Transport Characteristics of Soil Salinity in Saline－alkali Land under Water Storage and Drainage Conditions

Juan LI，Jichang HAN

1.Shaanxi Land Engineering Construction Group，Xi'an 710075，China；2.College of Forestry，Northwest A＆FUniversity，Yangling712100，China；3.Key Laboratory of Degraded and Unused Land Consolidation Engineering，the Ministry of Land and Resources of China，Xi'an 710075，China

1 Introduction

With the increasingly acute conflicts between world's population growth and land demand，how to reasonably use limited land resources and increase land resources has become a top issue of concern around theworld［1-3］.Currently，the saline-alkali land has great potential for development and utilization，and how to control soil salinization is one of the serious problems facing agriculture today［4-9］.In agricultural production，the saline soil is low-yielding soil，and it isa obstacle to building high-yielding fields.There is the threat of secondary soil salinization in themodern irrigation project［10］.The comprehensive control methods for saline-alkali land at home and abroad include physical，irrigational，chemical and biological improvement and other technologies and methods.In China，irrigational improvement is often used（namely using fresh water to restrain salt content and draining for desalinization of soil，and the drainage-based approach has reduced the salinity.Traditional saline-alkali land control has high operating costs，and leads to a waste of water，which to a certain extent can pollute downstream water bodies［11-16］.Lubotan，at the junction of Fuping County and Pucheng County in Shaanxi Province，is the lowlying land and the severe saline-alkali land formed naturally and artificially.Han Jichang etal.［17］report that the traditional salinealkali land control isbased on storage and it is necessary to establish the irrigation and drainage system，and propose a new model of"shifting drainage to storage，water-land coexistence，harmonious ecology"for the saline-alkali land control based on Comprehensive Control Project for Lubotan in Shaanxi Province.Under conditions ofwater storage and water drainage，we set the simulation experiment in the experimental base of Fuping County to analyze changes in soil salinity under different treatmentsand changes in salinity and anion and cation in different depth of soil，conduct comprehensive study of various treatments and perform the correlation analysis between salt ions，in order to provide a scientific basis for the rational developmentand utilization of saline-alkali land and harmonious ecological control.

2 Materials and methods

2.1 Overview of experimental stationThe experimental base is in Chuyuan Village，Ducun Town，Fuping County，Weinan City，ShaanxiProvince.The area featuresawarm temperate semi-humid climate，with the average annual rainfall of472.97mm.The rainfall in June-September accounts for 49%of the annual rainfall.The annual evaporation is1000-1300mm，the frost-free period is225 d，and the annualaverage temperature is13.4℃.The summermaximum temperature is 41.8℃，and the winter minimum temperature is-22℃.The total annual energy radiation is 123.9-127.8 kca／cm2.

2.2 Experimental designThe experimentwas implemented in October 2009.In order to simulate the land situation of Lubotan experimental area，wemake the experimental device by ourselves.It has the size of 23.0 m×1.5 m×2.0 m，and ismade by brick concrete.The experimental device includes two parts，namely water storage part and experimental soil bin.One end is the tank filled with water，and themiddle section is soil bin.The geotextile is placed between water and soil to prevent soil into the water.In order to ensure the experimental results，various interfaces are installed to do waterproofing.The experimental treatments include water storage treatment and water drainage treatment.Water stor-age treatment is to put some water into the ditch and use the dispersion effect between water and soil to control soil salinity transport；water drainage treatment is to use flood irrigation and take advantage of the convection effect between water and soil to drain for desalinization of soil.The same amountofwater isused forwater storage and water drainage treatments，and the transport and changes of salinity under different treatments are shown in Fig.1.The soil sample is the saline-alkali soil in Lubotan，and soil is taken at30-40 cm intervals.After crushing，grinding，air drying and sieving（5mm），the soil samples are put into the experimental soil bin.According to the test data on soil in the experimental area from Test Center of Northwest A＆F University in 2009，the average content of soil organicmatter，total salt，pH value，Cl-，HCO3-and SO24-is 0.70%，9.33，0.37%，0.08%and 0.06%，respectively.

2.3 Determ ination items and methodsAfter the cotton harvest in September 2013，in the above two treatments，6 sampling pointswere selected from each experimental plot，and the soil auger wasused to collect0-160 cm soilwith sampling intervalof20 cm.By laboratory experiment，the various indicators of 0-160 cm soilwere analyzed.Determination indicators and methods are shown in Table 1.

