Study On Evolvement And Evaluation Of Soil Quality And Early-warning System Of Degraded Seriphidium Transiliense Desert Grassland In The Middle Of The Northern Slope Of Tianshan Mountains

Seriphidium transiliense desert grassland locates at the transitional zone of the plains and mountains, which is the important grassland for grazing in spring and autumn, so it plays an important role in the protecting ecological environment and livestock production in Xinjiang. But due to long-term over-grazing, grassland degradation is widespread, even is extreme degradation in some areas. Taking the plain degraded Seriphidium transiliense desert grassland in the middle of northern slope of Tianshan Mountains in Xinjiang as a study object, the degraded grassland is classified into three groups: the moderate degradation grassland, the severe degradation grassland and the extreme degradation grassland. The typical plots were picked for studying the characteristics of the vegetation and soil physical, chemical and biological characteristics. Through sample analysis, data processing, main results were as follows:1. Increase in soil bulk density and organic matter content was result of the grassland degradation caused by overgrazing. The total salt content and pH were increased significantly (p<0.05). Soil organic matter, total phosphorus and potassium are relatively stable indicators, who are less affected by the situation. By impacts of plants and livestock excreta, soil nitrogen was significantly increased, and the changes of available P and K were in volatility so that the contents were highest in the extreme degradation stage, reaching significant levels (p<0.05). Therefore, the sensitive soil physical and chemical indicators to grassland ecosystem degradation are: soil bulk density, total salt, pH, total nitrogen, available nitrogen, available phosphorus and potassium.2. Soil particle fractal dimension had a significant positive correlation with clay particles content. The fractal dimension was gradually increased with the escalation of the degree of grassland degradation. Consequently, the soil did not show desertification at the present although the vegetation has been degraded. No significant correlations were observed between the fractal dimension and the soil physical properties, but there existed a significant or very significant positive correlation between the fractal dimension and chemical factors. Fractal dimension can represent the level of soil nutrient in a degree.3. The number of bacteria was largest, which was dominating, and followed by actinomycete and fungi. At 0-10cm soil layer, there was no difference of bacteria, desert actinomycete and fungi. At 10-20cm soil layer, the differences of the number of bacteria, actinomycete and the total number of microorganisms were notablely increased in the severe degraded grassland. At 20-30cm soil layer, the bacteria and the number of microorganisms were notablely increased in the moderate degradation grassland. Therefore, the influence depth of grassland degradation on soil microorganisms was mainly at 0-20cm soil layer.4. The activities of soil catalase, alkaline phosphatase, dehydrogenase and invertase were degressive along with soil depth. In 0-10cm soil layer, with the escalation of the degree of grassland degradation, the activity of catalase, urease, alkaline phosphatase, dehydrogenase and invertase increased. At 0-10cm soil layer, compared with the moderate degradation grassland and severe degradation grassland, the activities of soil alkaline phosphatase, dehydrogenase and invertase of extreme degradation grassland were notablely increased. Catalase Activity in severe degradation grassland was significantly less than it in the extreme degradation grassland. Urease activity had significant difference only at 10-20cm soil layer among the degraded grassland. The activities of catalase, alkaline phosphatase, dehydrogenase and invertase showed some differences in the soil profiles, but there was no difference about catalase activity. Therefore, the sensitive soil biological indicators for grassland degradation are: bacteria, actinomycete, fungi, alkaline phosphatase, dehydrogenase and invertase.5. The species diversity index and evenness index decreased in the degraded Seriphidium transiliense grassland, which indicated that the stability of grassland ecosystem had been reduced. There was a positive correlation between soil bulk density and species diversity index. There was a negative correlation between species diversity and soil chemical properties, in which the soil organic matter, total nitrogen and total potassium were especially notable, which illustrated that the changes in vegetation had a greater impact on soil organic matter and total nutrients.6. Using principal component analysis and cluster analysis method, the minimum data set for evaluating soil quality of grassland degradation was built at last, which includes bulk density, pH, available nitrogen, available phosphorus and the total salt. The soil quality was evaluated by means of scoring, the ranking are: the moderate degradation grassland > the severe degradation grassland > the extreme degradation grassland.7. Based on the frequency distribution of soil quality, the soil quality was graded into five warning states: no warning, slight warning, medium warning, sever warning and extreme warning. The majority of soil qualities in study area are in medium warning state, even more serious. Accordingly, the grassland soil conservation measures were proposed for the various warning states.