| According to the historic record, Qing-Tibetan Plateau is more sensitive to climate change than the plain area, and shows the forecast character. Through observation and historical analysis of data, the two period, it had been spotted the one of new warm period in China since1980 and another temperature variation data sheet since 1955 with a 10-year scale were recorded firstly in stations of Lizhi, Bomi and etc. which located in south east part of Qing-Tibetan Plateau, and then spreaded to the north and to the east. Comparing with eastern China, the record showed 4 to 8 years in advance, and a century-scale temperature change is also10 to 60 years earlier than the eastern region of China. Modern-scale climate observations showed that the Qing-Tibetan Plateau since the mid-50’s, the main stations are showing warming trend, and especially in winter season were much more obvious. The average linear warming rate was 0.16℃/10a, on an average of 0.32℃in winter/10a, the warming rate was more than the other area sharing same latitude in Northern Hemisphere.Soil matrix, as the foundation of Qinghai-Tibetan Plateau ecosystem, together with the specific atmospheric conditions in the highlands and the fauna and flora consisted the common form of carbon storage and exchange system. Therefore, in the Qinghai-Tibetan Plateau ecosystem scale, soil carbon pool of the quantity, distribution, dynamic change and its response to the different behavior of the system are not only a regional carbon cycle of the key elements of the system, but also important on the quantitative control on some management measures to achieve sustainable use of soil resources.The CENTURY model, developed by Parton and colleagues, is probably the most widely used and validated process-oriented model that has been tested successfully on grasslands worldwide to predict medium to long term dynamics in C, N and other nutrients in response to changes in climate, land use, and management. In this study, CENTURY model was parameterized by using realistic data in alpine meadow sites obtained from published works or by analysis of data gathered at Haibei station, CAS. Followed, the CENTURY model was run 10,000 years under undisturbed native meadows to obtain equilibrium of the CENTURY model on the scenario of Soil Oraganic Soil(hereafter as: SOC) accumulation from zero. For this simulation, the model was run by using yearly time step, and the final outputs were SOC in September. Because no data is available over such a lengthy period, the weather data was statistically generated by the CENTURY model sequencing based on the 45 years climate data on file.100.Model validation was also carried out by simulating the seasonal dynamics of CO2 flux from the steady-state soils of alpine meadows using observed monthly maximum and minimum temperatures, and monthly precipitation as driving variables. Regression analysis was then performed to test the simulation effectiveness by comparing measured and simulated CO2 fluxes. Results suggested that simulated CO2 flux from undisturbed native alpine meadow soil during this time was similar to observations, but over- or under-estimated during the entire simulation period, particularly in winter during which a rapid increase in CO2 flux occurred. Comparisons between the CENTURY model and the observations resulted in correlation coefficient r2 value of 0.89 (P<0.01; n=24) suggesting that Century model had potential to mimic CO2 flux in alpine meadow of Qinghai Province, and approximately 89% of the total variation in the data can be explained by the model.Following simulation suggested that SOC over this period was essentially constant and comparable with the initial soil carbon levels found in undisturbed native alpine meadows during its equilibrium:76.0～76.9 versus 77.0～78.0 t/hm2. The active, slow and passive SOC pool sizes, on average, were approximately 2.1±0.03,44.7±0.05 and 29.9±0.01 t/hm2, accounting for 2.8%,58.3% and 39.0% of the total soil carbon pool sizes, respectively. The total soil carbon pool sizes, including below-ground structural and metabolic pools, were approximately 77.7±0.07 t/hm2.Climatic change in alpine meadow site presented evidence of an increase in annual air temperature and constant statistical fluctuations in annual precipitation over the past 45 years (Fig.4). Simulated SOC showed a stable fluctuation in this period. Correlation analyses between simulated SOC and the mean annual air temperature resulted in a correlation coeffient r2 value of 0.54 (p<0.01). Precipitation, however, showed no effects on SOC and its fractions.Grazing reduced SOC content in alpine meadow soil, and with the increases of grazing intensity, the 0～20cm depth SOC declined significantly. The SOC loss were about 8.2%,10.6%,29.6% and 7.3% under mild, moderate, severe and winter grazing compared with the natural alpine meadow during the past 45 years. Cultivation effects on SOC contents were more intense than that of the grazing, but it may not be occurred under reasonable grassland managemental practices. Our simulation showed that SOC depreased after cutivating, whereare, the reducing rate was gradually easing and the SOC eventually reached to a new dynamic balance after experiencing a rapid increase and decrease during the past 45 years. The SOC contents in 0～20cm depth soil were about 64.4±0.57 t/hm2 under equilibrium, with the active, slow and passive SOC pool sizes were about 2.3±0.11, 32.2±0.67 and 29.9±0.01 t/hm2, which were accounting 3.5%,49.9% and 46.6% of the total SOC, repectively. The SOC loss was limited up to 14.0% without considering the changes of SOC in crop land during the past 45 years.Further simulation based on GCM under climate scenarios assumption in future showed that the warming in future might cause small losses of SOC in alpine meadow under nature condition. Grazing alpine meadow also presented yearly decreasing trend due to continuous intervention of grazing, but the losses would be less than in past 45 years. Comparing the variation of trend on SOC, the SOC in annual artificial grassland converted from alpine meadow reclaim relatively stable, the SOC was decreasing though the overall SOC change was not significant. The SOC during the period from 2006 to 2050 under AlB and B1 climate change scenarios were about 63.6～62.9 and 63.6～62.5 t/hm2,respectively, which ware 1.4% and 2.0% lower than level in year 2005.Artificial recovery measurements on degraded grassland might improve the soil carbon sequestration potentiality significantly. The SOC would accumulate in grazing alpine meadow; however, judging with the long-term trend of SOC under mid-grazing condition, SOC could not reach its initial level. And to heavily grazing alpine meadow, SOC increased by measurement of closure only limited up to 12.3%. Application of nitrogen would increase SOC significantly. After applying nitrogen at the level of 9.2 g N/m2·a for 45 years under nature condition, the SOC in depth of 0～20cm soil increased by 14.4% comparing in 2005. |
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