|Abstract:The carbon cycle research is the key point in Global Change Science. Its balance has become the frontiers and hot spots in the field of global change and earth science. Missing sink is the focus of the global carbon budget, finding and revealing the mechanism of its existence is of great significance to the study of global carbon cycle. In the global carbon researches, only consider the contribution of the ocean carbon library and the terrestrial ecosystem carbon library, but for the carbonate carbon pool in the earth lithosphere lack of adequate assessment. Research team headed by academician Yuan Daoxian who adopted the Carbonate-rock-tablet-test method, Hydrogen-discharge method and Diffusion Boundary Layer estimated the amount of carbon sequestration each year caused by karstification around the world for the first time, which account for one-third of the missing sink in the carbon cycle model. Modern karstology in China demonstrated karstification actively involved in the global carbon cycle. However, some scholars have cast doubt on the stability of karst carbon sink thought “carbonate dissolution is only a process of carbon migration but does not produce carbon sequestration”. Yet aquatic plants are an important part of aquatic ecosystems occupied the key interface of aquatic ecosystems, aquatic plants photosynthesis is of vital importance to the primary productivity and the carbon biogeochemical cycles. The paper is based on the Water-Rock-Gas-Organism new karst dynamic system model, taking typical karst rivers as study objects, random sampling method is used to survey the communities of aquatic plants in the watershed scale to determine the dominant species of aquatic plants in karst aquatic ecosystems. In order to reveal the interactions between aquatic plants and karst water environment in mechanism, taking the dominant species in the karst aquatic ecosystems as biological materials, making comparative studies of the growth of aquatic plants in karst and non-karst water, pH-drift method is used to study the inorganic carbon utilization strategy and its conversion efficiency for different aquatic plants. In the river basin scale, the Zhaidi river which fed by a typical karst underground river is taken as an example, by monitoring the water chemistry, dissolved inorganic carbon and stable carbon isotope change, combined with the stable carbon isotopic composition of aquatic plants, to analyze the dissolved inorganic carbon utilization strategy and stability of the carbon sink in natural conditions by aquatic plants. Dissolved oxygen method is used to evaluate the fixation amount of dissolved inorganic carbon by aquatic plants and make comparison of dissolved oxygen method and hydrogen-discharged method to estimate the accuracy of both methods. Finally, taking Zhaidi river as an example to preliminary establish the dissolved inorganic carbon migration model in a karst surface river. The results show: (1) The aquatic plants in Huaxi, Zhaidi and Chaotian river belong to Hydrocharitaceae, Potamogetonaceae, Haloragidaceae, Ceratophyllaceae and Characeae, all of them are submerged plants. The dominant species of aquatic plants are respectively Vallisneria natans, Ottelia acuminate and Hydrilla verticillata in Huaxi River, Vallisneria natans, Ottelia acuminate var. jingxiensis, Potamogeton wrightii, and Hydrilla verticillata in Zhaidi River, and Vallisneria natans, Potamogeton wrightii and Hydrilla verticillata in Chaotian River. Compared with the Chaotian river, the dominant species of aquatic plants distribution are relatively uniform, meanwhile in the Chaotian river, the differences between Vallisneria natans and other dominant species are relatively large. In the investigation of karst aquatic plants, it’s more accurate to determine the dominant of aquatic plants for the method of dominance rather than frequency. Matrix and water quality are the main factors influencing the distribution and community characteristics of aquatic plants. In karst surface rivers, Vallisneria natans community possesses the most wide adaption to the environment. To the condition of water quality and matrix types requirements are relatively low. Ottelia acuminate community only anchors in karst surface rivers, lives in high water quality environment and adapts to a wide range of matrix. Hydrilla verticillata community has higher requirements to the matrix and light conditions, for water quality is relatively low. Potamogeton wrightii can grow in different types of matrix and require less to the environment. (2) From the perspective of biomass and plant length, Hydrilla verticillata lived in karst water is respectively 2.6 times and 1.5 times than that in non-karst water. The result reveals that karst water has significant influence on the growth of aquatic plants, not only can promote the increase of plant length but also can promote the growth of plant roots and shoot tillering. Karst water has a significant “Fertilization effect” in the performance of promoting the growth of root devision and shoot tillering. Ca2+ is an essential mineral element in the growth of aquatic plants. When the Ca2+ concentration is 1.25mmol/L, the biomass of aquatic plants increased the most. High concentration of Ca2+ can promote the shoot