Effect of Acidification on Clay Minerals and Surface Properties of Brown Soil

Abstract

Globally, soil acidification is becoming a serious environmental and ecological concern, posing a major threat to ecosystem functions and services. In order to clarifying the acidification mechanism, evaluating acidification risk, and reconditioning soil acidification, the effects of acidification on clay mineral composition and soil surface properties should be evaluated. In this study, the surface charge, specific surface area (SSA), species and content of clay minerals were investigated using the ion adsorption method, methylene blue method, and X-ray diffraction (XRD) for brown soil samples, which collected from Muping, Shandong Province, China. The results showed that the clay mineral species and content varied with the degree of acidification. A small amount of montmorillonite was found in weakly acidic soils, and gibbsite was found in strongly acidic soils. Furthermore, although illite, kaolinite, vermiculite, and chlorite were commonly found in soils with different acidification degrees, their content differed. The negative charge (CEC₈.₂), permanent negative charge (CEC_P), variable negative charge (CEC_V), and SSA values decreased with increasing acidification, while anion exchange capacity values (AEC) decreased. The change of CEC_V was caused by soil organic matter, and the change of CEC_P was caused by illite content, which accounted for the largest proportion in clay minerals of brown soil.

1. Introduction

Globally, soil acidification affects up to 40% of the arable soils and 70% of the potentially arable land [1,2]. Thus, it poses a serious threat to ecosystem functions and services in non-agricultural and agricultural systems [3,4]. Recently, more attention has been paid to the negative impacts of soil acidification on nutrient balances, plant species richness, crop production due to the loss of base cations, and mobilization of Al and other potentially toxic elements in soil [5]. An earlier study reported that lower soil pH increases the activity of nutrients, such as Fe, Al, Mn, while a higher availability of micronutrients helps crops absorb them, the fertility of highly weathered soils are compromised or decline with time [6]. Additionally, a large amount of H⁺ enters the soil solution and then exchanges with exchangeable base cations on the soil colloids and then enters the soil colloid surface, which enables the soil to produce exchangeable H⁺ (EH⁺). When the adsorbed H⁺ on soil colloids exceeds a certain limit, the crystalline structure of these colloidal particles is disrupted and some Al octahedra even disintegrate. The Al ions detach from the octahedral lattice and become active Al ions, which are adsorbed on the negatively charged clay surface and are converted into exchangeable Al³⁺ (EAl³⁺) [7]. Hence, soil acidification can affect the structure and composition of clay minerals.

Clay minerals are important part of the soil. Their fine particles, large surface area, and negative charge have a significant effect on soil properties. Moreover, the changes in clay minerals caused by environmental influences affect soil fertility. Under soil acidification conditions, the weathering intensity may vary, depending on the type of clay mineral concerned. Studies have reported strong weathering of feldspar, mica, chlorite, and vermiculite under acidic conditions [8]. Xu et al. [9] reported that the soil was significantly acidified after the 54 years of tea planting, and soil weathering was enhanced by acidification caused by tea cultivation. Tao et al. [10] used X-ray diffraction (XRD) to scan cultivated brown soil after 23 years of targeted fertilization and found that that the transformation of clay minerals was accelerated by manure treatments. Compared to the no manure treatment, the increasing kaolinite and decreasing illite indicated that the rapid transformation from illite to kaolinite was dependent highly on the long-term manure application, which significantly influenced soil pH.

Soil acidity primarily exists in the form of exchangeable acid on the solid surface of soil. The charge status of the soil surface determines the buffering ability of soil to exogenous acid and the degree of soil acidification [11]. Previous studies have revealed the relationship between soil surface properties (hereafter surface properties) and pH. In the arid soil of southeast Iran [12] that has less variable charge, pH was positively correlated with the permanent negative charge and negatively correlated with the anion exchange capacity (AEC). Additionally, the change in charge was also related to the composition and content of clay minerals. In the soil developed from the tropical and subtropical regions, China [13], soil acidity showed a significant positive correlation with CEC, but a significant negative correlation with free iron oxide content. The degree of variation in the surface properties of brown-red soil, red soil, oxisol, yellow-cinnamon soil, and yellow-brown soil with pH was different in zonal soils in Central Southern China, which was related to the composition of the clay minerals and content of Fe and Al oxides in the soil [14]. Furthermore, soil acidification can lead to heavy metal element activation. On the one hand, H⁺ produced in the acidification process will dissolve the heavy metal ions into the soil solution, which can improve the availability of heavy metal ions. On the other hand, the reduction of clay content in acidified soil will reduce the adsorption of heavy metal ions, which will seriously affect the safety of agricultural products [15,16,17]. Over the past few decades, many studies have focused on the influence of soil acidification on clay mineralogy and surface properties. However, the effects of soil acidification in most studies were achieved by soaking in inorganic acids [18] or by a dynamic leaching test [19] of simulated acid rain. In nature, soil acidification is a slow and long-term process and is largely influenced by environmental factors. Therefore, the effects of different levels of soil acidification on the surface properties and clay minerals of natural soils need to be studied to improve our understanding of the mechanism of evolution of soil acidification.