Table1 Determ ination indicators and methods

2.4 Calculation and statistical analysisExcel2007 and DPS 7.05 software are used for statistical analysis，and Duncan's new multiple rangemethod is used formultiple comparisons.

3 Results and analysis

3.1 Com parison of 0－160 cm soilmoisture between water storage and drainage treatmentsIn June 2013，we conducted irrigation forwater storage and drainage treatmentswith consistent irrigation rate.In October 2013，the 0-160 cm soilmoisturewas measured during the cotton harvest.The soilwater storage capacity under water storage and drainage treatments in the autumn idle period is shown in Fig.2（a）.The0-160 cm soilmoisture shows the same trend underwater storage and water drainage treatments；in the 0-60 cm soil layer，soilmoisture decreaseswhile in the60-160 cm soil layer，soilmoisture increases.The 0-160 cm soil soil moisture on the average under water storage treatment is 4.47%higher that under water drainage treatment，and there are no significant differences between treatments.The experimental results show thatafter a long period ofwater transport，evaporation and use，the soilmoisture underwater storage treatment is slightly higher than thatunderwater drainage treatment，and the field soil compaction does not easily occur.

3.2 Analysis of mass fraction of TDSUnder water storage and drainage treatments，the profile distribution of TDS（Total Dissolved Solids）in 0-160 cm cotton field soil is shown in Fig.2（b）.Themass fraction of TDS in soil under water storage treatment is significantly higher than that under water drainage treatment，there are no significant changes with the increase of soil depth，and there are no significant differences between treatments.The averagemass fraction of TDS in 0-160 cm soil under water storage and drainage treatments is 0.906 and 0.487 g／kg，respectively.In the 0-40，40-100，100-160 cm soil，the mass fraction of TDS in soil is smallest in the upper layer，and shows an increasing trend；in the 100-160 cm soil，the mass fraction of TDS in soil is largest，14.98%and 15.59%higher than that in the 0-40 cm soil，respectively.This is related to the soilwater transport under the two treatments.Underwater storage treatment，the water usually enters into soil from themiddle part of soil，and moves with soil salinity up and down.Under water drainage treatment，the water enters into soil from surface，and the saline-alkalisoil surface is prone to compaction，so soil salinitymainlymoves down with thewater.The experimental data show that differentwater storage and drainage treatments can effectively reduce soil salinity possibly at soil depth of less than 60 cm.

3.3 Analysis ofmain salt ions in soil

3.3.1Anions.Under different treatments，the profile distribution of averagemass fraction ofmain anions in 0-160 cm cotton field soil is shown in Fig.2（c）.In 0-160 cm soil，the average mass fraction of HCO3-in soil under water storage and water drainage treatments is 0.019 and 0.018 g／kg，respectively；the average is highest in the lower layer and lowest in the upper layer.There are no significant differences between treatments（P＜0.05）.Themass fraction of SO24-and Cl-showsa similar trend，and themass fraction underwater storage treatment is significantly less than that under water drainage treatment.In 60-100 cm soil layer，the averagemass fraction of Cl-in soil underwater storage treatment is smallest，and the averagemass fraction of SO24-in soil under water storage treatment is 55.87%lower than that under water drainage treatment.The experimental data show that the desalination effect is obvious in themiddle soil under water storage treatment.

3.3.2Cations.Under different treatments，the profile distribution of averagemass fraction ofmain cations in 0-160 cm cotton field soil is shown in Fig.2（d）.Under water storage and water drainage treatments，the averagemass fraction of Ca2＋，K＋and Na＋in 0-160 cm soil shows the same trend，and the average mass fraction underwater storage treatment ishigher than thatunder water drainage treatment.There are no significant differences between treatments or various soil layers（P＜0.05）.The lowest averagemass fraction of K＋is in 120-160 cm soil，and it is lower under water drainage treatment.Under water storage and water drainage treatments，the averagemass fraction of Ca2＋in soil is 0.077 and 0.069 g／kg，respectively.In 0-160 cm soil，the averagemass fraction distribution of Na＋in soil under water storage and water drainage treatments is similar to that of HCO3-，and it is highest in the lower layer and lowest in the upper layer.Potassium is themajor crop nutrient，and themass fraction of K＋in soil is high under water storage treatment，which can effectively improve the crop nutrient.

