Variation Characteristics of Particle Surface Electrochemical Properties during the Improvement of Reclaimed Soil from Hollow Village in Loess Area

Abstract

Soil surface electrochemical properties, such as specific surface area and surface charge number, are important indexes to evaluate the agricultural soil quality change. However, there is not enough focus on the effect of different improved materials on the reclaimed soil surface electrochemical characteristics. Therefore, we selected maturing agent (TM), fly ash (TF), organic fertilizer (TO), maturing agent + organic fertilizer (TMO), fly ash + organic fertilizer (TFO), and no modified material (CK) treatment for 5 years of field location experiments to study the effects of different improved materials on the surface electrochemical properties of reclaimed soil from abandoned homestead. The results showed that, compared with CK treatment, the specific surface area, surface charge number, and surface charge density of reclaimed soil increased to 11.36–14.05 m² g⁻¹, 13.49–18.58 cmol kg⁻¹, and 1.14–1.76 C m⁻² after five years of application of different improved materials, respectively, and the number of surface charge under TFO, TMO, and TO treatment increased by 28.9%, 25.2%, and 37.7% compared with CK, respectively. Meanwhile, the specific surface area increased significantly (p < 0.05), showing an order of TFO > TMO > TO > TF > TM > CK. The surface electric field strength can reach the order of 10⁸ V m⁻¹. The statistical analysis results suggest that the contents of soil organic matter (SOM), silt, and clay were positively correlated with the soil surface electrochemical properties, which were the main factors for the changes of reclaimed soil surface electrochemical properties. Our research conclusion shows that in the process of reclamation of abandoned homestead in Loess Plateau, the application of different materials is helpful to improve the soil surface electrochemical properties, among which the organic–inorganic TFO treatment was a suitable improved material treatment for improving the surface electrochemical properties and fertility of reclaimed soil.

1. Introduction

Due to the acceleration of urbanization and industrialization in Loess Plateau, coupled with the widespread problems of “building new houses without demolishing old ones” which constantly occupies high-quality cultivated land and expands to the peripheral area, a hollow village has been formed, which has resulted in the destruction and occupation of a large number of cultivated land resources and the idle waste of high-quality land resources, seriously threatening the protection and promotion of cultivated land quality in Loess Plateau and becoming the main bottleneck affecting the regional food security and rural revitalization strategy [1,2,3]. Therefore, it is of great significance to carry out soil reclamation and quality improvement of abandoned homestead in hollow village for increasing the cultivated land resources in Loess Plateau, ensuring regional food security, and promoting rural revitalization [4,5]. However, the reclaimed soil of abandoned homestead comes from the old wall soil (raw soil) after the old houses are demolished, which loses the properties and functions of full-year cultivated soil. The low soil quality seriously limits the land productivity and utilization of reclaimed soil, and it is urgent to improve the reclaimed soil structure and fertility, and thus increase the productivity of reclaimed soil [6].

Previous studies have shown that the application of different soil improvement materials is an important way to improve the reclaimed soil quality in the abandoned homestead of hollow village in Loess Plateau, which can increase the soil nutrients and enzyme activities, improve the soil structure compactness, and enhance the water retention performance [7,8]. Organic fertilizer is rich in organic matter, nitrogen, phosphorus, potassium, and other nutrient elements, which can improve soil structure and pore condition, ripen soil, and boost soil fertilizer and productivity [9]. Fly ash is a solid waste residue discharged by coal-fired enterprises in the production process. With large specific surface area and strong adsorption capacity, it can promote the agglomeration and cementation of soil particles and enhance the ability of soil to retain water and fertilizer as a good soil improver and fertility filler [10]. Gao et al. showed that using ferrous sulfate as a soil amendment can improve soil physical and chemical properties, adjust soil pH, increase soil enzyme activity, loosen soil structure, and promote nutrient absorption of crops [11,12]. At present, there are many studies on the effects of different improved materials on soil properties and functions, mainly focusing on the changes of nutrient elements, aggregate stability, and water infiltration, but there are few studies on how to affect the electrochemical properties of reclaimed soil surface, with a lack of systematic understanding of the effects of soil surface electrochemical properties on soil water and fertilizer retention capacity and soil structure stability.

