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The Effects Of Global Climate Change On The Changes Of Alpine Grassland Ecosystem And Its Carbon Emission In Head-water Region Of The Yangtze River

Posted on:2012-10-21Degree:DoctorType:Dissertation
Country:ChinaCandidate:W BaiFull Text:PDF
GTID:1110330335966457Subject:Environmental Science
Abstract/Summary:PDF Full Text Request
As the major environmental problems, climate warming and nitrogen deposition have become the focus of people's attention in-recent years. According to the IPCC, the-global temperature will rise 1.5℃-4.5℃in 21st century. Because of the human activities such as burning fossil fuels, fertilizer use and industry emissions, lots of nitrogen compounds were released to the atmosphere. The nitrogen compounds accumulate in the atmosphere, and continuously deposit on terrestrial ecosystems and aquatic ecosystems. According to estimation, the reactive nitrogen deposit on terrestrial ecosystems is 43.47Tg each year, and the amount increases continuously. With the global change intensifying, climate warming and elevated nitrogen deposition will deeply affect the plant growth and physiological characteristics, community structure and function, and the key processes of carbon cycle in terrestrial ecosystems. The ecosystems in high latitude and high altitude of northern hemisphere are more sensitive to global change.As the largest and highest geomorphic unit of Eurasia, the Qinghai-Tibet Plateau is regarded as a sensitive region of climate change because of its unique geographical position and special variation characteristics of climate. The alpine grassland ecosystems, which are widely distributed in Qinghai-Tibet Plateau, might be more fragile and instable, making them especially vulnerable to global change and human disturbances. This study selected alpine meadow and swamp meadow, which are typical and representative in alpine grassland ecosystem as the research objects. In our experiment, open-top chambers (OTCs) were utilized to generate an artificially warmed environment and nitrogen fertilizer was applied to simulate nitrogen deposition, and the aim of this study was to study the influences of elevated temperatures, nitrogen deposition, and their interaction on biomass production, heat-water processes in the active layer, and ecosystem respiratory rate of the alpine grassland ecosystem. The results are as follows: (Ⅰ) To investigate the impact of simulated warming on alpine grassland ecosystem, the results showed that:1. The aboveground and underground biomass of alpine meadow increased significantly with temperature rising. But in vigorous growth period and the end of the growing season, the increases of above ground and underground biomass in OTC-1 were bigger than OTC-2. For the swamp meadow, the aboveground and underground biomass in OTC-1 and OTC-2 also increased significantly compared to Control manipulation. Moreover, due to the higher temperature, the increases in OTC-2 was bigger than OTC-1 in any period of growing season.2. The beginning time of freezing of the active layer was delayed with the temperature increase, and the beginning time of thawing was expedited, indicating that the entirely frozen period of shallow soil in the active layer was shorten with the increasing temperature. The maximum invasion depth and duration of the negative isotherm during the entirely frozen period decreased, and soil freezing was retarded when temperature raised. Meanwhile, the positive isotherm during the fully thawed period clearly increased, and the soil thawing was hastened, indicating that the elevated air temperature caused the thickening of the active layer of the permafrost.3. Ecosystem respiration in alpine meadow and swamp meadow under different manipulations showed a clear diurnal variation of a single peak. Meanwhile, in different phonological periods, the CO2 emission fluxes in the two types of meadow ecosystems showed evident seasonal variations under different manipulations. Moreover, in the vigorous growth period, the CO2 emission fluxes reached the peak values.4. CO2 emission from alpine meadow was significantly affected by elevated temperature. In different phonological periods, the CO2 emission fluxes in OTC-1 were 1.4%-122.5% higher than Control, and 29.1%-171.7% less than OTC-2. For the swamp meadow, the CO2 emission fluxes gradually increased with the rising temperature, and the increasing trend of ecosystem respiratory rate was larger when the temperature were higher.5. The CO2 emission fluxes in the two types of meadow ecosystems were significantly correlated with the environmental factors under different manipulations. There was an exponential correlation between the CO2 emission flux and air temperature, as well as the soil temperature at the depth of 5cm. And, the CO2 emission flux have quadric multinomial relation with soil moisture at the depth of 5cm. Meanwhile, from Control to OTC-2 plots, the goodness of fit and significance of the two kinds of models decreased with the rising temperature, but in swamp meadow ecosystem, the relationship between CO2 emission flux and air temperature did not show this regularity.