| In the Yellow River Delta,the infertile saline-alkali soil has severely restricted the development of the agricultural economy in this region.The main reason for low soil productivity lies in the degradation of physical properties,such as dispersed soil particles,poor soil structure,and low saturated hydraulic conductivity.Since the proposal of the rural revitalization strategy in China,the issues concerning"agriculture,rural areas,and farmers"have become the top priority of the agricultural work.Given the above status quo,it is of paramount importance to restore the saline-alkali soil in this region.With the saline-alkali soil in the Yellow River Delta as the study object,this paper seeks to study the physicochemical properties（bulk density,saturated hydraulic conductivity,electrical conductivity,pH,saltness,SAR,ESP,and mechanical composition）and microstructural characteristics（mineral composition,micro-morphology,and pore distribution）of the saline-alkali soils in the horizontal direction and on the vertical section of the coastal and inland area,respectively.To this end,physicochemical soil test and microscopic test techniques（X-ray diffraction,mercury intrusion porosimetry,and scanning electron microscope）are adopted.The following conclusions have been drawn through experimental research and analysis.1.Physicochemical properties and microstructural characteristics of the saline-alkali soil in the horizontal direction（1）From the coast to the inland,there is a decreasing trend in electrical conductivity,pH,saltness,SAR and ESP.However,no significant difference was observed in the saturated hydraulic conductivity,which changes within the range of（1.89～5.36）×10-5 cm·s-1.The mechanical composition is dominated by sands（0.02～2 mm）in coastal areas and by silts（0.002～0.02 mm）in inland areas.（2）In Yellow River Delta,the primary minerals,such as quartz and calcite,are the major mineral composition;while there are only a few clay minerals.Compared with the coastal saline-alkali soil,the inland one contains more clay minerals.（3）In coastal areas,the saline-alkali soil is mainly composed of single particles that are distributed densely.The pores are strip-shaped and separated by gaps.However,in inland areas,the soil is mainly composed of flat and platy-shaped particles that are compacted tightly.The pores are filled with a large amount of cement.（4）In the coastal saline-alkali soil,the fine and micro pores account for 30.51%and49.34%of the total pore volume,respectively.In the inland saline-alkali soil,the micro and ultra-micro pores account for 43.13～56.26%and 30.79～46.12%of the total pore volume,respectively.The pore surface area of this region is constituted by micro and ultra-micro pores,with a diameter of less than 2μm.The soil porosity ranges between 24.88%and 35.32%in the horizontal direction.2.Physicochemical properties and microstructural characteristics of the coastal saline-alkali soil in the vertical section（1）There is little difference in physicochemical indexes and mechanical composition of the coastal saline-alkali soil in the vertical section.Thereinto,the chemical parameters are much higher than the normal value.Silts and sands are the main particle sizes of the saline-alkali soil at different depths.（2）The coastal saline-alkali soil in the vertical section has a similar mineral composition,with a majority of primary minerals.The content of clay minerals is less than10%.（3）In coastal areas,the undisturbed saline-alkali soil in all layers shows the same micro-morphology.In the form of surface-to-surface contact,the framework grains are embedded with each other tightly,like a"brick wall",with no cement filled in them.In addition,most of the pores are intergranular ones that are poorly connected.（4）On the surface of the saline-alkali soil locates the fine and mesopores.However,in other soil layers,fine and micro pores account for the majority.The pore surface area is constituted by micro and ultra-micro pores.The porosity is 58.95%in the surface soil,while it is about 30%in other layers.3.Physicochemical properties and microstructural characteristics of the inland saline-alkali soil in the vertical section（1）The chemical indexes change little in the inland saline-alkali soil of all layers in the vertical section,showing a lower level.Except for the surface layer,saline soils’permeability increases with depth;while its bulk density shows a decreasing trend.Silty loam is the soil texture for all layers of saline-alkali soil.（2）The mineral composition has no relation with the depth in all layers of inland saline-alkali soil.The soil contains a large proportion of primary minerals and a small proportion of clay ones.In this region,illite is the main clay mineral and mixed up with a small amount of kaolinite and chlorite.（3）Basically,the inland saline-alkali soil has the same micro-morphology in all layers,containing dense structural planes and microcracks.Except for the surface layer,there are more microcracks as the depth increases.As the soil pores are filled with a large amount of cement,the framework grains are contacted indirectly.（4）The micropores are mainly located on the surface of the inland saline-alkali soil,accounting for 46.02%of the total pore volume.However,the soil at other depths is primarily composed of micro and ultra-micro pores.In all soil layers,the macropores only account for 2.39～9.67%of the total pore volume.The pore surface area is constituted by micro and ultra-micro pores.In addition,the deeper the soil is,the higher the porosity will be,except the surface layer.In conclusion,the physicochemical properties of the coastal saline-alkali soil have degraded severely.In contrast,the inland soil has been restored artificially,so its chemical parameters are improved.For the mineral composition,the saline-alkali soil contains a large proportion of primary minerals and a small proportion of clay ones.Therefore,in the Yellow River Delta,the degradation of physicochemical properties cannot be explained based on these minerals.There is a significant difference in the microstructure of coastal and inland saline-alkali soils.However,when it comes to each layer in the vertical section,the microstructural characteristics are the same.As shown by the results,the microstructure,which is the main factor affecting the degradation of physicochemical properties,can be used as an index to evaluate the soil restoration effect.This study provides a theoretical basis for the restoration of the saline-alkali soil in this region on a microscopic scale. |
PDF Full Text Request