| For mitigating the global warming, the reductions of greenhouse gas emissions have caused extensive concern of the international community. CH4 is an important greenhouse gas and its global warming potential is 30 times greater than that of CO2. The contribution of CH4 to greenhouse effect ranked only second to CO2. Constructed wetland is an important source of CH4. So studying the rules and factors of CH4 emission from constructed wetlands had important significance to understand the contribution of CH4 to greenhouse effect and evaluate the ecological efficiency of constructed wetlands.The pattern and factors of CH4 emissions from Xinxue River Constructed Wetland were studied by using the static chamber-GC theory. By long term systematically field sampling combined with subsequent laboratory analyses, CH4 emission status and variation trends in the constructed wetland were investigated. First I studied the diurnal variation of CH4 emission in the constructed wetland and found some interesting rules. In order to test the rules in larger scale, I studied the seasonal variation and spatial variation of CH4 emission, and then analized the influencing factors. But there were many uncertainties in field sampling, so I designed a water level control experiment to study the influence of water level. In the previous studies, I found that there were many research cases on the rules and factors of CH4 emission from wetlands in China, but they were lack of deep discovery and summary. So I also did the jobs on data mining and literature review to know the factors of CH4 emissions in larger scale. The main research conclusions are as follows:(1) Diurnal variations of CH4 emission in Xinxue River Constructed Wetland at upstream and downstream were consistent. The overall variation trend of CH4 emission had two peaks, one at 6:00-8:00, and another at 23:00-1:00. CH4 emission variation can divided into 3 parts in 24 hours. It increased gradually from 0:00 to 7:00; then decreased gradually from 7:00 to 14:00; and last increased again from 14:00 to 24:00, but there was a slight decline at 21:00 to 22:00. CH4 emission had positive correlation with water temperature and air temperature in summer, but it was not synchronous. The variation of CH4 emission delayed 6-11 hours and 7-14 hours behind the variation of water temperature and air temperature respectively. But they were both most relevant with CH4 emission when delayed 10 hours. The lag time of air temperature variation was longer than that of water temperature because water temperature variation is the direct reason. There was no correlation between CH4 emission and pH, but it was positive correlated with humidity and there was no lag time. So the influence of humidity to CH4 emission was faster than temperature.(2) The extent of CH4 emissions across four seasons was as follows:summer> spring> autumn> winter, with differences in daily emission trends during different seasons. CH4 emission in summer is significantly higher than that in other seasons. The average temperature in summer was 32.6℃, which was in the range of optimum temperature of methanogen. So CH4 production was high because of the strong activity of methanogen. CH4 emission in spring would be comparatively higher than that in autumn because CH4 accumulated in winter duo to frozen or low temperature would emitted with the temperature raise. Annual mean emission of CH4 was positive correlated with water temperature and air temperature. It was more correlated with water temperature than that with air temperature. CH4 daily emission presented an upward trend and downward trend in spring and autumn respectively. It showed an increased after an initial decreased in summer but had no obvious change in winter. The daily variation of CH4 emission in four seasons varied from each other.(3) Obviously the water quality at upstream was worse than that at downstream. Because water flow from upstream to downstream and was purified step by step when wastewater flowed in. Organic pollutants precipitated more in the upstream water than in the downstream water, so organism that be decomposed by microorganism at upstream was more. But in winter, the activity of methanogen was generally low because the temperature was below 10℃, so the decomposition capacity of methanogen was restrained. In theory, CO2 in atmosphere increased might directly promote the reduction of CO2 to CH4. In our study, CH4 and CO2 from Xinxue River Constructed Wetland in spring, autumn and winter had positive correlation. There was no significant correlation between CH4 and CO2 concentration in summer.(4) The controlled experiments on different water level showed that, when calculating CH4 fluxes, it was unreliable to do linear analysis using 4 values in 1 hour which was frequently adopted in some literatures. The linear analysis has the condition that values were linear distribution. When the water level was 10cm, CH4 emission increased first and then decreased in every hour at 9,10 and 11 am. When 20cm, CH4 emission decreased first and then increased. When 40cm, there was no rules. The initial CH4 concentration in every hour was as follows:20cm> 40cm> 10cm. CH4 concentration was positive correlated with water temperature and air temperature, but it was negative correlated with humidity.(5) Types of wetland had important effects on CH4 emission of wetlands in China. CH4 flux values can be arranged in order which were:rice paddy> marsh wetland> reservoir> riverine wetland> lake wetland> coastal wetland. CH4 fluxes in lake wetlands, marsh wetlands, riverine wetlands and reservoirs have no significantly difference. But CH4 fluxes in rice paddies and marsh wetlands are significantly higher than those in coastal wetlands. CH4 fluxes also have significant differences in different types of climate zones. CH4 fluxes in warm temperate zones and mid-temperate zones have no significant differences, but CH4 fluxes in plateau frigid zones are significantly higher than those in tropical and subtropical zones. Our study showed that wetland type and climate have significant interaction effect on CH4 emission from wetlands. In tropical zones, lake wetlands and marsh wetlands release CH4 but coastal wetlands absorb CH4. In subtropical zones, CH4 fluxes in rice paddies are significantly higher than those in lake wetlands and coastal wetlands. In mid-temperate zones, there are no significant differences between different wetlands. In plateau frigid, only marsh wetlands in Qinghai-Tibet Plateau are studied in China whose CH4 mean flux is 50.25gm-2yr-1.In general, time variation and spacial variation of CH4 from Xinxue River Constructed Wetland presented different rules. Water temperature, air temperature, pH, humidity and water level were all important influence factors of CH4 emission and the influence methanism of each one was different. In the review and research of CH4 emissions from wetlands in China, wetland type and climate have significant interaction effect on CH4 emission was found.Both the wetland type and the climate zone should be considered when estimating CH4 fluxes from wetlands in China as well as in other countries. |
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