Many studies have recently been conducted on soil acidification of red and yellow soil in South China [20,21]. However, relatively fewer studies have been conducted on the acidification of brown soil in the north. Furthermore, the acidification of intensive farmland and orchard soil in Jiaodong Peninsula not only leaded to crop production reduction, but also caused a series of non-point source pollution problems, which seriously restricted the construction of the blue economic zone in Shandong Peninsula. Therefore, the study on acidification of the brown soil in Jiaodong Peninsula urgently need attention. In this study, soil profiles with different acidification degrees were sampled from the Muping area of the Jiaodong Peninsula. The aims of this work were (1) to investigate the influence of the acidification degree on the species and content of clay minerals; (2) clarify the influence of acidification degree on the surface properties including soil surface charge and the specific surface area (SSA); (3) establish the correlation among indices of acidity, clay minerals, and surface properties. The results could provide fundamental theories for understanding the acidification mechanism, evaluating acidification risk, and reconditioning soil acidification.

2. Materials and Methods

2.1. Soil Samples

In our study, 9 sampling sites were selected and 27 soil samples (including 0–20, 20–40, and 40–60-cm layers) were collected in the Muping District, Yantai City, northeastern Shandong Peninsula (37°4′–37°30′ N, 121°9′–121°56′ E; Figure 1). The study area was characterized by a warm temperate monsoon climate, with an average annual temperature of 11.2 °C and an average annual rainfall of 737.2 mm. The parent rocks in this area are mostly granite. Soil parent materials are alluvial or residual alluvial. The sampling sites were all selected from farmland and a wheat-maize rotation system was adopted. The cropping years were all over 10 years. Each sample contained 1 kg of soil and had a mixture of five-point sub-samples collected using a soil auger. These were placed in sample bags and were air-dried and ground in the laboratory. One part of soil was passed through a 2-mm sieve for the determination of pH, EH⁺, EAl³⁺ and clay extraction, another part was passed through a 0.25 mm sieve for the determination of SOM, soil surface charge and SSA.

Based on the pH value of the 0–20-cm soil layer, the 9 sampled sites were divided into three groups according to the soil acidity grading standards of the Second National Soil Census [22,23]: strongly acidic, pH < 4.5 (Group I); acidic, 4.5 < pH < 5.5 (Group II); and weakly acidic, 5.5 < pH < 6.5 (Group III).

2.2. Sample Measurements

Soil pH was determined in a suspension with a soil to solution ratio of 1:2.5 using a pH meter (pH-3c, Shanghai REX Sensor Technology Co., Ltd.; Shanghai, China). Exchangeable H⁺ (EH⁺) and exchangeable Al³⁺ (EAl³⁺) were measured via the KCl exchange method. Soil organic matter (SOM) was determined via the dichromate method [24].

The Mehlich method [25] was used to measure the negative charge (CEC₈.₂) and permanent negative charge (CEC_P). Six portions of 0.5 g of 0.25 mm air-dried samples were weighed into a funnel and moistened with distilled water. Three portions were washed twice with 10 mL of BaCL₂-TEA (triethanolamine) buffer; the other three portions were washed 5 times with 10 mL of 0.05 mol/L HCL solution. Then 6 portions samples were washed with 0.3 mol/L BaCL₂ solution for 5 times, and rinsed 6 times with distilled water. The above eluent was discarded, then the samples were washed 5 times with 50 mL of 0.3 mol/L CaCL₂ solution, and 3 times with 40 mL distilled water. The eluent was collected in a 100 mL volumetric flask. The concentration of Ba²⁺ in the eluent was determined by spectrophotometer to calculate CEC₈.₂ and CEC_P. The variable negative charge (CEC_V) is the difference between CEC₈.₂ and CEC_P.

