Spatial-temporal Variation Of Greenhouse Gas Flux And Its Environmental Factors At Miyun Water Reservoir

Water reservoir is one important type of artificial wetlands with multi-functions, including water supply, hydro-power generation, flood control, transport and tourism. Within the context of global climate change debate, hydro-power development has been one of the important measures in national strategies to reduce carbon emission. However, since large area of forest or other land-use are converted to artificial wetlands due to water reservoir construction, whether hydro-power is one form of the clean energy has been questioned. Therefore, assess the role of water reservoir in global carbon cycle has been becoming an international focus recently. Understanding green house gas emission from water reservoirs can help to provide guidance on dam design and operation. Data from the study can provide science-based evidence for international climate change negotiations.This study focused on spatial-temporal variation of CH₄ and CO₂ flux at Miyun Water Reservoir in Beijing, P.R. China. Gas samples were taken at two zones of the water reservoir, namely, the seasonal flooded zone between the terrestrial land and the permanent water body (littoral zone), and the water body of the reservoir. The surrounding terrestrial land area was also monitored as control plots. Standardized static chambers were installed at the sites to assist gas sampling in August and October,2009, January and May 2010, when represent the four different seasons in Beijing (January for winter, May for spring, June and August for summer, and October for autumn). Meanwhile, key environmental factors that may contribute to the CH₄ and CO₂ emission were measured, including 22 factors about vegetation, water properties, soil properties, illumination and wind speed. It's the first time to study CH₄ and CO₂ emission under different spatial-temporal scales combining with several environmental factors among water reservoir in China.The results showed that:(1) Diurnal variation of greenhouse gas among water reservoirThe diurnal variation of CH₄ flux appeared two paterns:daytime single peak and nighttime single peak, daytime double-peak and nighttime single peak; The diurnal variation of CO₂ flux appeared three patterns:daytime single peak, daytime single peak and nighttime single peak, daytime double peak and nighttime single peak.The daytime peak of soil temperature in 5cm deep, surface temperature, illuminance and wind speed were consistent with that of CH₄ flux and CO₂ flux. Only the nighttime peak of wind speed was consistent with that of CH₄ flux and CO₂ flux.(2) Seasonal variation of greenhouse gas among water reservoir There was a little difference among seasonal variation of CH₄ flux and CO₂ flux in Miyun Water Reservoir. CH₄ emission mainly occurred in summer while CO₂ flux was high both in spring and summer when the metabolism of plant and soil microorganisms was active.CH₄ emission at littoral zone took place primarily in summer, whereas it is neglectable in rest of the year. The CH₄ flux in summer was 6 454.7,193.6,880.2 times higher than that in spring, autumn and winter, respectively, with the range of -0.003-19.364mg m^-2·h^-1; at permanent water area, CH₄ flux in summer was 1.9, and 3.2 times higher than that in spring, and autumn, respectively, with the variation range of 0.150-0.486mg m^-2·h^-1,CO₂ emission at littoral zone in growing season (spring and summer) is 4.6 times of that in non-growing season, with the variation range of 20.1-582.8mg m^-2·h^-1; whereas the CO₂ emission at water body areas in growing season was 84.1 times of that CO₂ sequestration at non-growing season, with the variation range of -1.0-43.5mg m^-2·h^-1.Compare to the water reservoir, surrounding land areas shown a huge contrast in CH₄. In the growing season, the land surface appeared to be CH₄ sink, whereas in non-growing season, it appeared slightly CH₄ sink, the difference was small, with only at-0.025-0.046mg m^-2·h^-1. However, the pattern of CO₂ emission at land areas was similar to water reservoir.(3) Spatial variation of greenhouse gasSpatially, the order of CH₄ flux in different zones was littoral zone>water area>land area, with the littoral zone the most important emission source, which is 21.8, and 754.1 times higher than that from the water body areas and terrestrial areas, respectively. On the other hand, the order of CO₂ flux appeared to be terrestrial area> littoral zones> water body. Terrestrial area became the major carbon source, which is 2.0, and 28.7 times of littoral and water body areas, with the littoral zones 14.1 times of the water body areas in CO₂ emission.(4) The important environmental factorsVegetation:Both CH₄ flux and CO₂ flux in areas covered with vegetation were higher than that of in areas without vegetation (p<0.01, df=97). The biomass correlated positively with the CO₂ flux (p<0.01, N=72). The relationship between biomass and CH₄ flux was determined by the CH₄ production capacity of the environment. The differences of vegetation cause differences of CH₄ and CO₂ emission. Optimization of the density, pattern and species of plant may also contribute to reduction of carbon emission.Water properties:Both CH₄ flux and CO₂ flux positively correlated with water temperature (p<0.05, N=33). CH₄ flux is significantly negative correlated with water depth (p<0.01, N=33). CO₂ flux is significantly negative correlated with pH (p<0.05, N=32). Both CH₄ flux and CO₂ flux negatively correlated with dissolved oxygen (p<0.01, N=31) significantly. Both CH₄ flux and CO₂ flux positively correlated with ammonia nitrogen (p<0.01, N=28). CO₂ flux positively correlated with nitrate (p<0.05, N=30). Based on the relationship between water depth and greenhouse gas emission, increasing water depth opportunely can decrease emission. Besides, alleviating eutrophication is another potential may contribute to reduction of green house gas emission.Soil properties:temperature of the soil on surface and in 5cm depth correlated positively with CO₂ flux (p<0.01, N=72). During suitable temperature range, both CH₄ flux and CO₂ flux increased with the temperature increase. The soil water content correlated positively with CH₄ (p<0.01, N=48) and correlated negatively with CO₂ flux (p>0.05, N=48). Nitrate correlated negatively with CO₂ flux (p<0.05, N=14) and ammonium salts correlated positively with CH₄ (p<0.01, N=14).