Influence of hydraulic project on the carbon cycle in the karst reservoir

In recently，lots of dams are built for hydropower，reservoir and flooding control in China at karst area. To understand the effect of hydraulic project on the greenhouse gas emission from karst water reservoir and the disturbance on the carbon cycle in karst area，this paper presents a comprehensive overview on the greenhouse gas emissions based on relevant published studies.Karst systems are sensitive to changes of many environmental factors，such as the atmospheric CO2 partial pressure. So，the dams will produce the waterfall effect when karst water flows through the spillway or hydraulic turbine，the stability of DIC and the atmospheric CO2 partial pressure will change，which will accelerate the emission of CO2 and promote the precipitation of CaCO3.Concerns about hydroelectric reservoir being important sources of greenhouse gases to the atmosphere have populated the specialized literature. The conversion of terrestrial land to an aquatic area for the creation of a reservoir is a major issue with regard to carbon cycle change in the production of carbon dioxide and methane. The impacts to carbon dynamics of impoundment in karst aquatic systems are mainly the result of three factors，(1) flooding of vegetation; (2) release of carbon from the flooded terrestrial biomass including soil organic matter; (3) the establishment of a new physical-chemical environment for carbon fixation and decomposition. So，the hydro-reservoirs produce biogenic gases through decomposing organic matter underwater. The bottom of the reservoir contains biomass that decomposes anaerobically，emitting principally CH4 and N2O，and secondarily CO2. In aerobic decomposition only CO2 and N2O are emitted.Moreover，the karst hydro-reservoirs will receive allochthonous nutrients from their catchment areas via streams and through groundwater and surface water inflow. Increased nutrient input into karst hydro-reservoirs causes eutrophication. The increased availability of easily degradable autochthonous organic matter increases decomposition and oxygen consumption，promoting oxygen depletion in the water column and sediment. Anoxic conditions can increase CH4 emissions from karst hydro-reservoirs by enhancing the CH4 production and/or decreasing the CH4 oxidation and low oxygen availability can also promote N2O production. Nitrous oxide generation in karst hydro-reservoirs generally requires steep oxygen gradients allowing sequential aerobic nitrification and anaerobic denitrification to take place. This implies that the karst hydro-reservoirs could also be supersaturated with CH4 and N2O.According to the results，the author provides the suggestion for the future study，(1) the qualitative analysis and quantitative calculation about greenhouse gas emissions from karst hydro-reservoirs must be carried out，and the emission rate from different carbon source should be compared; (2) the space-time analysis of the influence on the greenhouse gas emission should base on the karst carbon cycle and the life-cycle assessment methods for karst hydro-reservoirs should be considered.