The AEC was determined via phosphorus adsorption spectrophotometry [25]. The Ca saturated soil samples measured by CEC_P were dried at 40–45 °C. A certain amount of samples were weighed and placed into 50 mL centrifuge tube, and shaken with 20 mL of 0.006 mol/L Ca(H₂PO₄)₂ for 1 h. The samples were left overnight and then shaken for another hour. After centrifugation (4000 rpm) for 10 min, 5.0 mL of centrifuge solution were absorbed into a 25 mL volumetric flask, then 15.0 mL distilled water and 5.0 mL ammonium vanadate-ammonium molybdate solution were added. After 20 min, the absorbance at 432 nm wavelength was measured with spectrophotometer. The amount of phosphorus adsorbed by soil was calculated as the AEC of soil. The SSA was determined via the methylene blue method [26].

2.3. Clay Minerals Extraction and Determination

Soil samples were treated with H₂O₂ (35% at 40 °C) to remove SOM and the soil suspension was adjusted to pH 7–8 using 0.5 mol∙L⁻¹ NaOH. The clay fraction smaller than 2 μm was collected via a sedimentation technique according to Stokes’ law using repeated siphoning until the suspension withdrawn was clear [27]. Clay fractions were collected, air dried, and weighed and then ground with an agate mortar to pass through a 300-mesh sieve.

The tested samples were saturated with Mg-glycerol (Mg-gly) and potassium (K). The Mg-gly samples were examined at 25 °C using an X-ray diffractometer (D8 Advance, Bruker, Bremen, Germany). The K-saturated samples were tested at 25 °C and then successively heated to 300 and 550 °C as required, and diffraction scanning was conducted after a constant temperature was maintained for 2 h [28]. The XRD patterns were obtained with CuK_α radiation (incident angle = 3°, step size = 0.02°, scanning rate = 4°2θ∙ min⁻¹). The accelerating voltage and applied current were 40 KV and 40 mA, respectively.

The diffraction pattern of clay minerals was obtained by scanning the directional glass slides; clay mineral species were identified based on the clay mineral analysis method of Moore and Reynolds [29] and Środoń [30]. Comparative analysis of the changes of characteristic peaks under different treatments are shown in Table 1. The estimations of their semi-quantitavive proportions were performed as reported by Moore and Reynolds [29], a Gaussian curve with 9-point smoothness, deduction background and peak fitting were utilized for the diffraction pattern of Mg-gly. It calculated the integral area of each mineral diffraction peak and multiplied it by its proportional coefficient (Vermiculite × 2, illite × 3.5, kaolinite × 2, etc.). Then the relative content of each clay mineral was determined according to the percentage of the area of each characteristic peak in the total area [31].

2.4. Statistical Analysis

Microsoft Excel 2017 (Version 2017, Microsoft Corporation, Redmond, WA, USA) was used to initially organize the data. One-way analysis of variance was conducted using the SPSS 22.0 software (Chicago, IL, USA) and used to analyze the differences in the soil properties among different acidification degrees and across the soil layers. The relationship among soil basic properties, surface properties, and clay minerals was analyzed using the Pearson correlation and marked by p < 0.05. All data in the charts are presented as means ± standard deviations. The relationship among basic soil properties, clay minerals, and surface properties was studied using Canoco 5.0 (Microcomputer Power, Huston, TX, USA) for redundancy analysis (RDA) to illustrate the effects of basic soil properties and clay mineral on surface charge properties.

3. Results

3.1. Soil Properties

Soil properties varied with the degree of acidification and depth of the soil profile (Table 2). EH⁺ values decreased with increasing pH values. Especially in the 0–20-cm layer, EH⁺ values were significantly different among the three groups, decreasing from 0.55 to 0.13 cmol∙kg⁻¹. EAl³⁺ values also decreased with increasing pH values. The values of Group I were the highest and were 2.51–3.44, 3.57–7.67, and 7.63–15.27 times higher than those of Groups II and III, respectively. The variations in SOM content and pH value exhibited different trends among the three soil layers. In the 0–20-cm and 20–40-cm layers, the SOM contents of Groups I and II were similar and were significantly different from those of Group III. However, there were no significant differences among the three groups in the 40–60-cm layer. The clay content in the three layers increased with increasing pH values, and the clay contents of Groups II and III in the 0–20-cm layers were similar but differed significantly from those of Group I. There were no significant differences among the other layers.

