Changes Of The Swamp And Alpine Meadow Ecosystems And Its Carbon Balance In The Head-water Region Of The Yangtze River: Response To The Global Warming

As a region with fragile ecosystems and sensitive response to the global climate change,the Qinghai-Tibetan Plateau is vulnerable to the interference from human activities and the global warming impact.With the global warming and human activity intensifying,the high-cold ecosystems,in particular the grassland ecosystems,are suffering disturbance from unprecedented degradation,rodents,and the human destruction,the near-surface warming and such as factors,impacting on the ecosystem balance,on the Qinghai-Tibetan Plateau currently.As an important composition of the major research project funded by National Natural Science foundation of China-The response and the hydrological effect of the high-cold ecosystems on the global climate change in the headwater regions of the Yangtze and the Yellow River,this study selected two typical and representative ecosystems- swamp meadow ecosystem and alpine meadow systems-to sudsy especially in the Fenghuoshan region on the Qinghai-Tibetan Plateau.This paper investigated and studied on the following several aspects in detail:(1)to research and study the vegetation conditions,biomass under different conditions,soil types and climatic conditions of the study area;(2)to study the impacts of two high-cold ecosystems' degradation on soil physical and chemical properties and carbon fluxes from the ecosystems;(3)Under the condition of air temperature elevated,to study the influences of elevated temperature on the near-surface CO₂ concentration and the coupling relations between the CO₂ concentration and the environmental factors,at the same time,to identify the most sensitive factor to impact the change of CO₂ concentration;(4)to research the impact of elevated temperature on the net CO₂ exchange fluxes of high-cold ecosystems,and the coupling relations between the net CO₂ exchange fluxes and environmental factors;(5)to research the influences of elevated temperature on the carbon balance of two high-cold ecosystems.(â… )Research on the impact of degradation on the high-cold ecosystems,the results showed that:1.The soil properties of swamp meadow and alpine meadow with different degradation degrees showed significant differences(analysis of variance,F＞ 7.47,P＜0.05).The contents of organic matter,ammonia nitrogen and nitrate nitrogen in soil of non-degraded meadows are higher than those of the moderately and severely degraded meadows.With the degradation becoming aggravating,the nutrients in the soil run off seriously.The biomasses among different degradation degrees of the swamp meadow and alpine meadow have significant differences(analysis of variance,F＞4.63,P＜0.05).During the whole growing period,the biomasses of its aboveground and underground are behaving the increasing trend;however,among the different degradation degrees of the same ecosystem,gradually declining are the biomasses.2.The responses of CO₂ and CH₄ emissions from the swamp meadow are remarkable to the grassland degradation.Among them,the CO₂ emission fluxes from the non-degraded swamp meadow are 1.08-1.69 and 1.41-4.43 times higher than those from the moderately degraded and seriously degraded ones,respectively;the CH₄ emission fluxes from the non-degraded swamp meadow are 1.09-3.5 and 2.5-11.27 times higher than those from the moderately and seriously degraded one,respectively.The worse the degradation becomes,the lower are the CH₄ and CO₂ fluxes from the swamp meadow.For the alpine meadow,compared to the non-degraded and moderately degraded ones,the severely degraded meadow showed a higher CO₂ emission flux and a strong absorbing methane capacity.The CO₂ emission flux from the severely degraded ones is 1.05-78.5 and 1.04-6.28 times higher than that from the non-degraded and moderately degraded ones, respectively.The absorbing CH₄ flux of the severely degraded ones is 6.6-21 and 1.1-5.25 times higher than that of the non-degraded and moderately degraded ones,respectively.The more seriously degraded,the greater the intensity of the CO₂ emissions and methane absorption.3.Temperature,soil moisture and soil temperature are the major environmental factors to control the carbon fluxes from the swamp meadow and alpine meadow ecosystems.