Control Processes And Mechanisms Of DIC Stability As Carbonate Weathering Product

Global change has developed into an important issue crossing many fields such as politics,economy,and diplomacy since it was raised as a scientific issue in the 1980s.The Paris climate change agreement signed in 2016 proposed that controlling the increase of the global average temperature is below 2°C of the pre-industrial level,which caused attention of the researchers to the global carbon cycle again.At present,an important issue in global carbon cycle research is how to balance the atmospheric CO₂ budget.Ignoring the carbon sink effect of karstification may be one of the reasons for the imbalance of atmospheric CO₂ budget.Karstification rapidly and sensitively responds to environmental changes.Carbonate rocks,which account for 99.55%of the global carbon pool,actively participate in the global carbon cycle.However,the stability of dissolved inorganic carbon（DIC）as carbonate weathering product has been questioned due to the traditional carbon cycle model that treats geological action as a purely inorganic process.Karstification is not a purely inorganic geological process.DIC can be preserved in the relatively stable form of organic carbon（dissolved organic carbon（DOC）and particulate organic carbon（POC））by aquatic photosynthesis of submerged plants and microalgae/microbe in the surface water.Moreover,recent studies in the ocean have shown that heterotrophic micro-organisms not only use DOC,but also produce DOC.A part of DOC produced by heterotrophic micro-organisms is recalcitrant dissolved organic carbon（RDOC）,which constitutes carbon storage in waters.Karst carbon sink is also one of the important means to achieve goals of carbon peaking and carbon neutrality in China.However,the processes and mechanisms of the transformation of DIC into POC and DOC and the transformation of DOC into RDOC in karst water are still unknown.In this study,we chose a typical karst aquatic system,the Guilin section of the Lijiang River as our study area to reveal the processes and mechanisms of the transformation of DIC into POC and DOC and the transformation of DOC into RDOC,and to quantitatively evaluate the fluxes of autochthonous organic carbon in the karst aquatic system,which are expected to make new scientific contributions to the stability of karst carbon sink and to solve the problem of missing sink,and to provide countermeasures and scientific support for Chinese strategic decisions on climate change and economic development.In this study,twelve sampling sites（M1-M3 in the upper reaches,M4 and M5 in the middle reaches,M6-M8 in the lower reaches,and T1-T4 of four main tributaries）were selected along the Lijiang River.The control processes and mechanisms of DIC stability as carbonate weathering product in karst aquatic system were studied by combining field monitoring and incubation experiment in terms of water chemistry,isotope detection(δ¹³C_DIC,△¹⁴C_DIC,δ¹³C_POC,△¹⁴C_POC,δ¹³C_DOC,△¹⁴C_DOC andδ¹³C_RDOC)and 16S rDNA high-throughput sequencing.The results showed that:（1）Carbonic acid-dissolved carbonate was the dominant HCO₃^-source in the Lijiang River.The contributions of carbonic acid-dissolved carbonate to HCO₃^-of the river water gradually increased from the upstream to the downstream due to the spatial distribution of carbonate rocks in the Lijiang River basin,ranging from 57.32%to 89.45%,with an average of 73.24%.The contributions of exogenous acid-dissolved carbonate to HCO₃^-of the river water ranged from 6.33%to 30.17%,with an average of 18.37%.The contributions of carbonic acid-dissolved silicate to HCO₃^-of the river water ranged from2.24%to 32.94%,with an average of 8.31%.The contribution of silicate weathering to HCO₃^-can up to 30%in the upper reach（non-karst area）,while which was less than 10%in the mid-lower reaches（karst area）.The DIC of the Lijiang River was dominated by HCO₃^-.Theδ¹³C_DIC values were more negative in summer than that in spring and autumn.This may relate to the strong aquatic photosynthesis during spring and autumn;photosynthesis leads to an increase inδ¹³C_DIC of the river water by preferentially consuming ¹²C.Spatially,theδ¹³C_DIC values increased along the river due to the higher DIC concentrations and slow river flow in the mid-lower reaches,which could promote aquatic photosynthesis in karst aquatic system.（2）DOC was the main constituent of organic carbon in the Lijiang River.The DOC concentrations showed higher values during spring and autumn and lower values during summer.Spatially,the DOC concentrations of the mainstream were slightly lower than that in the tributaries.The DOC concentrations in the upper reaches（non-karst area）were lower than that in the mid-lower reaches（karst area）.The concentrations of POC had higher values in summer and lower values in winter,but there was no significant difference between the upper reaches and the mid-lower reaches.Theδ¹³C_POC andδ¹³C_DOC values ranged from-26.8‰to-22.6‰and-28.9‰to-23.8‰,respectively.Due to the influence of aquatic photosynthesis,bothδ¹³C_POC andδ¹³C_DOC values decreased along the river with slight fluctuating,while theδ¹³C_DIC values increased from the upstream to the downstream.Theδ¹³C_POC andδ¹³C_DOC values were the most positive in summer,and there was no obvious difference in the other three seasons.The POC/PN and DOC/DON of the Lijiang River water ranged from 7.1 to 12.1 and 7.0 to 10.5,respectively.Moreover,the POC/PN was slightly higher than DOC/DON.Spatially,both POC/PN and DOC/DON decreased from the upstream to the downstream.