| In recent decades, under the background of global change, it is controversial whether and how karst processes become a carbon sink, especially when impacted by anthropogenic activities. The study showed that dissolution of carbonate rocks and HCO3- in groundwater could not be solely attributed to the fact that carbon acid erodes carbonate rocks. With the increasing development of agriculture and urbanization, nitric acid and sulfuric acid from widely-used nitrogenous ferlizer, waster water and acid rain take part in dissolve carbonate rocks. It will not consume carbon dioxide that is originated from atmosphere or soil, but will increase the amounts of dissolved carbonate rocks and HCO3- concentration. It is obvious that the amount of carbon sink that was calculated by karst geological processes based on traditionally hydrochemical methods and standard limestone tests tends to be overestimated.In this thesis, a combined method of standard limestone tests and hydrochemistry-discharge as well as stable isotopes was applied to calculate the amount of karst carbon sink in Qingmuguan subterranean stream watershed, a region where has been suffered strong interference from anthropogenic agricultural activities.A series of instruments, such as CTDP300 multi-parameter sensors, WGZ-1 optical digital level gauge and HOBO automatic station were used to monitoring water quality, water level and rainfall. Samplings took place monthly from 2010 to 2011 for routine analysis of hydrochemistry,δ13C,δ34S, 87Sr/86Sr and monitoring of soil carbon dioxide and 813C. Based on the above data collected, we discussed karst process’s sensitivity responding to environmental changes and effects from agricultural activities on karst process and carbon sink. It was found that agricultural activities could exert important influences on karst process to change karst geological carbon sink.There were significant temporally and spatially physicochemical features of Qingmuguan subterranean stream. The hydrochemistry of groundwater, such as pH,Ec, T, Ca2+, Mg2+, K+, Na+,HCO3-,Cl-,SO42- and NO3- and dissolution rates under different types of land use showed different values at various time scales. The karst processes revealed changeable characteristics at different scales with the changing environment, which indicated that the rates of karst process are going fast and make quick response to environmental changes. So karst process actually plays a vital role in uptaking atmospheric carbon dioxide since it takes part in processes of carbon cycling in short-time scale.Soil pH, soil organic matters, soil CO2 and rates of dissolution under various land use types showed markedly values. The main influential factors to rates of dissolution were different under different land use types in different seasons. It was found that agricultural activities could have helped to dissolve carbon rocks, elevating 10.98t/km2·a in Qingmuguan subterranean stream. It consumed 19.14t/a CO2 which was equivalent to 6% CO2 consumed based on a calculation of standard limestone dissolution test in the whole watershed.Hydrochemistry and carbon isotopic compositions of groundwater proved that niric acid and sulfic acid originated from anthropogenic activities in Qingmuguan subterranean stream watershed did take part in dissolving carbonate rocks. The DIC concentrations in groundwater were derived from a joint dissolution of carbon acid, nitric acid and sufric acid. DIC from dissolution of carbonate rocks by carbon acid takes a large range of total DIC in groundwater, between 55.53% and 81.25%; the ratios were lower in the rainy period (62.98%) while higher in the drought period (74.86%). In contrast, DIC from dissolution of carbonate rocks by nitric acid and sulfic acid covered a range of 18.75%-44.47%; the ratios were higher in the rainy period (37.02%) than in the drounght period which was 25.14%. The annual yield of DIC from the dissolution of carbonate rockes by carbon acid in Qingmuguan subterranean stream amounted to 14.67×106mol/a. In this case, the net carbon sink based on karst process was 7.335×106mol/a or 0.323×10 g/a and the carbon sink intensity was 0.732×106 mol/km2·a. DIC from the dissolution of carbonate rockes by nitric acid and sulfuric acid in Qingmuguan subterranean stream was up to 7.48×106 mol/a mol/a, contributing to 33.8% of total DIC with a carbon sink intensity of 1.89×106 mol/km2·a.The 815N in groundwater verified that nitrate contamination of groundwater was mainly from fertilizer, soil organic nitrogen, manure and domestic sewage. The 834S values were in the range of 5.23‰～11.07‰with SO42- concentration between 0.52 and 0.97mmol/L.δ34S values in rainwater ranged from 2.34%o to 6.06%o with SO42- concentration in the midst of 0.09 and 0.17mmol/L. Thus, SO42- in groundwater should have been attributed to domestic sewage, rainwater and agricultural activities. The NO3-、SO42- did not completely participate in the reaction with carbonate rocks, which was in agreement with results calculated by hydrochemistry. The 87Sr/86Sr ratios tended to validate that the effects produced by nitric acid and sulfuric acid in different levels of fertilizing or acid rain showed differences, that is, higher solution of carbonate rocks by nitric acid and sulfuric acid in fertilizing periods and the rainy period than in the drought period. The isotopic proofs were consistent with results calculated from hydrochemical items. Moreover, the calculatedδ13CDic of groundwater were compliance with the monitored ones, approving the results were believable. The combined use ofδ15N,δ34S,87Sr/86Sr andδ13CDIC was to verify further that the research methods and conclusion were reliable. |
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