| In recent years, problems of saline soil and secondary salinization have become increasingly prominent, and have caused dramatic changes in soil structure, physical and chemical properties. Polyacrylamide has been populized in soil because of its active role in improving the structure of soil, retaining moistuer and preveting the loss of fertilizer. However, few researches have been conducted to study whether there is a positive effect of polyacrylamide on saline soil. With regard to this, we conducted this research by laboratory test and field trials to study the effect and mechanism on physical properties, hydraulic characteristics, soil moisture evaporation, adsorption capacity of slightly, moderately and severe salinized soil respectively after using polyacrylamide of8million,12million and16million molecular weight under different amount of applied aming at investigating the interaction law between polyacrylamide and different degrees of saline soil, at the same time, providing the necessary data and theoretical guidance to instruct the application of polyacrylamide in improving saline soil. The results are as follows:(1) After using polyacrylamide, saturated water content, field capacity increased of both non-salinized soil and saline soil and the aggregate content of>3mm,2-3mm,2-1mm, and1-0.6mm showed an increasing trend as well, while the aggregate content of<0.25mm and soil bulk density decreased.(2) After mixing polyacrylamide with slightly, moderately and severe salinized soil respectively, saturated hydraulic conductivity, horizontal diffusivity, water depth migration and infiltration rates of these varying degrees of salinization soil reduced; the application of polyacrylamide increased the water holding capacity of saline soil, showed as severe>moderately>slightly> no salt. Under the different molecular weight and the same amount of application, the larger molecular weight polyacrylamide was, the smaller the saturated hydraulic conductivity, horizontal diffusion rate and infiltration rate were; Under the same molecular weight and the same amount of application, saturated hydraulic conductivity, horizontal diffusion rate and infiltration rate of different soil salinization decreased with increase of the degree of salinity; Under the same type of molecular weight and the same degree of salinization, with the increase in fertilizer saturated hydraulic conductivity, horizontal diffusion rate, infiltration rate decreased, while water holding capacity of soil salinization enhanced conversely.(3) After mixed polyacrylamide with saline soil, cumulative evaporation volume of slightly, moderately and severe saline soil decreased with the increase of the amount of polyacrylamide application and salinization. and the cumulative evaporation volume curve of soil salinization is power function distribution. The evaporation of saline soil can be divided into two stages:the daily evaporation of comparison was lager than which in saline soil mixed with polyacrylamide in the first stage, and showed an opposite result in the second stage, the time difference between two stages was about12days.(4) Through the physical and hydraulic filter indicator, principal component analysis method was also used, an conlution was concluded that the positive effect of polyacrylamide was better in both non-salinized soil and slightly salinized soil than which in conparision, and the molecular weight of8million1:2500was better in non-salinized soil, the molecular weight of12million1:2500was better on slightly salinized soil; the improvement effect of Polyacrylamide on moderately and severe saline soil was not significant.(5) The optimal liquid to solid ratio of polyacrylamide of8milion molecular in slightly, moderately and severe saline soil was20:1, when polyacrylamide of different molecular weight was less than optimal ratio of liquid to solid, the amount of adsorption increased sharply with the increase of the solid-liquid ratio, but when it was lager than optimal ratio of liquid to solid, the adsorption amount of polyacrylamide kept constant with increase of solid-liquid ratio. Soil adsorption equilibrium time was6h of olyacrylamide of8milion molecular weight in slightly, moderately, and severe saline soil, which were both8h of12milion and16million, meanwhile,the adsorption capacity of the soil showed that non-salinized soil> slightly saline soil> moderately saline soil> severe saline soil. When the concentration of polyacrylamide of8million molecular weight was400mg/L, the adsorption of slightly, moderately, and severe saline soil reached the maximum, and when the concentration of polyacrylamide of12million and16million molecular weight was300mg/L, the adsorption of slightly, moderately, and severe saline soil reached the maximum.(6) The viscosity of polyacrylamide reduced with the increase of the concentrations of Na+and K+potassium, when the concentrations of sodium and potassium reached to500mg/L respectively, the viscosity of polyacrylamide were22.84and10.09mPa·s correspondingly which were minimums.Ca2+and Mg2+decreases the viscosity of polyacrylamide more significantly than Na+and K+. When the concentration of Ca2+and Mg2+reached800and500mg/L, the viscosity of polyacrylamide were5.65and5.26mPa·s correspondingly which were minimums. The viscosity of polyacrylamide increased with the increase of pH, and when pH>7, the viscosity kept a level at40mPa·s.(7) The impact of polyacrylamide on soil moisture of saline soil existed only in the soil layer used polyacrylamide, in other words, the use of polyacrylamide increased the moisture content of0-40cm soil layer, and no effect would appear below40cm. At the same level of irrigation, polyacrylamide helped to slow down deep seepage of fertilizer in saline soil. Besides, the effect of polyacrylamide on reducing nitrogen leaching was more obvious than which in phosphate. According to comprehensive appraisal method, considering production as the main factor in the results of the orthogonal test, S1F2P2was determined as the optimal combination by means of weighted composite score.(8) PAM is high polymer material, molecules in the process of dissolving can cause swelling that PAM molecules adsorbed water molecules, gathered a small scale soil moisture.And PAM dissolves in water to form a mesh structure, and the soil water solution viscosity increased, with tiny soil particles to form water membrane, inhibits the soil water movement.The existence of salt making PAM molecular chain curly and flocculant, coagulation blocked the porosity of the soil, when the soil moisture reduced viscosity increase adsorption small soil particles to form large aggregate. |
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