The surface electrochemical properties of soil particles include surface charge number, specific surface area, surface charge density, surface electric field strength, surface potential, and other parameters. The changes of these surface charge properties will cause changes in the interaction between soil particles, and have an important impact on many physical, chemical, and biological processes occurring on and around the soil particle surface [13,14]. Meanwhile, the electrochemical properties of the soil particle surface have a profound impact on the macro-processes of soil water and fertilizer retention, agglomerate breakage and stability, soil water movement, soil erosion, and agricultural non-point source pollutant migration [15,16,17]. The application of organic fertilizer, fly ash, and other improved materials is an important way to improve the quality of reclaimed soil in hollow village of Loess Plateau. The addition of these exogenous improved materials will inevitably have different effects on the electrochemical properties of reclaimed soil particles, which will affect the improvement effect of reclaimed soil fertility characteristics and structural stability [7,18]. Consequently, it is of great theoretical value and practical significance to study the effects of different improved materials on the electrochemical properties of reclaimed soil surface, and to accurately understand the variation characteristics of electrochemical properties of reclaimed soil surface, so as to thoroughly reveal the improvement mechanism of different materials on the fertility and structure of reclaimed soil in abandoned homestead. Therefore, this paper takes the reclaimed soil in hollow village of Loess Plateau as the research object, adopts the conjoint analysis of material surface properties to determine the surface electrochemical properties of soil particles treated with different improved materials, compares and studies the variation pattern of reclaimed soil surface electrochemical properties under the treatment of different improved materials, explores the correlation between the basic physical and chemical properties of reclaimed soil and the parameters of surface chemical properties after the application of improved materials, and discriminates the main factors for the change of electrochemical properties of soil surface during the improvement process so as to provide scientific basis for the improvement of soil structure and fertility in the hollow village.

2. Materials and Methods

2.1. Study Area

The long-term positioning test plot for reclaimed soil improvement in hollow village was set up in Fuping County Pilot Base (34°42′ N, 109°12′ E) in Weinan City, Shaanxi Province, and was completed on 15 June 2015. It is designed mainly for experimental research and engineering demonstration of key technologies for hollow village comprehensive improvement. The study area is located on the north side of Weibei Loess Plateau, with a climate type of warm temperate semi-humid continental monsoon climate zone, an average annual evaporation of 1154.2 mm, an average annual temperature of 13.3 °C and an average annual rainfall of 513.5 mm. The rainfall from June to September accounts for more than 60% of the annual rainfall. In the growing season of maize in 2020, the rainfall from June to September was 189.0, 133.4, 211.0, and 41.2 mm, respectively, and the monthly average temperature was 24.6, 25.4, 25.1, and 21.2 °C, respectively (Figure 1).

The reclaimed soil is from the backfilling of the old wall soil (raw soil) of the abandoned homestead land remediation project in Hollow Village. The backfill depth is 30 cm. After removing the gravel and other impurities, the reclaimed soil is cured and structurally improved by adding different improved materials to meet the requirement for the growth of food crops. The reclaimed soil is mainly developed from loess parent material. Before the experiment, the pH value of the surface soil was 8.5, and the soil texture was silty loam (USDA texture classification), with the organic matter content of 4.5 g kg⁻¹, the total nitrogen content of 0.16 g kg⁻¹, the available phosphorus content of 3.1 mg kg⁻¹, the rapidly available potassium content of 61.4 mg kg⁻¹, and the soil bulk density of 1.40 g cm⁻³. The soil quality is generally poor.

2.2. Experiment Design

The hollow village soil improvement experiment started in June 2015. Based on the survey and analysis of the characteristics of raw soil improvement materials and the review of published literature, this paper selected fly ash, organic fertilizer (decomposed chicken manure), and ferrous sulfate (FeSO₄) as the improvement materials of reclaimed soil to address the fertility and structural problems of reclaimed raw soil. The contents of environmental pollution indicators, As, Hg, Cu, Pb, Zn, and Cd, in fly ash were 13.59, 0.10, 91.6, 22.72, 57.81, and 0.06 mg kg⁻¹, respectively, which was in line with the soil environmental quality assessment standard (GB15618—2008, China) [19]. The experiment adopted a randomized block field experiment design with six treatments, maturing agent (TM), fly ash (TF), organic fertilizer (TO), maturing agent + organic fertilizer (TMO), fly ash + organic fertilizer (TFO), and no modified material (CK) treatment without the addition of improved materials. Each treatment has three replicates, totaling 18 experimental plots, with an 80 cm wide isolation belt between each group of treatments. The crop planting system is a two-year triple cropping system of winter wheat–summer maize rotation. The tested summer maize was sown in the first twenty days of June, with a sowing density of 6.5 × 10⁴ plants per hectare, and harvested in the first ten days of October. The planting variety is Xianyu 958. Before sowing, compound fertilizer of 1500 kg ha⁻¹ was applied to all maize treatments, in which the contents of nitrogen, phosphorus, and potassium were 15%, 10%, and 20%, respectively. Then, the improved materials with different treatments were uniformly mixed into the reclaimed raw soil, and the soil improved materials were applied to each treatment in one-time application. The irrigation amount, fertilizer treatment, and other daily management indicators of the six treatments were consistent. See