The soil-layer depth also affected the soil properties. The pH values of soil in the same profile increased with increasing depth, EH⁺ and EAl³⁺ values decreased with increasing depth, and SOM content in the 0–20-cm layer was evidently greater compared to that in the 20–40-cm and 40–60-cm layers. The clay content increased with increasing depth.

3.2. Species and Content of Clay Minerals

The XRD diffraction patterns of soil clay minerals in the same group were similar, so a representative pattern was selected for analysis in each soil layer and each group (Figure 2, Figure 3 and Figure 4). Ten significant diffraction peaks of the Mg-gly treated oriented glass slides were investigated using diffraction patterns, with d values of 14.2, 10.0, 7.1, 5.0, 4.70, 4.45, 4.24, 3.56, 3.52, and 3.33 Å.

The reflection at 14.2 Å was collapsed to 10.0 Å after K-saturation indicating the presence of vermiculite. The reflections at 10.0, 5.0, 4.45, and 3.33 Å were also observed on the Mg-gly X-ray patterns and remain unchanged after heated K-saturated samples (25, 300 and 550 °C), indicating the presence of illit [26]. As illite is a non-expansive mineral with a strong bonding effect on potassium ions and between crystal layers, it is difficult for other ions to enter the crystal layers. Therefore, illite has excellent stability and the diffraction peak does not disappear after potassium tablets are heated [32]. The diffraction peak at 7.1 Å disappeared after heating at 550 °C and other treatments had no effect, indicating that the peak was kaolinite. The diffraction peak at 4.70 Å was the characteristic spectrum line of chlorite; those at 3.56 and 3.52 Å were bimodal, which were kaolinite and chlorite, respectively [31]. The diffraction peaks at 4.24 and 3.20 Å were not affected by any treatment, indicating the presence of quartz and feldspar [26]. The sample diffraction patterns for sampling depths of 0–20 and 20–40-cm for Group III and 40–60-cm for Group II had peaks at 18.0 Å in the Mg-gly tablet, indicating the presence of montmorillonite. The Mg-gly diffraction patterns of group I samples in 0–20-cm and 20–40-cm layers had 4.34 Å diffraction peaks (Figure 5). They disappeared after heating at 300 °C, indicating that the peaks were gibbsite [33]. Its content was relatively small, 5.92%, 4.96% and 0.00%, respectively. Based on the above comparison and analysis, the clay mineral species were listed in Table 3.

The semi-quantitative analysis results of the main clay minerals are shown in Figure 6; illite, kaolinite, vermiculite, and chlorite were found in the test soils with different acidification degrees, and the contents of these soils varied with these degrees. Montmorillonite was present only in the soil of Group III in the 0–20-cm and 20–40-cm layers and the soil of Group II in the 40–60-cm layer. The illite content in the tested soil at different sampling depths was the highest in each layer, up to approximately 70%, decreasing with increasing pH values. This relationship was enhanced with soil depth (Figure 6a). The kaolinite content increased with increasing pH values, however, the relationship between these properties was not very strong in the three soil layers (Figure 6b). The vermiculite content aslo increased with increasing pH values, this relationship was more significant in 0–20-cm layer (Figure 6c). The relationship of chlorite to pH was not evident (Figure 6d).

3.3. Surface Properties

The total negative charge of test soil CEC₈.₂, permanent negative charge CEC_P, and variable negative charge CEC_V increased with increasing pH values (Figure 7). The relationship between CEC₈.₂ and pH was significant in all three soil layers (Figure 7a). The relationship between CEC_P and pH was much stronger in the 20–40-cm and 40–60-cm layers than that in 0–20-cm layer (Figure 7b). In 0–20-cm and 20–40-cm layers, the CEC_V values of Groups II and III were significantly higher than those of Group I (Figure 7c). The AEC values decreased with increasing pH values in all three soil layers (Figure 7d). The SSA of the soils increased with increasing pH values (Figure 7e). The relationship between CEC₈.₂ and pH was significant in all three soil layers (Figure 7e). The SSA in the 0–20-cm, 20–40-cm, and 40–60-cm layers increased from 54.96 to 82.85, 47.97 to 84.76, and 54.20 to 102.03 m²∙kg⁻¹, respectively. The relationship of CEC_V/CEC₈.₂ to pH was not evident (Figure 7f).

