Spatial Variability And Its Effect Mechanism Of Soil Salinity In The Low Plain Around The Bohai Sea

The low plain around the Bohai Sea is an important agricultural production region of our country, which is also the primary distribution area of saline and brackish water. To provide suggestions on utilizing brackish water for agricultural irrigation and mitigating the crisis of water resources, guide agricultural production, and further optimize planting structure by scientific theory and technique, it is very important to characterize the spatial distributions of soil salinity, in the low plain around the Bohai Sea. Thus, soil and groundwater samples over the low plain around the Bohai Sea were collected before and after the rainy seasons. Geostatistical method combined with GIS technique was used to analyze spatial variability and distribution of soil salinity in depths0-60cm, relationships of salt ions, effect factors., and the following conclusions have been achieved in this dissertation:(1) Besides the spatial variability of soil salinity in the low plain around the Bohai Sea, processes of soil salinity transfer, leaching, and accumulation were also exhibit spatial variability characteristics. Salinity accumulation area can be divided into three major parts, such as bicarbonates accumulation area, chloride salts accumulation area, intergradations and sulfate salts accumulation area. Due to the influences by Groundwater level (GWL) and total dissolved solid (TDS), soil salt contents changed similarly as TDS, while conversely with GWL. Influences on soil salt by GWL and TDS were much bigger in the top and bottom layers than that of the middle layers. Indicator kriging method was used to provide robust variogram. After analyzed by univariate and multiple-variable indicator kriging, it was found that soil salinization and secondary salinization increased from west inlands to east shorelands.(2) The primary anion and cation of the soil salts in the low plain around the Bohai Sea were HCO3-and Na+, while HCO3-was the major effecter for the soil alkaline. Semivariogram spherical, exponential and gauss models were used to analyz the soil salts. Results suggested that variations of soil salinity in the surface layer (0-5cm) belonged to highly variable, while the other layers belonged to moderate degree both in horizontal and vertical direction. Spatial correlated distances increased while the depth increasing, the values increased from35.3km for the surface soil to59.7km for50-60cm soil layer. Salinity type in the top layer was sulfate-chloride, while others were chloride-sulfate. Chloride contents in the saline water region were higher than that of the semi-saline water area. In the surface soil of the low plain around the Bohai Sea, the area distribution percentages of none salinity, slight salinity and medium salinity soils were almost equal, heavy salinity soil region was little. Salt content decreased after the rainy seasons, especially the top layer (0-5cm), it found that the chloride concentration reduced from25%to9.9%.(3) Salinity profile types in the low plain around the Bohai Sea can be classified into top enrichment, middle enrichment and bottom enrichment. The salinity top enrichment type can be recognized as that the salt contents from0to10cm higher than other layers, and this type was usually formed during the dry spring seasons, or the regions with small GWL and strong evaporation, which groundwater level was lower than5m, TDS was more than5g/L. Every year before the month of May, the area of this type could covered about34.4%of the total research region, with the area about3.07×104km2. The salinity middle enrichment type can be characterized as that its salt contents from15to30cm were higher than that of the top and bottom layers. This type was usually formed after precipitation or irrigation, or the regions with higher GWL, which GWL was higher than5m, TDS was bout2-5g/L. Area distribution percentage of this profile was35%, which was3.13×104km2. Brackish water in this region was suitable for irrigation in some extent. Characteristics of the salinity bottom enrichment type can be described as that the bottom layer salt contents were much higher than the other layers. This type was usually formed after the rainy seasons, or the regions with higher GWL, which GWL was more than one or several tens meters, TDS was about3g/L. Area distribution percentage of this type was about30.6%, which was2.73×104km2. The primary saline in the top enrichment type were NaCl and MgSO4, while in the middle and bottom profile types were Na2SO4, Na2SO4and NaHCO3respectively.(4) In the low plain around the Bohai Sea, the cumulative contribution of soil salinizaiton, soil alkalization, regional groundwater, TDS, Kainite and other factors reached to87.88%during the soil salts variation processes. During the cumulative process of soil salinity, groundwater and climatic conditions, topographic factors, soil texture, structure, farming systems and others were exhibited a certain impact. Through path method analysis, results showed that, saline root of soil depths5-10cm and20-30cm were the main effect factors to the top soil salinity enrichment, then was salt ions such as total alkalinity, sodium adsorption ratio, soluble sodium percentage, sodium calcium magnesium, and chlorine sulfur ratio. The other effect factor was the groundwater condition, such as GWL, pH, and TDS.