Research On The Mechanism Of The Effect Of Saline Water Irrigation On Soil Physical And Chemical Properties And The Construction Of Water Quality Assessment Method

The cation ratio of soil structural stability（CROSS）has a distinct advantage over sodium adsorption ratio（SAR）in evaluating irrigation water quality affecting soil physical properties.But CROSS still has no specified applicability.It is unknown whether irrigation with high-CROSS saline water will cause soil structural problems such as cracking,because previous studies on cationic effects on soil pore structure just focused,even stayed on the dispersion and flocculation of clay particles.This study attempted to quantify the formation of macropores and cracks caused by saline water with different cation compositions,appearing in various CROSS levels.An indoor soil column experiment was conducted.Saline water with the same EC of 4 d S m^-1,and different cation compositions of Na⁺-Mg²⁺（NM100）,Na⁺-K⁺-Mg²⁺（NKM67）,Na⁺-Ca²⁺-Mg²⁺（NCM38）,Na⁺-Ca²⁺（NC28）and K⁺-Ca²⁺（KC9）(the figure of each treatment name represents the value of optimal CROSS(CROSS_opt)),respectively,was applied taking deionized water as the control.The soil water and salt transport performance,hydraulic characteristics and pore structure were determined.The main conclusions are as follows:（1）Compared with CK,all saline water treatments all reduced saturated hydraulic conductivity,and the saturated hydraulic conductivity of NKM67 and NM100 was only 1/2 that of CK.With the continuation of irrigation,NKM67 and NM100 could increase the soil salination rate,while KC9,NC28and NCM38 could improve salt leaching efficiency.KC9 and NC28 were conducive to the formation of non-capillary pores in soil,reducing soil bulk density.The increase in total pore volume was mainly due to low CROSS saline water promoting the transformation of capillary pores to non-capillary pores in soil.K⁺-Ca²⁺saline water was conducive to soil structural stability,while Na⁺-Mg²⁺saline water could easily cause soil pore blockage and waterlogging.（2）The turbidity of soil suspensions became higher with the increase of CROSS_opt,and saline water with Na⁺-Mg²⁺dominance（NM100）was the most unfavorable for soil flocculation,while K⁺-Ca²⁺dominance（KC9）was the most preferable for soil structural stability.KC9 and NC28 were in favor of the formation of macropores and did not cause soil cracking.Vertical cracks presented substantially in NKM67.Vertical and horizon cracks intersected in NM100,and soil structure disintegrated,causing a sharp decrease in the number of macropores,narrow cracks transformed into cracks with relatively small aspect ratios.Under the Na⁺-Mg²⁺dominant condition,Mg²⁺could promote soil cracking besides soil dispersion caused by Na⁺,indicating the adverse effect of saline water with Na⁺-Mg²⁺dominance on soil pore structure.Thus,considering the healthy soil pore structure,Na⁺-Mg²⁺dominant saline water was least suitable for agricultural irrigation.（3）Taking the Hetao irrigation district as an example,according to the water quality assessment results of EC and CROSS,unsuitable irrigation water samples are mainly distributed in the northwestern part of the Hetao irrigation district and the main drainage area.Compared with saline water with high EC and low CROSS,saline water with low EC but high CROSS was more likely to cause soil cracking and soil disintegration.Compared with saline water with low EC and high CROSS,saline water with high EC and low CROSS was more conducive to soil structural stability and does not seriously reduce the infiltration performance of the soil.When assessing saline water quality by EC and CROSS,saline water with EC values ranging from 0-4 d S m^-1 and CROSS values exceeding the shreshold(38-67 mmol_c L^-1 of CROSS_opt,about 30 mmol_c L^-1 of CROSS_f and CROSS_d)were not suitable for irrigation.It is recommended to set more water samples in the EC value range to accurately calibrate the saline water quality assessment criteria in order to fully consider the effects of saline water irrigation on soil water infiltration,salt accumulation,soil cracking and disintegration.Summary,the research has clarified the mechanism of how saline water affects soil hydraulic characteristics,pore structure,and infiltration performance.It has proposed a water quality evaluation method based on EC and CROSS,which can provide a theoretical basis and technical means for the safe utilization and irrigation decision-making of saline water.