3.4. Correlation of Basic Soil Properties, Clay Minerals, and Surface Properties

The effect of soil basic properties on surface properties and clay minerals was analyzed by redundancy analysis (RDA) (Figure 8). The first and second axes explained 61.18% and 1.05% of the total variations, respectively. The values of SSA, CEC₈.₂, CEC_P and illite content had a strong positive correlation with pH values, while the values of AEC, kaolinite and vermiculite content had a significant negative correlation with pH values. The correlation between EH, EAl and these indexes was just opposite. SOM was positively correlated with CEC_V and CEC_V/CEC₈.₂. Soil pH was the main factor affecting surface properties (Table 4). According to importance ranking of basic soil properties on surface properties and clay minerals, the order was pH, clay content, EAl³⁺, EH⁺, and SOM.

The effect of soil clay minerals on surface properties was aslo analyzed by RDA (Figure 9). The first and second axes explained 34.70 and 22.80% of the total variations, respectively. The content of illite in the soil had a significant positive correlation with SSA, CEC₈.₂, and CEC_P, while kaolinite content had a significant negative correlation with SSA, CEC₈.₂, and CEC_P. The content of illite was the main factor affecting the surface properties (Table 5). According to the interpretation rates of clay mineral species to surface properties, the order was illite, chlorite, vermiculite, and kaolinite. Additionally, illite, chlorite, and vermiculite belonged to the 2:1 type clay minerals; this means that content changes of 2:1 type clay minerals had considerable influence on the soil surface properties.

The correlation heatmaps of soil pH, surface properties, and clay minerals (Figure 10) revealed that illite content was positively correlated with pH, CEC₈.₂, CEC_P, and SSA but negatively correlated with AEC. The kaolinite content was negatively correlated with the pH values and significantly positively correlated with the AEC values. The vermiculite content had significant negative correlation with pH and SSA values, while chlorite was not significantly correlated with soil properties. The content of clay minerals of various species was not significantly associated with the variable charge CEC_V.

4. Discussion

Soil acidification refers to the increase in soil H⁺ and Al³⁺, which is a natural soil-forming process [34]. Based on the different forms of H⁺, the acidity of soil can be divided into active acid and exchangeable acid [35]. The intensity of active acid is expressed by pH value, and its change can directly reflect the acidity of soil. Exchangeable acids are acidity caused by EH⁺ and EAl³⁺ adsorbed by soil colloids. They are two acids in the same equilibrium system, which can be converted to each other. In this study, EH⁺ and EAl³⁺ decreased with increasing pH values, similar results were obtained in earlier studies [36,37,38]. Furthermore, the EAl³⁺ value in the surface layer (0–20-cm) was close to zero in the red soil at the pH of 5.5 [39]. In this study, for brown soil with pH values between 5.5 and 6.5, the EAl³⁺ values were 0.54 cmol∙kg⁻¹ (0–20-cm layer), 0.27 cmol∙kg⁻¹ (20–40-cm layer), and 0.15 cmol∙kg⁻¹ (40–60-cm layer). This indicated that for the brown soil in this study, the EAl³⁺ still existed when the pH was greater than 5.5. This difference may be caused by different soil types and is related to different causes of acidification. Therefore, Al-induced toxicity to plants should not be ignored in the weakly acidified soil due to the presence of EAl³⁺.

The species and content of clay mineral varied with different acidification degrees. In this study, the clay mineral composition and content varied at different acidification degrees. A small amount of montmorillonite was found only in weakly acidic soils (pH = 5.6–6.5) and gibbsite was found in strongly acidic soils (pH < 4.5). Moreover, although illite, kaolinite, vermiculite, and chlorite were found in soils with different acidification degrees, their contents were different, which attributed to excessive H⁺ and Al³⁺ in soil solutions would inevitably affect the structure of the soil clay minerals [35]. Gibbsite is primarily the product of the decomposition and hydrolysis of aluminosilicate minerals, that is, the secondary mineral produced by the weathering of aluminosilicate minerals such as feldspar and clay kaolinite forms of Al₂O₃ or gibbsite [40]. Generally, tropical and subtropical climates have sufficient water and thermal conditions to favor the formation of gibbsite. For example, in the latosolic clay particles under tropical climates and yellow-red soil under subtropical humid climates in China, there have the phenomenon of Fe and Al enriched as gibbsite [32,41]. Brown soil contains large amounts of illite, as well as gibbsite, which is rare based on the order of soil clay formation. However, the climate of the Jiaodong Peninsula is warm, temperate, and humid, with an average annual relative humidity above 70%. When the soil acidity is strong (pH < 4.5), the clay illite desilication forms Al₂O₃, namely gibbsite. Al₂O₃ is generally stable but only exists as complex ions in strongly acidic or basic environments. Therefore, no gibbsite was found in the brown soil with a pH greater than 4.5.

