Response Of Salt Transfer And Transformation In Aerated Zone To Irrigation In Jinghuiqu Irrigation Area

The phenomenon of crop irrigation and fertilization is common in arid and semi-arid areas of Northwest China.A series of soil biogeochemical changes caused by irrigation and fertilization are very obvious.Long-term excessive irrigation and fertilization will lead to changes in soil moisture,pH and other environmental conditions,and dissolution or precipitation of soluble mineral salts in soil.Under the effect of irrigation water leaching,the dissolved salt components in the soil move to the deep soil layer to the groundwater,which not only causes the loss of soil nutrients,affects the growth of crops,but also leads to groundwater pollution.In order to apply fertilizer reasonably and effectively,and prevent soil and groundwater pollution,it is necessary to study soil salinity migration and transformation in irrigation area.The changes of soil salt content and existing forms under different irrigation and fertilization rates were studied in order to provide theoretical basis for irrigation methods and rational and effective irrigation and fertilization in irrigation areas.Based on field investigation and in situ test in Sanqu Town,Jinghuiqu Irrigation District,Shaanxi Province,and combined with laboratory test analysis,the main physical and chemical indexes of soil were obtained.Based on the numerical model software HYDRUS-HP1 and Visual MINTEQ,the law of soil salinity migration and transformation was simulated and calculated.Based on the simulation results of soil salinity migration and transformation under the existing irrigation and fertilization modes,different irrigation modes and different fertilization modes,the effects of irrigation and fertilization amount on the changes of soil chemical composition were analyzed.Each treatment mode can be divided into two different seasons,winter and summer.Firstly,the in-situ measured data and simulated data are fitted to determine the reliability of the model.Then,simulate and calculate the dissolution and precipitation of soil salinity,the main ion content of soil and the main existing forms of salt components under the three research modes.Finally,the influencing factors of soil salinity migration and transformation were analyzed.The following conclusions are drawn:(1)Irrigation leached the dissolved salt ions in soil profile and the adsorbed ions on the surface of soil colloid.The main salt ions in soil decreased with the increase of irrigation amount in winter and summer.Salt segregator content in soil layer above 100 cm was significantly reduced by irrigation,and ion content in soil layer below 100 cm was increasing.Before irrigation,due to evaporation,the salt content in the surface layer of soil in summer is higher than that in winter,so the leaching of salt components in the surface layer of soil in summer is more serious by irrigation.Influenced by the ground temperature of each soil layer,the salt dissolution of soil profile above 250 cm in summer is more obvious than that in winter,and the phenomenon of salt dissolution below 250 cm in soil layer is the same as that in winter.After irrigation,the proportion of salt fractions in summer is larger than that in winter.(2)After 30 days of irrigation and fertilization,the soil will reaching a dynamic equilibrium state.After urea hydrolysis,the changes of ammonium nitrogen,nitrate nitrogen and pH are different.The more fertilizer is applied,the higher the content of ammonium nitrogen and nitrate nitrogen is,and the lower the pH is.The change of pH will cause a series of soil water-rock reactions.The smaller the pH value is,the greater the dissolution trend of mineral salts in soil is,and the higher the content of salt fractions in soil is.The response of salt change to fertilizer application in summer was more obvious than that in winter.(3)Irrigation and fertilization affect soil pH,moisture,temperature and cation exchange capacity,thus affecting soil salinity migration and transformation.Irrigation and fertilization reduced soil pH value and reached the minimum value four days after irrigation,and soil salinity increased with the decrease of pH;the larger the irrigation amount,the higher the water content,the worse the soil permeability,the worse the absorption of nutrients by crops and the exchange and adsorption ability between mineral components;after irrigation,water retained in the soil longer in winter than in summer,leaching slower,and salt segregator content in winter.Higher than in summer,lower temperature will also affect the ionic strength of soil,resulting in lower pH value in winter than in summer,seasonal variation of salinity;soil organic matter content decreases with the increase of soil depth,and the surface content is the largest.The density of soil with high organic matter content is relatively small,which is conducive to water storage and circulation,and increases the absorption of salt separators by surface soil.