Characteristics Of Alkalinized Soil In Qitai Oasis In Xinjiang And The Investigation By Remote Sensing

Saline-alkali soil is a general designation of a series of soil which is affected by saline-alkali components and includes all kinds of saline soil, sodic soil and soil with different degrees of alkalization and Salinization. Xinjiang has the largest area of saline-alkali soil hence is called as"the saline-alkali soil museum in the world". Soil alkalinization and salinization have already become tone of the major land degradation reasons in oasis, the only land for the people to live in Xinjiang. The problem about soil alkalinization and salinization is also an issue related to the regional sustainable development and the environment improvement in Xinjiang's arid areas. Hence soil alkalinization and salinization have always been a focused research field. Alkali soil and salty soil have different chemical and physical properties, land surface symptoms, occurrence and development mechanism, therefore they belong to different soil types. So far, in Xinjiang, studies on the soil salinization and alkalinization mostly focuson southern areas where salt is significantly accumulated in topsoil but very few on larger areas of alkalinized soil in the Northern Slope Area of Tianshan Mountain in XinjiangBased on a large number of field investigations and soil, spectrum and vegetation samples collected from given spots, as well as the application of geography, agrology, environmentology, ecology and remote sensing theory, this thesis systematically discusses the physical and chemical characteristics of alkalinized soil, the correlations between soil alkalinization and environment factors, the subdivision of degree of soil alkalinization, the characteristic s of spectra of alkalinization soil in different scales and the investigation by remote sensing on the alkalinized soil in Qitai Oasis in Xinjiang by the 3S techniques and quantitative and qualitative method s.The first chapter mainly introduces the background and the purpose of the soil alkalinization study summarizes the domestic and international advances of soil alkalinization research, and then demonstrates author's research concepts, research contents, research procedures and methods in detail. The second chapter primarily introduces general situations of the study area, and specifies the procedures of field investigations, data collection and sample tests. The third chapter discusses soil physical and chemical properties in the study area, investigates into the distribution of the alkali -saline soil and the correlation between this spatial pattern and the topographic factor extracted from SRTM data and analyzes the spatial variation and distribution based on the regional topographic factor using the data from monitoring the soil alkali content in Qitai oasis. The fourth part studies the relationship between halophyte coverage and soil alkalinization indices and builds the subdivision of the degree of soil alkalinization based on the restraining effect on halophyte from different levels of soil alkalinization. The fifth part focuses on the correlation between soil pH and spectrums in three different scale (field-measured spectrum,laboratory-measured spectrum and the image spectrum). A new quantitative remote sensing monitoring index pHI for soil alkalinization is built. The last chapter summarizes the whole thesis and lists the achievements, difficulties and prospect s of this study.Main conclusions and developments as follows:(1) Alkaline-earth in the study area presents a typical desalination-alkalinization feature. The average value of soil pH is more than 8.8, and the soil within the study area is belong to sodic alkali soil. The alkalinization and salinization of soil occurs simultaneously in the study area because a small area near the Gurbantunggut Desert still has the phenomenon that salt is being accumulating in topsoil.At a regional scale, topography plays an important role in the spatial distribution of the alkali -saline soil, and salt is accumulated within the slightly sloped area below the elevation of 680m. Alkalinization occurred in the steeply sloped area above 740m. With the altitude decreasing, the soil salt content increases and the PH value drops concurrently, so the evolvement of salination has a reversal pattern as alkalinization. Elevation has more impact on the soil salinization-alkalinization degree and the spatial distribution of ions of salt compared with slope.(2)There is a significant negative correlation between halophyte coverage and each soil alkalinization index. The correlation coefficient between halophyte coverage and soil pH is the greatest of 0.810 and the next highest is ESP. Soil alkalinization is the major restrained factor to halophyte coverage; therefore halophyte coverage could be a good indicator of the soil alkalinization level. Based on halophyte coverage, the subdivision of the degree of soil alkalinization is built and the degree standard has good response to other soil alkalinization degree standards in domestic and overseas.(3)Both the filed-measured reflectance and the image reflectance multivariate linear regression models were built to the evaluate soil alkalinization level. Filed-measured reflectance had good potential ability to rapidly and accurately estimating changes of the soil alkalinization and the model obtained the best effect with a R² of 0.873. If building model by TM reflectance directly, the accuracy of fitting is lower than the model of the filed-measured spectrum because of the vegetation distraction in the image spectrum. The Laboratory -measured spectrum is significantly negatively correlated to the soil OM, but not correlated to the soil pH.(4)From the analysis and comparison of the response characteristics of the spectrums in three different scales (field-measured spectrum,laboratory-measured spectrum and the image spectrum) of alkalinized soil, pH-NDVI space-based remote sensing synthesis index models for monitoring soil alkalinization was built. The result of extracting alkalinized soil shows a high accuracy of 94% by pHI method. The extracted pH from the image by pHI is in good agreement with the measured soil pH with the average error of 0.25 pH unit. The application of extracting alkalinized soil in the study area based on pHI covers the area of lightly alkalized soil and medium-alkalized soil that represent 84% of the total wasteland. The lightly alkalized soil with pH8.0～8.5 has little free Na₂CO₃ and NaHCO₃ , so it is easy regarding to improvement and utilization. If irrigation water supply problem can be solved, Qitai oasis will has great potential in ploughland developments.