The degree of acidification also affected the conversion of soil clay minerals. The correlation analysis of the relative contents of clay minerals in different acidified soils (Figure 10) revealed that illite was negatively correlated with kaolinite and vermiculite, indicating inter-conversion among illite, kaolinite, and vermiculite in the soil under the acidification conditions. This is consistent with the observations of previous studies on the weathering of clay minerals in the soils of southern China [9]. As the weathering products of primary minerals in specific environments, clay minerals evolved from primary minerals into 2:1 type minerals, then into 1:1 type minerals, and subsequently into Fe and Al oxides. This process typically follows the direction illite → vermiculite → montmorillonite → kaolinite → gibbsite [42]. However, the evolution sequence of clay minerals in different zonal soils is different. In this study, montmorillonite occurred only at pH values greater than 5.5 in the 0–20-cm and 40–60-cm soil layers. Hence, at pH > 5.5, a small amount of illite was converted into montmorillonite and at pH < 5.5, illite was converted into kaolinite or vermiculite. Kaolinite is formed in medium acidic environments and has a high weathering degree. In the acidic environment, illite content decreased because K⁺ between the illite layers was replaced by other cations, and the kaolinite content increased. When the pH of the soil reached a strongly acidic level (pH < 4.5), clay minerals were completely hydrolyzed to gibbsite. This was consistent with the results of Lindsay [43] that Al in clay mineral kaolinite was activated under strong acidic conditions and then formed gibbsite with H₂O. This phenomenon exists even in brown soil.

Clay minerals are the main substrate for soil formation and their mineral types and composition determine other soil properties. The results of RDA (Figure 8 and Figure 9) indicate that the acidification degree (pH) and clay content were the main factors affecting surface properties, especially for CEC₈.₂, CEC_P, AEC and SSA as shown in Figure 10. Moreover, the effect of acidification degree on soil surface charge was different in different profiles. The soil surface negative charge CEC₈.₂ was divided into permanent negative charge CEC_P and variable negative charge CEC_V. In the 0–20-cm layer, the CEC₈.₂ values increased significantly with increasing soil pH, which was responsible for the increased variable charge CEC_V. This has also been proven by Khawmee et al. and Li et al. [12,13]. The pH values had a significant positive correlation with SOM, which could be inferred that SOM contributed significantly to the negative charge on soil surface. This is consistent with the results of Wisawapipat’s study on soil in Thailand [44]. In 20–40-cm and 40–60-cm layers with less SOM, we found that the pH value had a significant positive correlation with CEC_P, while the correlation with CEC_V decreased, indicating that the degree of acidification affected the permanent negative charge. The amount of these negative charges is also an important factor affecting the adsorption of heavy metal ions on the mineral surface. With the increase of the negative charges, the adsorption capacity of heavy metal cations is enhanced. Clay minerals control the migration of metal ions in the soil system through their own charge surface, thus reducing the effectiveness of heavy metals in the soil, which will help to improve the quality of the crops. In addition, the increase of pH will be beneficial to the increase of negative charge and specific surface area, which will increase the absorption of nutrient base ions such as Ca²⁺, Mg²⁺, K⁺, available N, and available P, to improve the fertility of soil [45].

Many studies have indicated that the permanent negative charge does not change with the pH value and that the permanent negative charge of soil primarily originates from the isomorphism substitution; soils containing 2:1 clay minerals have more isomorphic displacement and higher permanent negative charge [13]. The RDA results (Figure 9) and heatmaps (Figure 10) revealed that the contents of illite was the main factor that affected the surface charges CEC₈.₂ and CEC_P. Illite belongs to 2:1 clay mineral, which occupies the highest proportion in brown soil. Therefore, the permanent negative charge was not directly affected by pH, but the clay minerals changed from 2:1 to 1:1 under long-term acidification conditions, which led to a decrease in the permanent negative charge. Just as a previous study found that the long-term acidification in tea gardens results in a decrease in the content of 2:1 clay minerals and an increase in the content of 1:1 clay mineral kaolinite, which primarily contributes to the significant decrease in CEC of the soil [9].

