盐阳离子类型及浓度对土壤持水及干缩开裂的作用效果

为探索阳性盐离子对土壤持水性能的影响,同时定量分析失水过程所致的土体收缩及裂缝特征,选取陕西粉黏壤土作为供试土壤,分别采用含有K+、Na+、Ca2+和Mg2+4种离子的盐溶液(浓度均为5、30和100 g/L)对土样进行饱和处理,采用离心机法测定土-水曲线,进一步对土壤持水能力进行评价;同时测定土体沉降高度,采用数字图像处理技术获取面积密度和长度密度等裂隙度量指标,对土体收缩和开裂水平进行定量分析。结果发现:1)van Genuchten模型适用于盐溶液浸泡土壤的土-水曲线拟合;2)4种盐离子均基本导致土壤持水能力降低(5 g/L Na+除外),且持水性与离子浓度呈负相关关系;同时使得土壤饱和含水率、残余含水率和进气吸力降低,其中土壤饱和含水率与离子浓度呈负相关关系;3)K+和高浓度Na+有利于减轻土壤轴向收缩度,且土体轴向收缩应变与K+(P0.01)和Na+(P0.05)浓度呈负相关关系;在收缩过程中,不同离子对土壤容重的影响程度表现为Mg2+Ca2+Na+K+;4)4种离子均可减轻土壤开裂程度,且裂缝面积密度和长度密度与K+(P0.01)、Na+(P0.01)和Ca2+(P0.05)浓度呈负相关关系,与Mg2+浓度呈正相关关系(P0.01)。研究可为盐碱土壤持水能力评价、制定灌溉制度提供参考。

Impacts of type and concentration of salt cations on soil water retention and desiccation cracking

Abstract: The soil water characteristic curve (SWCC) defines the relationship between soil suction and volumetric water content, and is commonly used to evaluate the size and distribution of soil pores and the soil water availability and holding capacity for investigating the functions of unsaturated soil with various properties and for modeling the transport of soil water and solutes. Besides, the SWCC is usually effectively used to evaluate soil water retention, which is always considered as a basic factor for determining reasonable irrigation schemes. The SWCC can be obtained via centrifuge method; however, soil samples often shrink and crack during the SWCC measurement. Of various influencing factors on water retention, salt ions with different types and concentrations have various effects on water-holding capacity. The present study therefore aims to explore the differences in the characteristics of the soil water retention and the shrinkage and cracking during drying process resulting from 4 types of salt cations with different concentrations. In order to achieve the objectives, silty clay loam from Shaanxi Province was selected as experimental soil. Four replicates of the samples were air dried, sieved through a 2-mm mesh, and then compacted into cutting rings at a bulk density of 1.40 g/cm3. Four powder reagents, namely potassium chloride (KCl), sodium chloride (NaCl), calcium chloride (CaCl2) and magnesium chloride (MgCl2), which included 4 kinds of salt cations (i.e., K+, Na+, Ca2+ and Mg2+), were dissolved in distilled water at the concentration of 5, 30 and 100 g/L respectively. Distilled water was used as the control (CK). And all soil samples were first saturated in the solutions for 48 h before the experiment. The SWCCs were then constructed using a centrifugal method and used to quantitatively assess soil water holding capacity. At the same time, the deposit height of soil samples for each tested soil suction was measured using a vernier caliper, which was used to calculate linear shrinkage ratio and axial shrinkage strain for quantitatively evaluating soil shrinkage during the drying process. Cracks gradually occurred with soil water decreasing and were obtained using digital image processing technique in this paper, which were used to quantitatively evaluate the desiccation cracking based on crack area density and length density. The experimental results showed that first of all, salt cations had no effects on the SWCC shape, and the van Genuchten model was suitable for fitting SWCC saturated by saline solution with the R2 of high than 0.99. Furthermore, the 4 kinds of salt cations could weaken soil water retention, and water-holding capacity was correlated negatively with the concentration of salt cation. According to the fitting results by the van Genuchten model, the 4 kinds of salt cations could also weaken saturated water content, residual water content and air suction, and water-holding capacity was correlated negatively with saturated water content. Besides, K+, Na+ with high concentration and Mg2+ with low concentration were helpful to decrease soil axial shrinkage. And the axial shrinkage strain was correlated negatively with the concentration of K+ (P<0.01), Na+ (P<0.05) and Ca2+ and positively with Mg2+ concentration. Shrink of soils during the drying process was accompanied with the increasing of soil bulk density, and the effect of salt cations ranked in the order of Mg2+>Ca2+>Na+>K+. Finally, the 4 kinds of salt cations were helpful to weaken cracking based on the quantitative analysis of crack area density and length density. And these 2 crack indicators were significantly negatively correlated with the concentration of K+, Na+ and Ca2+ and positively with Mg2+ concentration. These findings are helpful to provide the theoretical and practical guidance for the evaluation of water retention and the determination of irrigation scheme, and to provide some references for developing the engineering practice on saline soils. The further study should add focuses on the effects of salt anions and smaller concentration gradient on soil water retention and desiccation cracking.