| By taking samples from artificial alfalfa grasslands which had been planted for1,3,4,5and8years along the eastern foot of Helan Mountain in Ningxia, we analyzed soil carbon storage and its vertical distribution characteristics at the depths of0-10,10-20,20-40,40-60,40-60,60-80and80-100cm on April,2012. And the fluxes of CO2at the sample plots were measured by using static enclosed chamber-GC/MS technique. Also, we expounded the law of seasonal variation of CO2fluxes in different plots, and the relationship between CO2fluxes and its influencing factors. The main results showed as following:1. With the increase of planting ages, soil carbon storage inhanced significantly (P<0.05), but soil carbon storage of alfalfa planting8years was lower than the other samples, the order of the concentration and density of SOC (Soil Organic Cardon, SOC) and SAOC (Soil Active Organic Carbon, SAOC) in the0-100cm soil layer were8a<la<3a<4a<5a, the concentration and density of SOC and SAOC had found significant difference at5%level. The alfalfa plots showed significant SOC and SAOC sequestration ability, and carbon sequestration potential. Vertical distribution characters of concentration and density of SOC and SAOC showed the surface gathering, planting age was the main factor that influenced the pattern of vertical distribution of soil carbon storage.2. Alfalfa planted from la to8a, soil profile SOC concentration mainly showed a significant accumulation in the0to40cm soil depth, accounted for69.7%,65.8%,73.8%,70%and67.2%from0to100cm, respectively, but it sharply declined below40cm,0-100cm mean SOCD (Soil Organic Carbon Denisity, SOCD) were1.15,1.12,1.23,1.40and0.84kg.m-3from la to8a, respectively; and the surface soil SOCD (0to40cm) accounted for about54.8%to61.8%of total SOCD from0to100cm. Soil profile SAOC concentration indicated the same accumulation in the0to40cm soil depth, and accounted for90.4%,93.4%,93.9%,78.3%and89.9%from0to100cm, respectively; SAOCD(Soil Active Organic Carbon Denisity) accounted for89.9%,94.2%,90.5%,84.9%and89.8%of total SAOCD from0to100cm, respectively.3. The CO2fluxes of soil system and soil-vegetation system of alfalfa pasture displayed the same irregular peak curve with the minimum value in autumn and the maximum value in summer, and followed the pattern with4a>5a>8a>3a>la. On the season scab, the emission of CO2were significantly positively correlated to soil temperature (R=0.794, n=10, P<0.01) and air temperature (R2=0.768, n=10, P<0.01). Therefore, the soil temperature and air temperature were very important factors to CO2emission of soil-vegetation system.4. The average observation value of CO2fluxes of soil-vegetation and soil system was 490.86mgm-2h-1and291.91mgm-2h-1.Compared to CK (planting1year plot), the average value of C02flux of soil-vegetation system increased by19.29%,47.56%,43.97%and39.69%after planting for3,4,5,8years respectively, at the each observation date, and CO2flux of soil system increased by19.32%,49.39%,48.38%and44.74%, respectively. The CO2fluxes of soil-vegetation system were significantly higher than the CO2fluxes of soil system, it indicated that plant respiratory action significantly contributed to CO2emission.5. The CO2fluxes of soil system were significantly positively correlated to the concentrations of SOC, SAOC and C/N, and the fluxes of CO2were closely positively correlated to the contents of TN CR2=0.949, n=10, P<0.01). That meant the emission of CO2would be intensified by the increase of C, N concentrations after planted alfalfa and similar environmental factors. Content of soil water was an important factor that affected soil CO2flux. The study found that CO2flux and10cm soil moisture content had a parabola relationship:Y=-0.373X2+28.013X-357.35,R2=0.443, CO2emission reached the maximum value when the soil water content was about35%. Therefore, the emission of CO2would be intensified by microbial activity when the condition of soil water was better. In addition, flux of CO2and content of up-ground biomass indicated a same trend. So that, the growth of generous up-ground vegetation was also a very important factor to CO2emission.6. The concentrations of SAOC increased after planting3to5years, but the increase of SOC were far less than that of SAOC, which meant the alfalfa grassland accumulated more SAOC, SAOC was an important index to characterize the change of soil organic matter. Soil carbon storage of alfalfa planting8years was lower than the other samples, the reasons that dued to long years of alfalfa growth, the average coverage was low, the higher increase of soil temperature, accelerate the decomposition of soil organic carbon because of soil aerobic microbial activity after irrigation to a certain extentIn a word, we drawed the conclusion as below. First, the alfalfa pasture plots showed significant SOC, SAOC sequestration and great potential, and their vertical distribution characters of concentration and density of SOC and SAOC showed the surface gathering. Secondly, the CO2fluxes of soil system and soil-vegetation system of alfalfa pasture indicated a same trend with the minimum value in autumn and the maximum in summer, which followed the same pattern with the4a>5a>8a>3a>la. Thirdly, there were a great many factors affected CO2emission. |
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