Evaluation Of Carbon Sink Effects Of Ecological Engineering In The Littoral Zone Of The Three Gorges Reservoir

The littoral zone caused by the water level changes of the rivers, lakes and reservoirs is the eco-tone of terrestrial and aquatic ecosystems. The littoral zone of the Three Gorges Reservoir(TGR) is an important part of this area, which is directly related to the ecological security of the TGR and even to the whole Yangtze river basin. In order to ensure the ecological safety of the reservoir, we carried out a series of projects, such as the littoral-forest and dike-pond project. Under the background of global change, the reservoir is considered to be one of the important sources of greenhouse gas emissions. Through the dynamic monitoring of greenhouse gas emissions, we used the life cycle assessment method to evaluate the carbon sequestration benefits of the ecological engineering.According to the characters of the plant community, the experiment sites can be divided into four parts: the traditional agricultural area, the natural littoral zone, dike-pond project and littoral-forest project. We carried out greenhouse gases sampling during the growing season and flooding season, and used the static boxes method in the land area and the floating boxes method in the submerged area to carry out field experiment, respectively. We used the gas chromatographic method to monitor CH4、CO2 and N2 O concentrations. Finally, the carbon sequestration benefits are calculated by the method of biomass measurement.With the water level decline in spring, plants will grow quickly and microbial metabolism of soil is very active. Under the condition of different land use, greenhouse gases emissions is high, the growing season contributes to more than 80% of the total amount of greenhouse gases emissions, which is significantly higher than the emissions of the flooding season in winter. CH4 emission fluxes in the dike-pond and rice paddies significantly greater than the natural littoral zone and littoral-forest project. CH4 production is closely related with the condition of anaerobic environment that is beneficial to the methanogens metabolic activity. Compared with the natural pond, the presence of vascular plants favors CH4 gas directly emission into the atmosphere through the plant respiration, avoiding oxidation in the water.During the submerged season, the emissions of CH4、CO2 and N2 O in the littoral zone show weak emission, and this kind of emission contributes to the total annual emissions is limited. The emission flux of Cynodon dactylon plant community is less than 10%. The emission flux of the dike-pond project is high in winter, changing between 23% and 10%. Greenhouse gases emissions is directly or indirectly influenced by the plant community, soil temperature, water temperature, dissolved oxygen(DO), redox potential and p H. The winter low temperature that affects microbial metabolic activity is one of the important reasons that cause emission flux decrease.Research shows that the littoral zone of different land use with different carbon sinks, the net greenhouse gases emission of the planting rice is 3.22 t-CO2.ha-1.year-1. However, the net carbon sink of the Cynodon community is 1.07 t-CO2.ha-1.year-1, and the net carbon sinks of the Sagittaria and Canna generalis are 19.74 t-CO2.ha-1.year-1 and 16.33 t-CO2.ha-1.year-1, and the net carbon sinks of the Aeschynomene indica and Cyperus community are 1.90 t-CO2.ha-1.year-1 and 3.40 t-CO2.ha-1.year-1, respectively. The carbon sequestration ability of the littoral-forest is significantly affected by understory plant communities. Comnyza canadensis and Cyperus communities show weak carbon sources, with 1.33 t-CO2.ha-1.year-1 and 2.58 t-CO2.ha-1.year-1 release respectively. As the trees gradually adapt to the periodic flooding conditions, the forest canopy density will increase, the competitive capacity of the forest will enhance, and the littoral-forest engineering will gradually enhance its capability of carbon sequestration. From the economic point of view, the littoral zone with rice planting with release of 1kg of CO2 will produce direct economic value of 0.23 yuan. The production of Nelumbo nucifera and Canna generalis will produce direct economic value of 0.26 yuan and 1.43 yuan respectively. Under the condition of the near natural management method, the littoral-forest engineering with fixed 1kg of CO2 will produce direct economic value of 0.30 yuan.