| The physical and chemical properties of soil, soil enzyme, forage qualities and trace elements of soil - grass - animal system of the temperate desertification grassland at different grazing intensities and different grazing seasons were determined, based on which carried on the research of nitrogen tracer in middle grazing and high grazing areas. The results showed as follows:1 Seasonal dynamics of soil physical and chemical propertiesThe soil moisture declined as the season went by. It was the lowest in summer at different grazing areas except control area. With the increase of soil depth, the soil moisture in grazing areas had the rise trend. The soil pH in winter, spring and autumn were 8.32-9.15 which showed weakly alkaline while in summer was 7.9 closed to neutral. Soil bulk densities in grazing areas have cumulative effect except specific soil layer. Namely, they enhanced with the soil depth increased. And also in winter and spring soil bulk densities increased with the grazing intensities increased. In 0-30cm they were LGwinter and spring > autumn (p <0.01) except a few soil layers in all grazing areas. They did not show a certain regularity. With the grazing intensity increased, the total soil nitrogen of three seasons were LG> MG> CK> HG (p <0.01). It was the lowest in 20-30cm and only about 0.03% in HG. The soil available nitrogen of all grazing area did not show some seasonal variation. The surface soil available nitrogen content increased with the soil depth increases especially in summer and fall. In MG, the soil available nitrogen content of three layers declined with the season went on. Namely they in summer were significantly higher than in winter and spring and autumn. With the soil depth increased, its content gradually decreased. They were LG> MG> CK> HG in winter and spring with the grazing intensity increased. The change trends of the soil available phosphorus were similar with the available nitrogen in grazing areas except MG. The change trend of the soil available K of all grazing areas was different. They decreased with the seasons went on in HG and were significantly higher in summer than in winter and spring and autumn in MG and HG, and with soil layers enhanced the contents decreased. The ratio of soil C / N in 0-30cm declined while N / P increased with the grazing intensity increased and the C / P increased in winter and spring and autumn except summer.2 Seasonal dynamics of enzymes in soilThe urease activities in soil of all grazing areas in summer was significantly higher than in spring and winter and autumn (p <0.01), and decreased with the depth of soil in summer, while there are no certain differences among levels of soil in the other two seasons. With the increasing of grazing intensity, the urease activities showed no difference in spring and winter exception of individual soil depth; They in 0-30cm soil in summer are MG> LG> CK> HG , but LG> CK> MG> HG in autumn; The soil alkaline phosphatase activities of grazing areas rose first then decreased as the season goes, and with the increasing of soil layer its activities declined. It showed LG>MG>HG>CK in spring and winter , but LG>CK>MG>HG except the soil surface in summer; The catalase activities of LG, MG and HG of 10-30cm layers were significantly higher in summer than in winter and spring and autumn (p <0.01), and declined with the increasing of the soil depths, particularly in HG.Correlation analysis indicated that the three soil enzymes and the main soil fertilities showed significant(p<0.05) or very significant (p<0.01)differences among different grazing intensity treatments, which showed the soil enzymes can be used to characterize the status of soil fertility; Principal component analysis got the same result: The three soil enzymes and the main soil fertilities showed good correlation.3 Seasonal dynamics of community characteristicsThe total cover and gross density of vegetation in temperate desertification grassland are LG>CK>MG>HG in summer with the increasing of grazing intensity(p<0.01), the total cover decreased significantly but the gross density showed LG>MG>CK>HG in autumn; Biomass over ground decreased with grazing intensity in summer and autumn exception of LG>CK>MG>HG in spring and winter. The dominant species of plant communities were different under different grazing intensities and changed with seasons. So the nutrient values of mixed forages from different grazing areas are different which showed no obvious regularity. The results of test showed that CP of all seasons in CK are higher than in other grazing areas, CF of LG are higher than others, CA of HG are highest, other indexes showed no obvious trend; CP in spring and winter are higher than in summer and autumn, but EE, CF, P, NFE showed contrary in all grazing areas.4 Seasonal dynamics of trace elements in soil-forage-animal systemThe trace element contents changed with the seasons under different grazing intensities in soil-forage-animal system:4.1 Trace elements in soilThe contents of Cu in soil of LG, MG and HG were very significantly higher in summer than in another reasons (p<0.01) except CK, and they were significantly higher in CK and LG than MG and HG(p<0.01) in spring and winter and autumn. Except MG, the contents of Mn in soil decreased significantly with season going (p<0.01).they were significantly higher in MG than in other grazing areas in summer while in spring and winter and autumn the contents of Mn in soil were significantly higher in LG than in other grazing areas (p<0.01). In HG, Fe in soil were significantly (p<0.05) or very significantly (p<0.01)higher in spring and winter than in summer and autumn while they were higher in summer than in spring and winter, and autumn in other grazing areas. The contents of Zn in soil were significantly (p<0.05) or very significantly (p<0.01) higher in spring and winter than in summer and autumn in other grazing area except LG.. There were no definite trends among the grazing areas in three reasons.Correlation analysis and principal component analysis among soil trace elements, three soil enzymes and the main soil fertilities indicated that the soil trace elements could affect the soil fertility, but it located in subordination station only.4.2 Trace elements in forage and serum of sheepThe contents of Cu, Mn, Fe and Zn(except LG) were significantly(p<0.05)or very significantly(p<0.01) higher n spring and winter than in summer and autumn in all grazing areas. But they changed differently with different grazing areas in the same season; Contents of Cu, Fe and Zn in sheep serum in HG had no certain regularity with different seasons. Namely, the Cu were very significantly (p<0.01) higher in summer than in autumn and no obvious difference (p>0.05) with spring and winter; Fe were very significantly (p<0.01) higher in autumn than in summer than in spring and winter while the Zn were very significantly (p<0.01) higher in spring and winter than in summer and autumn.5 Study on nitrogen tracer5.1 The influences of nitrogen application to the aboveground biomass and the belowground biomass in surface layer were significant. The aboveground biomass and the belowground biomass in surface layer of middle grazing intensity grassland were higher than high grazing intensity grassland.5.2 The recovery rates of the labelled 15N were ranged from 3.84% to 11.67% in the plant pool of temperate desertification grassland. With increasing of nitrogen rates, the recovered amount by aboveground and belowground organs significantly increased while the recovered rate decreased, and the labeled nitrogen recovered amount from aboveground organs of high grazing grassland were less than middle grazing grassland.5.3 In one growing season about 18.19%-48% of the labeled nitrogen derived from fertilizer while about 52%-81.81% derived from soil. Soil nitrogen was still the main nitrogen source which was absorpted and utilized by plants.5.4 After one growing season about 50.43%-84.41% of the labeled nitrogen was retained in the soil pool, which was distributed equally to the 0-20cm soil layer. They increased significantly with increasing of nitrogen application rates and retained in the surface soil pool were less than in the under soil.5.5 The loss amount and loss rate of fertilizer nitrogen were differed significantly among different nitrogen rates in two grazing intensities grasslands and showed an increasing trend with the increasing of nitrogen rates.5.6 Loss/recovery analysis showed that fertilizer application rate of 12.25 g N·m-2 was associated with the lowest risk/benefit ratio under the climatic and soil conditions prevailing during this experiment, so it was the most economical nitrogen application rate among the nitrogen rate treatments. These results provide a reference for the future management strategies of grassland ecosystem.
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