| The Loess Plateau is a typical transitional zone between cropping and pastoral production. Grassland accounts for37%of the total land areas in the region, where it was used mainly by sheep grazing. In this thesis the responses of soil microbial functional diversity, nitrifying bacteria population, nitrification potential, soil properties and enzyme activity to sheep grazing intensities were studied by field trials and laboratory experiments at Huanxian County, eastern Gansu Province. The main results are summarized as the follows:1,The Biolog ECO Plate TM system with31-carbon-substrate was used to determine soil microbial functional diversity in grassland paddocks grazed by sheep in various stocking rate. The results showed that amount of carbon substrates utilized by soil microbial community increased with incubation time. Values in average well-color development (AWCD) were lower at paddocks with8.7and2.7sheep/ha than at paddocks with0and5.3sheep/ha. Soil microbial community metabolic functional diversity was significantly different under grazing intensities after cultured72h. Principal component analysis of utilized carbon substrate type by soil microbial community showed that the main carbon substrates used were7carboxylic acids,5amino acids,4carbohydrate,16carbon substrate and2polymer in total. The comparison of soil microbial community metabolic functional diversity under different grazing intensities indicated that the differences of both Shannon index and richness were significant (P<0.05).2,Grazing increased nitrification potential and culturable ammonia oxidizing bacteria (AOB) populations compared with ungrazed treatments. Ammonia-oxidizing bacteria populations increased from155colony/g dry soil with0sheep/ha stocking rate to16218colony/g dry soil with8.7sheep/ha. Grazing led to an increase in population of AOB at0-10cm soil depth, but had no such effects at10-20cm soil depth. Nitrification potential increased from1.21mg NO3-N/kg soil/d in ungrazed treatments to2.86mg NO3-N/kg soil/d with8.7sheep/ha. Soil nitrification potential and the size of culturable AOB populations were depend on grazing intensity, soil depth and season. This information is potentially important for the optimal selection of stocking rate for grazing ecosystems.3,The soil net nitrogen mineralization and nitrification rates under the four different grazing intensities exhibited significant temporal variations during the growing season. Net nitrogen mineralization rates ranged from-0.14to1.31mg kg-1'd-1with the highest in August. Consistent higher values in net nitrogen mineralization rates were observed from the paddock under the grazing intensity of8.7sheep ha-1. The mean soil net nitrification rate under the four grazing intensities was positively correlated with the soil C/N ratio and pH, respectively. On the other hand, the mean nitrogen mineralization rate was negatively correlated with soil organic carbon, but was positively correlated with the soil C/N ratio. Our study demonstrated that net nitrogen mineralization and nitrification rates were strongly linked to grazing intensity, soil temperature and moisture content.4,The effects of different levels of synthetic sheep urine on nitrification were studied in grassland soil microcosms. Low, medium and high Concentrations of synthetic sheep urine applied to the pots included500kg-N/ha,250kg-N/ha,50kg-N/ha and0kg-N/ha, which represented high. Medium and low concentration, and control treatment, respectiely the increase in ammonium concentration immediately after synthetic sheep urine addition was accompanied by an increase in soil pH. Soil pH then decreased, as ammonium was converted to nitrite and nitrate. The nitrification potential was similar under four sheep urine addition treatment. The application of the urine substrate significantly increased the nitrification potential. Synthetic sheep urine is likely to influence the spatial distribution and composition of ammonia oxidizer communities, with consequent effects of nitrate accumulation.5,The effects of sheep grazing, plant species and grazing-plant species interaction on soil mineral nitrogen concentration, enzyme activity and nitrification potential were significant. Ammonium values were higher under Stipa bungeana and Artemisia capillaries at paddocks with8.7sheep/ha than others. Nitrate values were higher in the rizhospere soil of Lespedeza davurica than those of Stipa bungeana and Artemisia capillaries, although the nitrate concentration was low at grazing intensities paddocks. With increasing grazing intensity, nitrification potential under Lespedeza davurica was first decreased and then increased, Stipa bungeana showed a gradual increase. Soil protease and invertase activity were significantly decreasing with increased grazing intensity. However, soil urease activity was differed significantly among plant species.6,Soils collected from paddocks with different grazing intensity were spiked with Zn2+and Cd2+solutions for4weeks. Thorough mixing work was done to ensure the solutions were evenly distributed within the soil samples. Experiments were conducted to examine the effects of Zn and Cd toxicity on the nitrification potential in soils. It was found that heavy metals have a significantly stronger inhibitory effect on soil nitrification potential. Nitrification rates varied significantly between different grazing intensity. The higher nitrification rates appeared in the paddocks with8.7sheep/ha. The low concentration of Cd2+(0.5mg/100g) had no effects or a slightly effects on of nitrification potential, while high concentrations of Cd2+(>0.5mg/100g) were a significant inhibitor of the nitrification process. The nitrification rate was similar among different grazing intensity when the concentration of Zn2+was150mg/100g.
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