(â…¡)Research on the impact of warming on the near-surface CO₂ over the high-cold ecosystems,results showed that:For the swamp meadow ecosystem: 1.Under the conditions that the near-surface temperatures increased 3-5℃and 1-2℃respectively,the soil temperature at a depth of 5cm was higher 3.67℃and 1.65℃respectively than that at the nature condition;The soil moisture at a depth of 5cm was lower 2.1%and 7.9%respectively than that in the controlling sites;At the same time,the higher the near-surface temperature, the initial melting rate of the permafrost faster.2.During the whole observation,at the sample plots that the near-surface temperature increased 3-5℃and 1-2℃,respectively,the average CO₂ concentration was higher 29.3ppm and 9.8ppm,respectively than that over the nature sample plots.3.Air temperature,soil temperature and moisture at a depth of 5cm are the major environmental factors to impact the CO₂ concentrations.Sensitivity analysis shows that:the changes of CO₂ concentration was most sensitve to the soil temperature and moisture content at the depth of 5cm.For the alpine meadow ecosystem:1.At the conditions that the near-surface air temperatures were average 4.76℃and 2.00℃higher respectively than that at the natural state,the soil moistures at a depth of 5cm were average 6.97%and 2.68%lower than those at the natural condition,respectively.2.During the whole observation period,the near-surface CO₂ concentrations over the sample plots,which temperature was elevated 4.76℃and 2.00℃, respectively than that at the natural sample plots,were average 37.78ppm and 21.89ppm higher than those over the natral sample plots.3.Air temperature,soil temperature and moisture at a depth of 5cm are the main environmental factors to impact the near-surface CO₂ concentrations on the alpine meadows.Sensitivity analysis shows that:the changes of CO₂ concentration was most sensitve to the soil temperature at the depth of 5cm.(â…¢)Research on the impact of warming on the net ecosystem carbon exchange (NEE)and carbon balance of the high-cold ecosystem,results showed that:For the swamp meadow ecosystem:1.The influences of the near-surface air warming were very significant on the net ecosystem CO₂ exchange of the swamp meadows.In the growing seasons, the net ecosystem CO₂ exchanges of the swamp meadows gradually increased with the warming gradient ascending.2.At the sample plots treated with warming,the changes of the average monthly net CO₂ exchange fluxes from the swamp meadows all showed a single peak curve and the net CO₂ exchange fluxes from the swamp meadow plots all reached the most low values at the time from the end of July to the beginning of August.From early June to mid-August,the swamp meadows both at the plots with 3-5℃elevated and those at natural conditions showed a capacity of absorbed;and at the sample plots with 1-2℃elevated,the swamp meadows performed the characteristics for CO₂ emissions during the whole growing seasons.3.Regression analysis showed that:the net CO₂ exchange fluxes from the swamp meadows with three different treatments were significantly correlated with the soil temperature at a depth of 5cm and the correlation coefficients were greater than 0.58.4.The swamp meadow ecosystem in the Fenghuoshan region is a carbon sink. At the sample plots treated with 3-5℃,1-2℃elevated and natural state,the carbon sequestration rates of the swamp meadows were 5044.77±4.02g·m^-2·a^-1,3242.4±1.70g·m^-2·a^-1and 1836.6±4.02g·m^-2·a^-1, respectively.The near-surface temperature warming enhanced the respiration of the ecosystem,and took a positive feedback effect on the carbon balance. The warmer the near-surface temperature,the stronger the effect of carbon sinks.For the alpine meadow ecosystem:1.The influences of the near-surface air warming were also very significant on the net ecosystem CO₂ exchange of the alpine meadows.In the growing seasons,the net ecosystem CO₂ exchanges of the alpine meadows gradually increased with the near-surface temperatures ascending.2.At the sample plots treated with warming,the higher the near-surface air-temperature elevated,the greater the slope of the curve,which showed the changes of the net CO₂ exchange fluxes from the alpine meadow ecosystem. This denoted that the respirations of the ecosystem were greater and the loss rate of soil carbon would be accelerated.3.Regression analysis showed that:the net CO₂ exchange fluxes from the alpine meadows with three different treatments were also significantly correlated with the soil temperature at a depth of 5cm and the correlation coefficients were greater than 0.53.4.The alpine meadow ecosystem in the Fenghuoshan region is a weak carbon sink.At the sample plots treated with 3-5℃,1-2℃elevated and natural state, the carbon sequestration rates of the alpine meadows were 898.67±2.38g·m^-2·a^-1,765.24±2.21g·m^-2·a^-1and 543.80±1.99g·m^-2·a^-1, respectively.The near-surface warming on the alpine meadow ecosystem has also taken a positive feedback effect on the carbon balance,and strengthened the effect of carbon sink.