（ie non-karst area to karst area）.Seasonally,the POC/PN and DOC/DON showed similar variations with theδ¹³C_POC andδ¹³C_DOC values.TheΔ¹⁴C_DIC was generally high in rainy season（-97.9‰～-26.2‰）and low in dry season（-135.0‰～-71.6‰）.Spatially,theΔ¹⁴C_DIC values were more negative in the mid-lower reaches than that in the upper reaches due to the geological background of the Lijiang River basin.TheΔ¹⁴C_POC values were-141.6‰～-84.3‰and-312.1‰～-161.8‰in the dry season and rainy season,respectively.TheΔ¹⁴C_DOC values were-266.6‰～-134.3‰and-362.3‰～-183.2‰in the dry season and rainy season,respectively.BothΔ¹⁴C_POC andΔ¹⁴C_DOC in the rainy season were more negative than those in the dry season.Moreover,the correspondingΔ¹⁴C_DOC andΔ¹⁴C_POCvalues were more negative thanΔ¹⁴C_DIC values.（3）The contributions of soil,submerged plants and microalgae/microbe to river water POC of Lijiang River ranged from 27%to 60%,20%to 37%and 20%to 38%,with an average of 39%,29%and 32%,respectively.The contributions of soil,submerged plants and microalgae/microbe to river water DOC of the Lijiang River ranged from 22%to 48%,29%to 38%and 23%to 45%,with an average value of 32%,33%and 35%,respectively.Autochthonous organic carbon was the dominant source for both POC and DOC in the Lijiang River.The contributions of autochthonous organic carbon to POC and DOC were 61%and 68%,respectively.DIC input may be the dominant factor that limits aquatic primary production（the formation of autochthonous organic carbon）in the Lijiang River.Moreover,compared to PO₄^3-,aquatic primary productivity was more sensitive to NO₃^-.Diurnal monitoring results revealed that DIC and NO₃^–transformations were primarily controlled by metabolic processes（photosynthesis and respiration）of subaquatic communities,accompanying autochthonous organic carbon formation,in the Lijiang River.The consumption of DIC and NO₃^–by aquatic photosynthesis was in the ratio of 9:1（mol/mol）to produce autochthonous DOC,accompanying the enrichedδ¹³C_DIC,δ¹⁵N-NO₃^–andδ¹⁸O-NO₃^–,with a daily variation of 7.9‰,10.6‰ and 11.2‰,respectively.（4）The concentrations of RDOC were stable at about 2.10 mg/L in the Lijiang River,accounting for 67%of DOC.Theδ¹³C_RDOC values ranged from-29.7‰to-27.2‰,with an average value of-28.4‰,which were lower about 1.9‰than the correspondingδ¹³C_DOC values.The spatial variations ofδ¹³C_RDOC were not obvious.Seasonally,theδ¹³C_RDOC values showed a similar variation withδ¹³C_DOC values.The average contributions of soil,submerged plants and microalgae/microbe to the RDOC were 17%,23%and 60%,respectively.Autochthonous organic carbon was the dominant source for RDOC in the Lijiang River.The contribution of autochthonous organic carbon to RDOC can up to 83%.Compared with the sources of DOC,the contribution of soil to RDOC significantly decreased;the contribution of submerged plants to RDOC also decreased,but the reduction was lower than that of soil;while the contribution of microalgae/microbe to RDOC significantly increased.Autochthonous RDOC in the Lijiang River accounted for about 82%of autochthonous DOC,indicating that most of autochthonous DOC can be converted into RDOC.Limnohabitans,Sediminibacterium,Flavobacterium and Sphingorhabdus are heterotrophic bacteria,which were the key microbial genera for converting DOC into RDOC in the Lijiang River.The average sum of relative abundances of Limnohabitans,Sediminibacterium,Flavobacterium and Sphingorhabdus can up to 30%.（5）The annual fluxes of autochthonous POC,DOC and RDOC at the Guilin section were 2394.78 t C/yr,11924.38 t C/yr and 8071.84 t C/yr,respectively,with 87%,84%and 82%of the fluxes of autochthonous POC,DOC and RDOC produced in the wet season,respectively;the annual fluxes of the autochthonous POC,DOC and RDOC at the Yangshuo section were 3316.45 t C/yr,18504.06 t C/yr,and 13858.44 t C/yr,respectively,with 88%,81%and 82%of the fluxes of autochthonous POC,DOC and RDOC produced in the wet season,respectively,which indicated that although the shorter residence time,higher turbidity and weaker light in the rainy season could limit the aquatic photosynthesis to a certain extent,the accumulation of autochthonous organic carbon from the photosynthesis of subaquatic communities during the rainy season was still dominant in the Lijiang River.The autochthonous POC,DOC and RDOC fluxes in the Yangshuo section were 1.38 times,1.55 times and 1.72 times of that in the Guilin section,respectively,indicating that autochthonous organic carbon was accumulated along the river.The proportions of autochthonous POC,DOC and RDOC fluxes to total inorganic carbon fluxes in Yangshuo section were 2%,10%and 7%,respectively,indicating that the consumption of DIC by aquatic photosynthesis is very considerable and the karst carbon sink is stability.