tillering, but show no promoting on the plant length and shoot tillering. HCO3- provide sufficient carbon source for photosynthesis of aquatic plants. When the HCO3- concentration reaches 4mmol/L, the biomass of aquatic plants increases the most; when the HCO3- concentration is 8mmol/L, the promoting effect reaches the climax for aquatic plants. High concentration of HCO3- in karst water environment promotes the root growth which guarantees the aquatic plant growing in the matrix dominated by sand. HCO3- is the dominant factors in controlling the “Fertilization effect” for the growth of aquatic plants in the karst water environment. Ca2+ can strengthen the ability of aquatic plants to utilize dissolved inorganic carbon. pH drift experiments reveal that the dominant species of aquatic plants have the ability to utilize the HCO3- as carbon source. Ottelia acuminate has the highest ability to utilize HCO3- than any other dominant species. In the process of photosynthesis, Myriophyllum spicatum and Hydrilla verticillata have priority to utilize CO2 than HCO3-, at the same generate precipitation in the form of CaCO3. The karst carbon sink conversion ratios of Myriophyllum spicatum and Hydrilla verticillata are 54.29FfFf and 54.43FfFf, respectively. Through the photosynthesis of aquatic plants, a part of unstable dissolved inorganic carbon in karst water are converted into organic carbon in the form of biomass, another part are precipitated together with HCO3- in the form CaCO3. Karstification remove atmospheric/soil CO2 into water supplied for photosynthesis. Therefore, in karst water ecosystem, aquatic plants play an important role in fixing unstable inorganic carbon, it’s crucial to take consideration of primary producers represented by aquatic plants in karst water ecosystem for estimating karst carbon sink. (3) The discharge of underground river is the main sources of dissolved inorganic carbon to Zhaidi aquatic ecosystem. The river matrix is mainly sand grains with the diameter of 0.075~2mm, and pH are alkaline between 8.23 to 8.88. The organic matter content range from 0.12FfFf to 0.95FfFf, the endogenous inorganic carbon content generated by the decomposition of organic matters are far lower than that of the input of exogenous inorganic carbon. The average outgassing content is 4581.87mg/m2/h in Zhaidi river and decrease with the flow distance, the maximum is 6719.80 mg/m2/h at the inlet and the minimum is 2935.89 mg/m2/h at the outlet. The concentration decrement of free CO2, HCO3- and Ca2+ are 2.86mg/L, 7.05mg/L and 0.38mg/L, respectively. The stable carbon isotopic composition shows a constant positive and a trend of “Gradient Distribution” along the flow direction. Water chemistry, dissolved inorganic carbon, dissolved oxygen and pH show a significant diel change consistent with the change of stable carbon isotopic composition of dissolved inorganic carbon and aquatic plant, all of them demonstrate that photosynthesis of aquatic plants are the dominant factor in controlling the change of each parameter. Outgassing effect, precipitation effect and photosynthesis effect are the main migration ways for dissolved inorganic carbon in the karst aquatic ecosystem. Among them, outgassing effect and precipitation effect have good coupling relationships and both mainly subject to hydrology and water chemistry. Through outgassing and precipitation effect, a part of dissolved inorganic carbon is transformed in vertical section in the form of free CO2 exchanged with atmosphere and CaCO3. A part of dissolved inorganic carbon is converted in longitudinal section and the conversion efficiency is mainly limited by environmental factors. Along with the flow direction, dissolved inorganic carbon are constantly fixed by photosynthesis of aquatic plants in surface river and converted unstable inorganic carbon into stable organic carbon, then entering the cycle of ecological system. (4) Dissolved oxygen method can be a very good approach to assess the fixed amount by aquatic plants. The daily average primary production is equivalent to 32.70mg/L O2 and the respiration rate is equivalent of 21.42mg/L O2, the net primary production in Zhaidi river is of 11.27mg/L O2. Photosynthesis and respiration are the dominant factors influencing the variation of dissolved oxygen. Dissolved oxygen method is used to calculate the conversion and fixation amount of dissolved inorganic carbon by aquatic plants, which are equivalent of 1.02mmom/L and 0.35mmol/L O2, respectively. However, the reduction of HCO3- is only 0.12mmol/L with the method of hydrogen-discharge method. The total utilization amount by aquatic plants is 36.32FfFf and the fixed amount is 12.52FfFf which occupied the total input dissolved inorganic carbon. Taking the fixation amount as an example, the calculation results of hydrogen-discharge method account for 34.29FfFf than that of dissolved oxygen method. Therefore, the hydrogen-discharge method is seriously underestimated the fixation amount of karst carbon sink caused by aquatic plants. To sum up, in the karst aquatic ecosystem, aquatic plants have significant effect to the stability of dissolved inorganic carbon. The karst carbon sink caused by aquatic plants can’t be ignored not only in the small scale but also in the watershed scale.