Mechanism Of Saline-sodic Soil Reclamation By Saline Ice Irrigation And Meltwater Leaching In Songnen Plain

Saline-sodic soil is a serious problem of soil environmental degradation,which is widely distributed in the world,especially in the Songnen Plain of Northeast China.The deteriorating physicochemical properties of saline-sodic soil adversely affects the plant growth,agricultural economic production and the sustainable development of ecological environment.Therefore,the amelioration of saline-sodic soil is of great significance to the rational use of derelict land and the protection of ecological environment.It is an emerging strategy to improve the saline-sodic soils through salt leaching after melting of the freezing saline water.In order to study the effect and mechanism of saline water freezing and brackish ice infiltration on saline-sodic soil with different exchangeable sodium percentage(ESP),there are 3 parts of experiments in this dissertation:the experiment of salt distribution during saline water freezing and thawing process,the simulation experiment of saline ice irrigation to improve saline-sodic soil and the paddy improvement experiment of saline water icing and thawing infiltration.These experiments studied and discussed the mechanism of saline ice meltwater irrigation and its effects on soil physical,chemical and biological properties.Firstly,six salinities of saline water were prepared by adding different amounts of Ca Cl₂ to the saline groundwater in saline-sodic soil area,and then the saline water was frozen and thawed to study the salt and ion distribution during the freezing thawing process.Secondly,the 16,26,36 and 46 mmol_c/L saline ices were prepared by adding Ca Cl₂ to the underground salt water in the saline-sodic soil area,these 4 salinities of saline ices were used to irrigate and leach the ESP 20,ESP 40 and ESP 70 saline-sodic soils.This part of the soil column experiment was conducted to study the amelioration effect of saline ice melting and leaching on saline-sodic soil with different alkalinity.Finally,through the covering of saline ice combined with the application of 25%gypsum requirement(GR),50%GR,75%GR and 100%GR of flue gas desulfurization gypsum(FGDG)on saline-sodic soil for rice planting and amelioration,we studied the field improvement effect and agricultural benefits.The results showed that:(1)Both freezing and thawing processes can achieve desalination,but the desalination effect of the thawing process was greater than that of the freezing process.The effect of excess amounts of salts and Na⁺in underground water of Songnen Plain can be alleviated through adding 7.78 g/L Ca Cl₂(analytical pure)and freezing-thawing process.(2)For the saline-sodic soil with ESP 20,when the salinity of melt water used for irrigation is low(16 mmol_c/L),increasing the volume of saline ice(irrigation water volume)will increase the p H,EC,Na⁺content and alkalinity of the surface soil in comparison to the initial value,thus aggravate the degree of salinization of surface soil.Adding Ca Cl₂ can make the salinity of underground brackish water reach 46mmol_c/L in Songnen Plain,and using 2.5 PV of 46 mmol_c/L saline ice with Na/Ca<0.80 can achieve the best improvement of ESP 20 saline-sodic soil.(3)The saline-sodic soil with high ESP(e.g.ESP 70)is not sensitive to the salinity of saline ice.When the volume of saline ice reaches 2.5 PV,irrigation with low salinity saline ice(16 mmol_c/L)can get a thorough improvement effect.When the volume of saline ice is less than 2.5PV(2/3PV and 1.5PV),the saline-sodic soil with ESP 70 showed clear stratification effects among different layers.(4)Compared with the control,the infiltration rate,soil moisture and bacterial diversity of saline-sodic soil were significantly increased by saline water freezing-thawing infiltration;soil bulk density,soil electrical conductivity(EC),soil alkalinity,sodium adsorption ratio(SAR)and Na⁺content were significantly decreased,and the abundance of bacterial community changed structurally.(5)Saline ice combined with 100%Gypsum requirement(GR)can achieve the best improvement effect and save fresh water.For the final rice yield,compared with fresh ice meltwater irrigation,saline ice meltwater irrigation can save 7%gypsum application amount.(6)After saline ice irrigation and leaching,soil p H,alkalinity and other chemical parameters can predict rice yield in the vegetative growth period of rice;but,before rice transplanting,bacterial diversity in saline-sodic soil have a strong correlation with rice yield,and could accurately predict rice yield.Therefore,monitoring soil microbial diversity before rice planting can predict rice yield earlier and guide field management.(7)When 100%GR was applied,the amelioration effect of saline water freezing and fresh water freezing on soil physical and chemical properties was similar,indicating that saline water can replace fresh water for freezing irrigation.But,the EC of soil was higher after saline ice melting and infiltration,and it is necessary to leach the salts with fresh water to prevent secondary salinization.To sum up,through the study of the theory and practice of saline ice melting and infiltration,we clarified the mechanism and optimal way of saline ice to improve saline-sodic soil,revealed the biological,physical and chemical properties of the ameliorated saline-sodic soil and its predictive effect on rice yield.This study will provide theoretical basis and technical guidance for the improvement and utilization of saline-sodic soil.