盐基离子对土壤持水及收缩特性的影响

不同盐离子对土壤持水能力具有不同程度影响,且在土壤水分特征曲线测定过程中,土体随失水发生收缩和开裂。在已获取研究成果基础上进行扩展,进一步探索8种低浓度盐离子(K~+、Na~+、Ca~(2+)、Mg~(2+)、Cl~-、HCO_3~-、CO_3~(2-)、SO_4~(2-))对土壤持水能力和收缩特征的影响。选取陕西粉黏壤土并分别采用含有此8种离子的盐溶液(浓度均约为1 g·L~(-1))对土样进行饱和处理,以无盐离子处理作为对照(CK);采用离心机法获取土壤水分特征曲线,使用游标卡尺测定离心过程中的土体轴向沉降高度,编写MATLAB程序对裂缝图像提取裂隙度量指标,据此对各处理土壤持水能力和收缩特征进行对比分析及评价。结果发现,在此浓度条件下:(1)K~+、Na~+及4种阴性盐离子在一定程度上均可提高土壤持水能力,且阴阳盐离子中CO_3~(2-)和K~+效果最显著,分别较CK提高35.81%和3.68%;(2)Mg~(2+)、SO_4~(2-)和CO_3~(2-)有利于减小土体轴向收缩度,且阴阳盐离子的作用效果分别表现为HCO_3~-Cl~-SO_4~(2-)CO_3~(2-)和Na~+K~+Ca~(2+)Mg~(2+);(3)Na~+、Mg~(2+)及4种阴性盐离子在一定程度上均有利于减轻土壤开裂程度,可同时减小土壤裂隙总长度、总面积以及长度密度和面积密度,且阴阳盐离子中CO_3~(2-)和Na~+效果最显著。研究可为不同类型盐碱土壤持水能力评价及其干缩开裂机理认知提供理论指导。

Effects of anions and cations on soil water retention and shrinkage behavior during dehydration process

Soil shrinkage and cracks commonly occur during soil water characteristic curve (SWCC) measurement, and the soil water retention varies with the type of salt ions (i.e., anions and cations) in the soil. A laboratory study was therefore conducted to make further analysis on the effects of four salt cations (K⁺, Na⁺, Ca²⁺, and Mg²⁺) and anions (Cl^-, HCO₃^-, CO₃^2- and SO₄^2-) with a low concentration of 1 g·L^-1 on water retention and soil shrinkage in silty clay loam saturated by salt solutions (1 g·L^-1), based on existing findings. The SWCCs were measured by the centrifuge; meanwhile, during dehydration, the heights of soil samples were measured using calipers to estimate the change in soil bulk density and the axial shrinkage. Two indices, crack length and crack area, were directly obtained from the processed binarized images by using MATLAB. The results showed following. (1) The cations, K⁺, Na⁺, and four salt anions could improve water retention, and CO₃^2- and K⁺ increased soil moisture by 35.81% and 3.68%, respectively, relative to the control treatment. (2) Mg²⁺, SO₄^2-, and CO₃^2- could decrease axial soil shrinkage. The influences of anions and cations on soil shrinkage were ranked in order that HCO₃^->Cl^->SO₄^2->CO₃^2- and Na⁺>K⁺>Ca²⁺>Mg²⁺, respectively. (3) Na⁺, Mg²⁺, and four salt anions could reduce crack length, crack area, crack length density and crack area density. And the performance of CO₃^2- and Na⁺ was the best. The findings provide theoretical and practical guidance for evaluation of water retention and cognition of the mechanism of desiccation cracking.