Table 1. Experimental treatments of reclamation soil improvement in hollow village.

Number	Treatment	Improved Materials	Application Amount
1	CK	Control (no improved material)	0
2	TM	Maturing agent (ferrous sulfate)	0.6 t ha−1
3	TF	Fly ash	45 t ha−1
4	TO	Organic fertilizer (chicken manure)	30 t ha−1
5	TMO	Maturing agent + organic fertilizer	(0.6 + 30) t ha−1
6	TFO	Fly ash + organic fertilizer	(45 + 30) t ha−1

for specific experimental treatments and the application amount of improved materials.

2.3. Soil Determination Indexes and Methods

After the summer maize harvest of this experiment in early October, 2020, 0–20 cm reclaimed soil surface samples were collected according to the experimental treatments, and three repeated soil samples were randomly taken from each treatment to analyze the effects of different improved materials treatments on the physical and chemical properties and surface electrochemical properties of reclaimed soil. After the soil sample is naturally air-dried indoors, impurities such as plant residues and gravel are removed, and the soil sample is ground through 0.25 mm, 1 mm, and 2 mm sieves. The basic physical and chemical properties of soil were determined by conventional classical analysis methods, including soil organic matter content by the K₂Cr₂O₇ heat capacity method [20], total nitrogen by the Kjeldahl method [21], available phosphorus by the sodium bicarbonate extraction–molybdenum antimony colorimetric method [22], rapidly available potassium by the ammonium acetate extraction–flame photometer method [23], and soil pH by the electrode method (water–soil mass ratio of 1:2.5). The bulk density and water content of soil are determined by the cutting ring method and drying method [24,25], and the soil particle sizes by MS3000 laser particle size analyzer. According to the American standard, the soil particles are divided into three grades: sand (2–0.05 mm), silt (0.05–0.002 mm), and clay (<0.002 mm).

2.4. Determination and Calculation of Soil Surface Electrochemical Properties

The soil surface electrochemical properties mainly include surface charge number, specific surface area, surface charge density, surface potential, and surface electric field strength. The soil surface electrochemical properties were determined by the conjoint determination method of material surface properties established by Li et al. [26]. Firstly, 200 g soil samples of 0.25 mm particles with different treatments were weighed and put into a 1000 mL beaker, and 600 mL of 0.5 mol L⁻¹ hydrochloric acid solution was slowly added. A glass rod was used to fully stir them until no obvious bubbles were generated when they were made standing. The supernatant was sucked with a straw, then the hydrochloric acid solution was added again. This process was repeated for many times until no carbon dioxide bubbles were produced in the soil samples, thus removing the calcium carbonate from the soil. Secondly, the soil without calcium carbonate was centrifuged to discard the supernatant, and 600 mL of 0.1 mol L⁻¹ HCl solution was added to shake for 5 h before it was centrifuged to discard the supernatant. The hydrochloric acid solution with the same volume and concentration was continuously added, and the above operation was repeated for 3 times. After the last centrifugation, 600 mL of deionized water was added to oscillate until there was no Cl⁻ in the centrifuged solution, and then the soil sample was dried at 65 °C to obtain the hydrogen saturated soil sample. Finally, 5–10 g hydrogen saturated sample was weighed and put into a 100 mL centrifuge tube, and 40 mL of 0.0075 mol L⁻¹ NaOH and Ca(OH)₂ mixed solution was added to oscillate on a shaking table for 24 h (240 r min⁻¹, temperature 25 °C). Then, the pH of the mixed solution was adjusted to 6–8 with 1 mol L⁻¹ HCl. After shaking for 12 h, the pH of the mixed solution was determined until reaching 7.0. The supernatant was obtained by centrifugation, and the concentrations of Ca²⁺ and Na⁺ in the mixed solution were determined by atomic absorption spectrometer. The measurement of each treated soil sample was repeated three times. The measured data were brought into Formula (1)–(6) [26,27,28] to calculate the surface electrochemical properties, such as surface potential (φ₀, V m⁻¹), surface charge number (σ₀, C m⁻²), surface electric field strength (E₀, V m⁻¹), specific surface area (SSA, m² g⁻¹), and surface charge density (SCN, cmol kg⁻¹).

