with high DOC and DIC concentrations is discharged into the main stream in the form of spring water and seepage,resulting in higher concentrations of DOC and DIC in the source of permafrost areas,and the lower half is affected by the inflow of tributaries,and the dilution effect makes the DOC concentration show a downward trend.In the early stage of spring permafrost thawing,in the first half of the permafrost area(bedrock area),the source river water is mainly replenished by the rapid confluence of ice and snow melt and precipitation,dilution and insufficient water and rock,resulting in the lowest concentration of DIC concentration.In the second half of the permafrost area(alluvial plain),the main stream river water begins to replenish groundwater,the proportion of tributary inflow increases,because the runoff path is shallow,the tributaries mainly flow through the shallow organic soil layer,so DOM is mainly terrestrial source,and the aromatization and humification degree of DOM is high.At the initial stage of thawing period,the snow meltwater leached the soil DIC into the river,and the frozen salt in autumn and winter melted in spring,so that the concentration of DIC in the river water showed a significant upward trend.During the thawed period,due to the increase in the proportion of slope runoff,the concentration of DOC and DIC in the river water was low.(2)In the seasonally frozen soil area,Quaternary pore groundwater is the main source of river water moisture in seasonally frozen soil area.Pore aquifers regulate the migration of DOCs.On the one hand,the DOM of the water body is degraded by microorganisms and adsorbed by minerals during the transport of pore water,and the content of microbial source DOM in groundwater is high,and the content of microbial source C3 in river water in seasonal frozen soil area is increased after groundwater replenishment,the land-based source DOM is greatly reduced,the concentration of DOC is reduced,and the aromatity and humification degree of DOM are reduced.On the other hand,the high groundwater flow rate and insufficient water-rock interaction in the aquifer under the huge altitude difference,coupled with the dilution effect of meltwater from mountain glaciers on both sides and precipitation infiltration recharge aquifers,lead to low DOC and DIC concentrations.Affected by groundwater recharge,the concentrations of DOC and DIC and DOM components from terrestrial sources in river water were low and the seasonal variation was small.(3)In the seasonally frozen rock area,the water flow path is shallow,dominated by overland flow and/or shallow underground flow that mainly flow through the regolith,resulting in a significant upward trend in the concentration of DIC in the river water along the process,while the concentration of DOC and DOM components only shows a slight upward trend.The microbial degradation processes that occur in aquifers in seasonal permafrost lead to a higher proportion of microbial sources of DOM in groundwater.The proportion of DOM sources of microbial sources in the mainstream river water of the basin after groundwater recharge is also higher than that in the Arctic permafrost area.This feature will accelerate the mineralization process of dissolved organic carbon and enhance the positive feedback of climate warming in alpine mountains.The freeze-thaw cycle process in permafrost and the hydrological regulation and storage of pore aquifers in seasonal permafrost affected the DOM output characteristics of alpine mountainous areas,which made the output law of DOC in alpine mountainous area significantly different from the output law of river DOC in arctic and subarctic regions.3.Influence of frozen soil degradation on water flow pathThere are obvious differences in the hydrochemical characteristics of outlet river in sub-basins with different permafrost area proportions.With the decrease of the permafrost area proportion in the basin,the concentration of major anion and cation in the river water tends to rise,and TDS increases,especially at the initial stage of thawing period(April)and the thawed period(July).It indicates that with the degradation of permafrost,the gradual thickening of the active layer leads to the deepening of the water flow path,the prolongation of the retention time,and the strengthening of water rock interaction,So that the concentration of inorganic solutes in the deep mineral layer increases.In the initial stage of thawing and thawed periods,the contribution rate of groundwater to runoff may increase significantly with the decrease of the proportion of permafrost,indicating that frozen soil degradation may cause the increase of groundwater recharge,and more groundwater will be converted into surface water.4.Effect of permafrost degradation on dissolved organic carbon export in alpine mountainsAt the initial stage of permafrost degradation,the surface runoff increases,exposing the carbon contained therein to the carbon cycle process between soil and water.At the same time,the increase of vegetation coverage leads to the increase of plant-based carbon and the accelerated release of DOC.The concentration,aromaticity,molecular weight and humification degree of DOM in the water body increase.More soil organic carbon enters the water body of the basin through the process of production and concentration,which may lead to the increase of DOM flux in the basin.With the further melting of frozen soil,the active layer thickens,the flow path deepens,and the water flows through the deeper mineral soil layer,resulting in the adsorption of organic carbon by minerals and microbial degradation,and the output of DOC decreases.In addition,with the degradation of permafrost and the increase of groundwater recharge,DOM from microorganisms in groundwater and DOM with low aromaticity enter the river water,which also makes the DOC concentration in the river water low.However,the degradation of frozen soil will also release water,leading to the increase of vegetation coverage.Therefore,the degradation of frozen soil is accompanied by the change of carbon source,which increases the source of carbon and affects the concentration of DOC in river water.DOC concentration is affected not only by the melting depth of frozen soil,but also by vegetation coverage,underlying surface conditions(bedrock and loose rock)and watershed topography(slope,etc.).With the degradation of frozen soil,the thickness of the active layer increases,the retention time increases,the water rock interaction increases,and the water flows through the deep mineral layer.At the same time,the new hydrological channel and the deeper groundwater flow depth are likely to increase the DIC composition due to the enhanced weathering.When the active layer gradually melts to the deep layer,more soil water and CO2 enhance mineral weathering reaction and DIC output,and DIC concentration and flux increase with the thawing of the active layer.In general,DIC is significantly affected by the ablation depth of frozen soil.Compared with DOC output,DIC output is not affected by vegetation coverage.The change of DOM content in the basin on the time scale is mainly affected by the characteristics of river runoff,while its change on the spatial scale is closely related to the distribution range of permafrost,vegetation characteristics,geographical location of the river,soil texture,topographic and geological conditions and other basin characteristics.The presence of porous aquifers may slow the effect of permafrost degradation on river dissolved organic carbon output.However,in general,permafrost dominates the output laws of DOC,DOM and DIC in water bodies in small watersheds.The results of this study are helpful to understand the response mechanism of dissolved organic carbon output of middle and high latitude rivers to frozen soil degradation under the background of climate warming. |