5. Conclusions

In this study, the acidification had effect on clay mineral species, content, and surface properties of brown soil. The content of illite in clay particles decreased with the increase of acidification degree, and it was weathered into vermiculite or kaolinite. Kaolinite increased with the increase of acidification degree, and a small amount of gibbsite was formed by complete hydrolysis under the condition of strong acid. Furthermore, with the increase of acidification degree, the CEC₈.₂, CEC_P, SSA values of brown soil were lower, but AEC values were higher. The variation of CEC_P values was caused by the decrease in type 2:1 clay minerals illite with increasing acidification degree. which occupies the highest proportion in brown soil. Our study provides theoretical data on the effect of acidification on clay minerals and surface properties of brown soil, and also give guideline for farmers to take measures, such as the application of organic fertilizers or binding materials to enhance mineral formation, to increase the amount of negative charge that alleviated the soil acidification in the agricultural production.

Author Contributions

N.Y. conducted the experiment, statistical analyses, and wrote the paper; N.G. conducted the experiment and statistical analyses; T.W. conducted the statistical analyses; H.W. provided software; H.P. conducted formal analysis; Q.Y. conducted formal analysis; Y.L. contributed to the study design and reviewed the paper; Y.Z. contributed to the study design and reviewed the paper. All authors have read and agreed to the published version of the manuscript.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

No applicable.

Data Availability Statement

All data generated or analyzed during this study are included in this published article.

Conflicts of Interest

The authors declare no competing interests.

Footnotes

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Figures and Tables

Figure 1. Sampling site location of Brown soil in Muping County.

Figure 2. Plots exhibiting X-ray diffraction patterns of soil pH range at 0–20-cm soil layer. The pattern of one sample was selected for each group. Mg-gly represents saturated magnesium ions and saturated glycerol. K-25, K-300, and K-550 °C represent saturated potassium ions at 25, 300 and 550 °C after drying, respectively.

Figure 3. Plots exhibiting X-ray diffraction patterns of soil pH range at 20–40-cm soil layer. The pattern of one sample was selected for each group. Mg-gly represents saturated magnesium ions and saturated glycerol. K-25, K-300, and K-550 °C represent saturated potassium ions at 25, 300 and 550 °C after drying, respectively.

Figure 4. Plots exhibiting X-ray diffraction patterns of soil pH range at 40–60-cm soil layer. The pattern of one sample was selected for each group. Mg-gly represents saturated magnesium ions and saturated glycerol. K-25, K-300, and K-550 °C represent saturated potassium ions at 25, 300 and 550 °C after drying, respective.

Figure 5. Characteristic peak of gibbsite in Mg-gly diffraction pattern for samples of Group I. The colored lines are software auto-fitting lines for the peaks.

Figure 6. Graphs showing relationships pH between (a) illite; (b) kaolinite; (c) vermiculite; (d) chlorite; ** p < 0.01.

Figure 7. Graphs showing relationships pH between (a) the negative charge (CEC8.2); (b) the permanent negative charge (CECP); (c) the variable negative charge (CECV); (d) anion exchange capacity (AEC); (e) Ratio of a variable negative charge to the negative charge (CECv/CEC8.2); (f) specific surface area (SSA); * p < 0.05, ** p < 0.01 and *** p < 0.001.

Figure 8. Redundancy analysis (RDA) ordering charts of soil physio-chemical properties, surface properties and clay minerals. 1, 2, and 3 represent Group I, Group II and Group III. The red arrows represent the impact factors.

Figure 9. Redundancy analysis (RDA) ordering charts of soil clay minerals and surface properties. I, Ch, V, and K represent illite, chlorite, vermiculite, and kaolinite, respectively. 1, 2, and 3 represent Group I, Group II and Group III. The red arrows represent the impact factors.

Figure 10. Heatmap of soil basic properties, surface properties, and clay minerals. Correlations were assessed by Pearson’s correlation coefficient analysis: p < 0.05 (*) and p < 0.01 (**).

Table 1

X-ray diffraction characteristics of d₀₀₁ (Å) of the major clay minerals.