(Ⅱ) To investigate the impact of simulated nitrogen deposition on the CO2 emission in alpine grassland ecosystem, the results showed that:1. In different nitrogen fertilizer levels, the CO2 emission fluxes in the two types of meadow ecosystems showed evident seasonal variations. Starting from May (the beginning of growing season), the CO2 emission fluxes gradually increased, by August (the vigorous growth period) it reached maximum value and then gradually decreased in September (the end of growing season).2. Under the condition of continuous nitrogen input, the CO2 emission fluxes of alpine meadow under N5 manipulation were always higher than control plots(NO) in growing season, which indicates that simulated nitrogen deposition facilitate to the ecosystem respiration to some extent, and it was found that the promoting effect was significant only in the early days of applying nitrogen fertilizer. For the swamp meadow ecosystem, the CO2 emission fluxes under N5 manipulation were always obviously higher than NO manipulation throughout the growing season, and the differences between the two manipulations were extremely significant in all stage of growing season. Therefore, simulated nitrogen deposition has a remarkable positive impact on the ecosystem respiration of swamp meadow.3. In different nitrogen fertilizer levels, the CO2 emission fluxes in the two types of meadow ecosystems were significantly correlated with the environmental factors. For the alpine meadow, there was an exponential correlation between the CO2 emission flux and air temperature, as well as the soil temperature at the depth of 5cm. And, the CO2 emission flux have quadric multinomial relation with soil moisture at the depth of 5cm. For the swamp meadow, there was an exponential correlation between the CO2 emission flux and soil temperature at the depth of 5cm, the CO2 emission flux have quadric multinomial relation with soil moisture at the depth of 5cm, and there was favorable linearity relation between CO2 emission flux and air temperature. Meanwhile, from NO to N5 manipulation, the goodness of fit and significance of the three kinds of models decreased with increment of nitrogen-applying level.4. Based on Stepwise linear regression analysis, the variation of ecosystem respiratory rate in alpine meadow under NO manipulation was caused by the common influence of soil temperature and air temperature; Under N5 manipulation, soil temperature and moisture were the major controlling factors of the CO2 emission flux. For the swamp meadow, the variation of ecosystem respiratory rate under NO manipulation was due to the interaction of soil temperature and moisture; Under N5 manipulation, air temperature and soil moisture were the major controlling factors of the CO2 emission flux.(Ⅲ) To investigate the combined effect of simulated warming and nitrogen deposition on the CO2 emission in alpine grassland ecosystem, the results showed that:1. Ecosystem respirations in alpine meadow ecosystem showed evident seasonal variations under 4 different manipulations(CK, N, T, and NT), and reached maximum values in the vigorous growth period (August); The CO2 emission fluxes under NT (elevated temperature and nitrogen applying) manipulation were always higher than that under other manipulations throughout the growing season, the CO2 emission fluxes under CK (ambient temperature and ambient nitrogen deposition) manipulation were always lower than that under other manipulations in all stage of growing season, but under N (nitrogen applying) and T (elevated temperature) manipulations, there was no such evident regularity of ecosystem respiration; Based on the main effect analysis, both warming and nitrogen applying have significant impact on the CO2 emission flux, and there was significant interaction between them. For the swamp meadow ecosystem, ecosystem respiration rates showed evident seasonal variations under 4 different manipulations; In different phonological periods, the CO2 emission fluxes under CK, N and T manipulations showed the same trend:vigorous growing period> beginning of growth> withering stage, they reached maximum values in August; Under NT manipulation, although the CO2 emission flux reached maximum value in August, but its minimum was in June; Meanwhile, under the 4 manipulations, the order of CO2 emission flux throughout the growing season was:NT>N>T>CK; Based on the main effect analysis, both warming and nitrogen applying have significant impact on the ecosystem respiration rate, and there was notable interaction between them.2. Under NT manipulation, the CO2 emission fluxes in the two types of meadow ecosystems were remarkably correlated with the environmental factors, but the correlation was weaker than the other 3 manipulations. Meanwhile, in the two types of meadow ecosystems under NT manipulation, there was an exponential correlation between the CO2 emission flux and air temperature, as well as the soil temperature at the depth of 5cm. And, the CO2 emission flux have quadric multinomial relation with soil moisture at the depth of 5cm. Moreover, the goodness of fit and significance of the two kinds of models under NT were lower than the other 3 manipulations. Especially in the swamp meadow, the goodness of fit of the regression equation between CO2 emission flux and soil moisture was very low, and the determination ratio was as low as 0.289. Therefore, the relationship between CO2 emission flux and the environmental factors in the alpine grassland ecosystem have become more complex due to the combined effect of warming and elevated nitrogen deposition.3. Based on Stepwise linear regression analysis, the variation of ecosystem respiratory rate in alpine meadow under NT manipulation was caused by the common influence of soil temperature and moisture; For the swamp meadow ecosystem, soil temperature and air temperature were the major controlling factors of the CO2 emission flux under NT manipulation.
Keywords/Search Tags:Global climate change, Warming, Nitrogen applying, Alpine meadow, Swamp meadow, Heat-water process, Biomass, Carbon flux, Ecosystem respiration, Qinghai-Tibet Plateau
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