Soil Water-salt Dynamics Under Root Restriction In Saline-sodic Soils In Songnen Plain,Northeast China

Impacted by the climate conditions and human activities,the saline-sodic land formed by both primary and secondary salinization-alkalization has extended in a large scale in western Songnen Plain,which becomes a significant eco-environmental problem limiting agricultural development.Due to the dynamic changes of groundwater and climate conditions in western Songnen Plain,it is unclear whether the root restriction soil will be salinized and what measures can be taken to delay the salinization-alkalization.In the field experiment,we setted up five different treatments:control,without treatment around the sandy soil(T0),laying film in lateral(T1),laying film in lateral and straw in bottom(T2),laying film in lateral and hole film in bottom(T3),laying film in lateral and bottom(T4),traced the source of plant water and soil water by hydrogen and oxygen isotopes;and there were four groundwater tables at the depths of 0 cm,20 cm,40 cm and 60 cm in the simulated experiment,using Hydrus-1D to simulate the soil water-salt transport,the objective of this study was to investigate the soil water-salt transport under root restriction in saline-sodic soils,and provide data support for water and salt management in saline-sodic land.The main conclusions were as follows:1.In the experiment of soil water-salt transport under root restriction in different treatments,the soil water content and salt content showed obviously seasonal differences,the strong evaporation in dry season led to the decrease of soil water content,and the soil salt content gradually accumulated on the surface soil with the water moving upward;however,in wet season,the soil water content was sufficient,and soil salt was leached by precipitation,so the overall soil salt content was lower than that in dry season.The soil water content of T2 was relatively lower,while T3 and T4 were relatively higher,that was to say,the closed bottom inhibited the sandy soil water discharged,which led to the over saturation of soil water under root restriction;the degree of soil salinization-alkalization was increased with the passage of time,and the process of the surface soil salinization-alkalization was earlier than that of the deep soil.In wet season,the soil salt content was increased with the increase of soil depths,however,in dry season,the soil salt content were decreased with the increase of soil depths.Compared with T0,T2 treatment can alleviate the soil salinization-alkalization process.2.In the experiment of stable isotope tracing,the stable isotopes of the surface soil water were mainly affected by the stable isotopes of precipitation,while those of the deep soil water were mainly affected by groundwater.There were significantly positive correlations between?¹⁸O,?D in soil water and EC,Na⁺in soil solution in dry season,and there were non-significant correlations between?¹⁸O,?D in soil water and EC,Na⁺in soil solution in wet season.In wet season,the plant mainly used the 0-40 cm depths of soil water,and 0-10 cm depths of soil water mainly came from precipitation;in dry season,the plant mainly used 40-80 cm depths of soil water and groundwater,similarly,0-10 cm depths of soil water mainly came from 40-80 cm depths of soil water and groundwater.3.In the experiment of soil water-salt transport under different groundwater tables,when the soil was below the phreatic water level,the soil water was basically saturated.When the soil was above the phreatic water level,the soil water content was lower as the soil farther away from the phreatic water level.The degree of soil salinization-alkalization were increased with the decrease of the groundwater tables.Compared with saline-sodic soil,the sandy soil was more prone to salinization-alkalization,the speed of sandy soil salinization-alkalization was faster,and the salinization and alkalization process of soil didn't occur at the same time,salinization before alkalization.After 12months of soil water-salt transport,the soil bulk density was increased.The bulk density,total porosity and field moisture capacity of sandy soil were lower than that of saline-sodic soil.There were non-significant influences on physical properties of sandy soil between different groundwater depths.However,the groundwater depths had significant influences on bulk density and field moisture capacity of saline-sodic soil,the bulk density and field moisture capacity were increased with the decrease of groundwater depths.4.Through the numerical simulation of soil water-salt transport in different groundwater tables,the results showed that the shallower the groundwater table was,the higher the water evaporation was,and the evaporation of 0 cm treatment was 1.9times of the 60 cm treatment.The water evaporation was increased with the increase of the temperature.The soil salt content was increased with the process of water evaporation,and gradually accumulated on the surface soil.When the depth of groundwater was 60 cm,the simulation result of Hydrous-1D model on sandy soil water content was effective.Hydrus-1D can be used to simulate sandy soil salt transport under root restriction in different groundwater tables.Hydrous-1D was used to predict the dynamic changes of soil salt in different groundwater depths.The results showed that groundwater had significant influences on surface soil salt of sandy soil when the depth of groundwater was lower than 150 cm;groundwater had non-significant influences on surface soil salt of sandy soil when the depth of groundwater was higher than 150 cm.It's suitable to adopt the root restriction in the saline-sodic land where the groundwater table depth is higher than 1.5 m.