3.4 Analysis of soil pHUnder water storage and drainage treatments，the changes in pH of various layers of soil are shown in Table 2.The soil pH underwater storage treatment is less than that under water drainage treatment.In the 0-40 cm soil layer，the average soil pH under water storage treatment is 12.41%less than that under water drainage treatment.It can be seen from Table 4 that under different treatments，the soil pH is high in the middle and lower layer but low in the upper layer，but it has no effect on the growth ofalkali-resistant cotton，and thereareno significant differences between treatments，indicating that the two treatments have a little effecton soil pH.Compared with traditionalwater drainage treatment，thewater storage treatment can effectively reduce the pH value of the soil layer.

Table 2 Soil pH value comparison under water storage and drainage treatments

3.5 Soil salinity andmajor ionsTo further analyze the reason for differences in soil salinity distribution and test the changes in soil salinity under water storage and drainage treatments，we perform the statistical analysis of soil salinity and its composition.Themain statistical characteristics of salinity and its composition under different treatments are shown in Table 3.It can be found that soil salinity ismainly based on bicarbonate and sulphate.The multiple linear regression analysis of salt ions and total salt shows that the correlation coefficient（R2）between total salt and only two ions（HCO3-and Na＋）is high，reaching 0.9661.

Table3 Descriptive statistics of soil salinity and main ions Unit：g／kg

As can be seen from Table 4，the correlation coefficient between cations and anions is relatively large.Therefore，we use principal componentanalysis to extract common factors.Table5 is the componentmatrix of the first threemain common factors.

Table 4 Correlation matrix between main ions

Table5 Componentmatrix

Table 5 shows that common factor 1 mainly depends on the contribution of Na＋，K＋and Ca2＋，and is largely affected by Cl-，while common factor2mainly depends on the contribution of Cl-，Ca2＋and SO24-，indicating that the local salinity ismainly based on bicarbonate and sulphate，but the main ions affecting difference in the salinity distribution are Na＋and Cl-.By compa-ring the threemain common factors underwater storage and drainage treatments，it is found that for the 60-100 cm soil，saltmay gather and theremay be salinity enrichment and secondary salinization.

4 Conclusions and discussions

Traditional draining for desalinization of soil is to pour fresh water to dissolve various statesof salt in soil，and emphasizes the role of washing and convection［18］，while water storage for desalinization of soil is to give play to the diffusion effect.To test the change in saline-alkali land soil salinity and salinity transport under water storage and drainage treatments，we established the experimental model in Fuping County of Shaanxi Province in 2009，to analyze the salinity change in 0-160 cm soil under two treatments.The experimental results show that the soilmoisture under water storage treatment is significantly higher than under water drainage treatment，and soil is not likely to be compacted，which is conducive to crop growth.It is consistent with the findings of Han Jichang［19］.The salinity decrease under water storage treatment is mainly due to the diffusion effect；the salinity decrease underwater drainage treatment ismainly due to the convection effect.The salinity，pH and ion concentration of soil decreases when compared with the initial stage of experiment，indicating that water storage treatment has better desalination effect than water drainage treatment.Themain salt ions affecting the soil salinity distribution under water storage and drainage treatments are Na＋and Cl-，and themass fraction of Na＋in soil underwater storage treatment is high，which is not conducive to the improvement of soil structure and crop growth，similar to the findings of Zai Songmei et al.［20］.Water storage treatment ismainly based on water level of impounding reservoir，changing concentration and dynamic balance between water and soil salinity，to achieve accelerated circular leaching of salt on saline-alkali soil，reducing the secondary pollution and hazards to soil and water，so it has good application prospect［9］.The domestic studies on saline-alkali land control mostly focus on the comparison of different control measures and ecological restoration effect of saline-alkali land［21-22］.For the study on soil salinity change under water storage and drainage treatments，the foreign studies on saline-alkali land controlmainly focus on the control of water drainage and agricultural non-point source pollution［23-25］，and it lacks water storage treatment.Sun Bo etal.［26］make the experimenton soil-water salt transport，and conduct an in-depth analysis of saltwhereabouts after the salinity reduction under water storage conditions.The study of Han Jichang et al.［27-28］shows that after water storage，the soil organic matter content increases，and the soil quality is improved significantly.This experimental study has not yet finished，because it only involves 0-160 cm soil，it lacks water salinity monitoring and subsoil indicator research under water storage treatment，and the quantitative analysis of various experimental factors is not deep.Through the study of this experimentalmodel，we'llmonitor water and salt dynamic system and conducta long-term analysis of water and soil distribution，transport and soil quality in order to gradually improve the research results.

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