Research On Characteristics And Driving Mechanism Of Desertification In Maqu Alpine Meadow Region

Desertification is one of the main factors which causes grassland degradation and productivity decline, perhaps is a crucial environmental problem on rangelands in the western China. The alpine meadow in Maqu plays important roles in conservation of water resources in the upper reaches of the Yellow River. Due to some natural and artificial factors such as global warming and overgrazing, the area of land desertification increases, influencing regional eco-environment security and economic development in the past decades. Studies on desertification characteristics and driving mechanisms would be helpful to undestand and evaluate desertification degree and impact on regional environment, and provide scientific basis for ecological control and rehabilitation practice of desertified alpine meadow.Combining ecological and economical data collected from local government and field survey, the method of spatial-temporal substitution was applied to analyze the relationships between soil variables and vegetation variables at different desertification stages and to draw conclusions of desertification mechanism and primary driving forces. Finaly,some suggestions on ecological rehabilitation ways of alpine meadow in Maqu County in aspects of both technology and policy were put forward. The main results and conclusions showed as follows:1. The soil particles of alpine meadow became coarser, the soil bulk density increased, and the soil porosity decreased along with the desertification development. Compared with non-desertified meadow, the sand content percentage in topsoil layer (0~20cm) of desertified meadow at light, moderate, severe, and very severe stages of desertification increased by 9.19%, 13.32%, 13.45% and 19.08% respectively, the soil bulk density of the same soil layer increased by 6.43%, 9.29%, 12.86% and 12.14% respectively, while the soil porosity decreased by 7.19%, 10.43%, 14.61% and 13.91% respectively. These results indicated that alongside the gradually serious desertification of alpine meadow, its soil physical properties tented to be worse and, moreover, this condition developed downward deep layers from surface soil layer, especially at severe and very severe stages of desertification, the harmful changes of the soil physical properties in the deep soil (20~40cm) had taken place more significantly.2. The impact of desertification on soil moisture and soil water storage capability of meadow increased with the development of meadow desertification. Compared with non-desertified meadow, the soil moisture in the topsoil layer (0～20cm) of desertified meadow at light, moderate, severe, and very severe stages of desertification gradually reduced separately by 57.65%, 43.95%, 64.77% and 60.84% in the middle of July, the soil water storage capacity decreased 54.92%, 38.73%, 60.17% and 55.98% respectively, and reduction in top soil was greater than that in deep soil layer. Difference of the soil moisture and soil water storage capability between top and deep soil layers were significantly decreased at severe and very severe stages of meadow desertification. The meadows at different desertification stages were arranged in order of its soil moisture and soil water storage capability as follows: non-desertified meadow > lightly desertified meadow > moderately desertified meadow > severely desertified meadow > very severely desertified meadow. These results indicated that the water conservation function of alpine meadow decreased with its desertification development.3. The soil organic matter and N content significantly reduced at different desertification stages. Compared with non-desertified meadow, the organic matter in the soil layer of 0～20 cm of desertified meadow at light, moderate, severe, and very severe stages of desertification decreased separately by 70.85%, 80.31%, 82.27% and 80.24%, the active organic matter reduced separately by 82.50%, 95.43%, 93.63% and 97.24%, and the soil available N reduced by 80.12%, 81.95%, 82.08% and 86.75% respectively. The results also indicated that reduction of active organic matter was greater than that of organic matter and the available N, and the decreasing trend was more obvious in topsoil layer than that in deep soil layer. The influences of meadow desertification on soil P and K content were not significant before the severe stage of desertification, but showed significant reduction at very severe stage of desertification. The effect of meadow desertification on the soil available nutrition was greater than that of the total nutrition in the desertification process of alpine meadow. The soil pH increased slightly along with the alpine meadow desertification.4. Grassland degradation was consisted of the some changes of both physical and chemical features of soil. Correlation analysis indicated that the total N content in soil was significantly positively correlated with the organic matter. Soil sand content showed significantly negative correlation with soil nutrition. On the other hand, there was a positive correlation between soil moisture and organic matter content, and soil N and P. However,there was a significant negative correlation between the soil bulk density and the soil organic matter, as well as N, and P and K in the top soil layer.5. Kobresia spp. were the dominant plants in alpine meadow. The proportion of Kobresia spp. decreased and was replaced by other plants in the process of alpine meadow desertification. Composition and structure of communities changed completely at severely desertified stage compared to non-desertified community. Correspondingly, the species richness and Shannon-Weiner index descended remarkably, but evenness index increased, and then decreased in the development of alpine meadow desertification. The (3 biodiversity index was higher at the initial stage of desertification and relatively low in grassland desertification. At the same time, the standing biomass reduced sharply, indicating that grassland was unsuitable for grazing at severely desertified stage.6. There was a negative feedback effect between the soil degradation and the vegetation degradation due to interaction between soil variables and vegetation variables. Correlation analysis showed that standing biomass and biodiversity index were positively correlated with the total N, P and K contents and the silt component, but negatively correlated with sand particle content, soil pH and the soil bulk density. The changes of vegetation diversity and the reduction of biomass were the result of interaction between vegetation degradation and soil degradation.7. There are the positive correlation between the desertification rangeland area of Maqu alpine meadow and the variation of the total output of animal husbandry, mean annual temperature, sheep and goats amount on hand, population, beef and mutton productivity, big livestock amount on hand, mean annual precipitation. However, there are the negative correlation between the desertification rangeland area and the variation of the annual runoff of Yellow River, mean annual wind velocity and mean annual gale days.8. Gray correlation analysis showed that desertification area was affected by following factors in order of different correlation degree as: total output of animal husbandry > population > mean annual temperature > sheep and goats amount on hand > beef and mutton productivity > mean annual precipitation > big livestock amount on hand > annual runoff of the Yellow River > mean annual gale days > mean annual wind velocity. The increasing mean annual temperature was the most important natural driving force causing alpine meadow desertification, and had bigger effect on the alpine meadow degradation than changes of mean annual precipitation and wind velocity.9. Comprehensive analysis of human active factors and natural factors indicated that the human active factors contributed to desertification 61.55%, while nature factors took up 38.45%. These data disclosed that over-gazing and irrational utilization of land are the key factors causing meadow desertification in alpine region, but the changes of regional climate and hydrological factors were merely the external causes, which accelerated the development of alpine meadow desertification.10. Based on desertification situation, analysis of social economy development condition, the characteristic of the alpine meadow and the driving mechanics, the author suggested out a series of measures to control and manage alpine meadow, including: make effort to obtain the investment from central government for ecological rehabilitation, control the livestock number, strengthen the natural grassland protection and optimize the production pattern of stock raising and so on.