Mechanism And Technology Of Using Subsurface Pipe Drainage To Improve Soda Saline-Alkali Soil

Soda saline-alkali soil improvement is a worldwide problem.As an important grain producing areas in China,Songnen plain is one of the three concentrated distribution areas of soda saline-alkali soil in the world.Soda saline-alkali soil of Songnen plain,covers an area of 3937 thousand hectares and the annual rate of growth is 1.4%.Characteristics of strong alkalinity,poor permeability,poor structure and poor physical and chemical properties of soda saline-alkali soil seriously restricted the development of local agriculture.Through the efforts of several generations,great advance in soda saline-alkali soil improvement was made.In the various improvement measures,the most commendable is by planting rice.But it needs a large amount of water and a long washing time.The"desalination layer"formed by planting rice is very shallow,once the water-course disappear,salt will reverse.Therefore,how to remove the salt from the soil completely with high efficiency is a problem we must consider."Salt comes with water,salt goes with water"is the basic rule of soil salt and water movement.Subsurface pipe technology is a saline-alkali soil improvement project based on water and salt transport law,through laying underground drainage pipe,so that soil desalination.In the irrigation or rainfall conditions,leaching saline water moves to the underground pipe,through the underground pipe discharged soil,so as to achieve the effect of washing,and the pipe can be the underground water level control in the critical depth,can effectively inhibit soil capillary water upward and avoid the soil salt,fundamentally improve saline-alkali soil.According to the previous research results and combined with the actual situation,we chose to use the subsurface pipe drainage,improve soil fertilizer rice planting in combination with the comprehensive management of soda saline-alkali soil in Songnen plain.The study was conducted between Anguang and Honggangzi village in Da'an.Soil permeability is 1.15×10^-3 mm/min,pH is 10.26,total salt content is 4.43g/kg,electrical conductivity is 1.07 ms/cm,total alkalinity is 6.645cmol/kg,exchangeable sodium percentage is 66.56%,sodium adsorption ration is 17.7?mmol/L?^1/2,soil organic matter and nutrient content are low.Under the conditions of rice cultivation in the experimental plot,the influence of different subsurface pipe treatment and soil fertilizer on pH,electrical conductivity,total salt content,total alkalinity and rice yield of soil,subsurface pipe water and rice paddy water were analyzed.The water discharge of different subsurface pipe treatment were simulated by HYDRUS-1D and comparative analysis was made between the simulation value and test value,the main results are as follows:?1?The main technical parameters of subsurface pipe drainage in Songnen plainAccording to theoretical analysis,information collected and engineering practice,respectively set 0.8,1.0 and 1.2m as buried depth,set 5,10,20,40m as spacing.Double-wall corrugated tubes with 110mm outer diameter made of high-density polyethylene?HDPE?with a PE level of 100 were selected.The diameter of the infiltrating hole in the pipe wall is 1-3mm and the pitch-row is 10-20mm.Choose sand as the filter material,sand filter should be laid evenly around the subsurface pipe,and the thickness is 6-8mm.?2?Supporting technology and mechanism of modified soil fertilizer for subsurface pipe drainageMethods of subsoil,pressure sand,organic fertilizer and MM amendment were adopted for supporting subsurface pipe drainage in the study.Among the methods,subsoil and pressure sand can directly improve the permeability of soda saline-alkali soil,as well as organic fertilizer and MM amendment can further improve itsphysicochemical properties.The comprehensive measures?subsoil 0.6m+pressure sand 500m³/hm²+fertilizer 34m³/hm²+MM amendment 15t/hm².?in the study can significantly increased the soil permeability to 0.045 mm/min,organic matter content to 14.58 g/kg,and effectively reduce pH,electrical conductivity,total salt content,exchangeable sodium percentage,total alkalinity,sodium adsorption ratio.?3?Numerical simulation of water discharge of soda saline-alkali soil in Songnen plainAccording to the comparison between simulated peak and the corresponding measured peak,the HYDRUS-1D numerical simulation results with the initial conditions and boundary conditions in the paper is satisfied.The maximum difference between the simulated peak and the measured peak is 0.006 cm/d,with a minimum of0.00001 cm/d.As the model has good prediction accuracy,it can be used to predict water discharge under different subsurface pipe treatment conditions.?4?Drainage effect and mechanism of different kinds of subsurface pipe treatmentUnder the conditions of comprehensive supporting measures,subsurface pipe water discharge were 0.028cm/d?0.0075cm/d?0.0035cm/d in turn when the buried depth were 0.8m?1.0m?1.2m sequentially and the spacing was 5m.The reason for the sequentially decrease of the subsurface pipe water discharge is that the path through which the water flows from the surface of the soil to the subsurface pipe gets longer,and the action of obstructing water movement increases,which causes the water slows down the movement to the subsurface pipe.Under the conditions of comprehensive supporting measures,subsurface pipe water discharge were 0.028cm/d,0.0035cm/d,0.0018cm/d,0.00048cm/d in turn when the spacing were 5m,10m,20m,40m sequentially and the buried depth was 0.8m.The reason for the sequentially decrease of the subsurface pipe water discharge is that the slope through which the water flows from the surface of the soil to the subsurface pipe slows down,the speed of water movement decreases,which causes more water stranded in the soil and can't be expelled from the subsurface pipe.?5?The mechanism of alkali change and yield-increasing effect under different kinds of subsurface pipe treatment conditionsOn the whole,the combination of subsurface pipe and comprehensive supporting measures integrated with rice planting has synergistic effect on the improvement of soda saline-alkali soil.It is hardly discharge water just laying out subsurface pipe without comprehensive supporting measures,so there is no improvement in soda alkaline soil.Comprehensive supporting measures integrated with rice planting can change the texture and construction of soil.With the increase of time,the physical and chemical properties of soil gradually improved.The embedding of the subsurface pipe is beneficial to the migration of soil moisture which causes the salt discharged and the water discharged increases gradually with the improvement of the physical and chemical properties of the soil.On the whole,the discharge of the subsurface pipe increases with the shallow burial and density.The greater the water discharge,the physical and chemical properties of soda saline-alkali soil better.Under the comprehensive supporting measures,the best effect of alkali change and yield-increasing occur when the buried depth was 0.8m and the spacing was 5m.The yield can reach 7500kg/hm²In conclusion,under conditions of rice cultivation,physical and chemical properties of soda saline-alkali soil in Songnen plain can be improved by shallow buried and densely covered subsurface pipe and comprehensive supporting measures,as well as soil productivity.