Treatment	Montmorillonite	Vermiculite	Illite	Kaolinite	Chlorite	Gibbsite
Mg-gly	18	14.2	10	7.1	14.2	4.34
K-25 °C	10–12	10	10	7.1	14.2	4.34
K-300 °C	10–12	10	10	7.1	14.2	disappear
K-550 °C	10	10	10	disappear	13.8	disappear

Mg-gly represents saturated magnesium ions and saturated glycerol. K-25, K-300, and K-550 °C represent saturated potassium ions at 25, 300 and 550 °C after drying, respectively.

Table 2

Basic properties of brown soil under different acidification degrees.

Layer(cm)	Group	pH Value	EH+(cmol∙kg−1)	EAl3+(cmol∙kg−1)	SOM(g∙kg−1)	Clay(%)
0–20	Group I	3.98 ± 0.20 Ba	0.55 ± 0.03 Aa	1.86 ± 0.03 Aa	10.48 ± 1.25 Ab	4.61 ± 0.80 Bb
	Group II	4.78 ± 0.08 Bb	0.30 ± 0.04 Ab	0.74 ± 0.06 Aab	11.93 ± 0.76 Ab	7.51 ± 1.13 Aa
	Group III	5.76 ± 0.24 Bc	0.13 ± 0.03 Ac	0.54 ± 0.13 Aab	15.83 ± 0.85 Aa	8.07 ± 0.57 Aa
20–40	Group I	4.21 ± 0.15 Ba	0.37 ± 0.04 ABa	2.07 ± 0.18 Aa	4.25 ± 0.33 Bb	6.08 ± 0.90 ABb
	Group II	5.95 ± 0.22 Ab	0.28 ± 0.01 Aa	0.58 ± 0.03 Aab	4.97 ± 1.07 Bb	7.68 ± 0.82 Aab
	Group III	6.79 ± 0.02 Ac	0.12 ± 0.01 Ab	0.27 ± 0.05 Bb	7.66 ± 2.24 Ba	8.41 ± 0.66 Ba
40–60	Group I	5.34 ± 0.08 Aa	0.28 ± 0.04 Ba	2.29 ± 0.47 Aa	6.82 ± 0.69 Ba	7.52 ± 1.17 Aa
	Group II	6.23 ± 0.23 Ab	0.21 ± 0.02 Ba	0.30 ± 0.05 Bb	6.13 ± 0.42 Ba	8.29 ± 1.91 Ba
	Group III	7.08 ± 0.02 Ac	0.06 ± 0.01 Bb	0.15 ± 0.02 Bb	5.02 ± 0.22 Bab	8.79 ± 0.15 Ba

The results are the mean values (n = 3) with standard deviations. Lower case letters after values indicate the significance at p < 0.05 in the same soil layer for different groups; upper case letters indicate the significance at p < 0.05 for the same group among different soil layers. Group I: strongly acidic (pH < 4.5), Group II: acidic (4.5 < pH < 5.5), Group III: weakly acidic (5.5 < pH < 6.5).

Table 3

The major clay minerals of soil with different acidification degrees at three sampling depths.

Layer (cm)	Group	Montmorillonite	Vermiculite	Illite	Kaolinite	Chlorite	Gibbsite
0–20	Group I	-	+	+	+	+	+
	Group II	-	+	+	+	+	-
	Group III	+	+	+	+	+	-
20–40	Group I	-	+	+	+	+	+
	Group II	-	+	+	+	+	-
	Group III	+	+	+	+	+	-
40–60	Group I	-	+	+	+	+	-
	Group II	+	+	+	+	+	-
	Group III	-	+	+	+	+	-

“+”: detected; “-”: not detected.

Table 4

Importance ranking and significance test results of interpretation of soil properties factors.

Name	Importance Ranking	Explanatory Quantity (%)	Pseudo-F	p
pH	1	50.2	25.2	0.002
Clay	2	5.8	3.2	0.088
EAl	3	3.8	2.2	0.128
EH	4	2.0	1.1	0.302
SOM	5	0.7	0.4	0.56

Table 5

Importance ranking and significance test results of interpretation of soil soil clay minerals and surface properties.

Name	Importance Ranking	Explanatory Quantity (%)	Pseudo-F	p
I	1	24.0	7.9	0.018
Ch	2	6.7	2.3	0.158
V	3	2.9	1.0	0.368
K	4	1.0	0.3	0.586

I, Ch, V, and K represent illite, chlorite, vermiculite, and kaolinite, respectively.

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