$${\phi }_0=\frac{2RT}{(2{\beta }_{\mathrm{C}\mathrm{a}}-{\beta }_{\mathrm{N}\mathrm{a}})F}\ln \frac{a_{\mathrm{C}\mathrm{a}}^0{N}_{\mathrm{C}\mathrm{a}}}{a_{\mathrm{C}\mathrm{a}}^0{N}_{\mathrm{C}\mathrm{a}}}$$

(1)

$${\sigma }_0=\mathrm{sgn}({\phi }_0)\sqrt{\frac{\epsilon RT}{2\pi }\left[a_{\mathrm{Na}}^0\left(e^{\frac{{\beta }_{\mathrm{Na}}{F}_{{\phi }_0}}{RT}}-1\right)+a_{\mathrm{Ca}}^0\left(e^{\frac{2{\beta }_{\mathrm{Ca}}{F}_{{\phi }_0}}{RT}}-1\right)+\left(a_{\mathrm{Na}}^0+2a_{\mathrm{Ca}}^0\right)\left(e^{\frac{F_{{\phi }_0}}{RT}}-1\right)\right]}$$

(2)

$$E_0=\frac{4\pi }{\epsilon }{\sigma }_0$$

(3)

$$SSA=\frac{N_{\mathrm{N}\mathrm{a}}k}{m{a}_{\mathrm{N}\mathrm{a}}^0}{e}^{\frac{{\beta }_{\mathrm{N}\mathrm{a}}F{\phi }_0}{2RT}}=\frac{N_{\mathrm{C}\mathrm{a}}k}{m{a}_{\mathrm{C}\mathrm{a}}^0}{e}^{\frac{2{\beta }_{\mathrm{C}\mathrm{a}}F{\phi }_0}{RT}}\times {10}^{-2}$$

(4)

$$SSA={10}^5\frac{s{\sigma }_0}{F}$$

(5)

$$m=0.5259\ln \left(\frac{c_{\mathrm{N}\mathrm{a}}^0+c_{\mathrm{H}}^0}{c_{\mathrm{C}\mathrm{a}}^0}\right)+1.992$$

(6)

where φ₀ (mV) is the surface potential; σ₀ (C m⁻²) is the surface charge density; E₀ (V m⁻¹) is the surface electric field strength; SSA (m² g⁻¹) is the specific surface area; SCN (cmol kg⁻¹) is the surface charge number; R (J K⁻¹ mol⁻¹) is the universal gas constant; T (K) is the absolute temperature; F (C mol⁻¹) is the Faraday constant; Z is the charge of each ion species; β_Na and β_Ca are the corresponding modification factors of Z for Na⁺ and Ca²⁺, respectively; ε is the dielectric constant for water (8.9 × 10⁻⁹ C² J⁻¹ dm⁻¹); β_Na = 0.0213ln(I^0.5) + 0.7669, β_Ca = −0.0213ln(I^0.5) + 1.2331; κ (dm⁻¹) is the Debye–Hückel parameter; I (mol L⁻¹) is the ionic strength; κ = (4πF²∑Z_i²α_i⁰/εRT)^1/2; and $c_{\mathrm{Na}}^0$ (mol L⁻¹), and $c_{\mathrm{Ca}}^0$ (mol L⁻¹) are equilibrium Na⁺ and Ca²⁺ concentrations in the bulk solution, respectively.

2.5. Statistical Analysis

The experimental data were organized and statistically analyzed by the software of Microsoft Excel 2013 and SPSS22.0, the mapping was made by Origin 2019, and redundancy analysis (RDA) was carried out using the software of Canoco 5. The least significant range (LSD) method was used to analyze the correlation between the physicochemical properties and particle surface electrochemical properties of reclaimed soil, with p < 0.05 indicating significant level and p < 0.01 indicating extremely significant level.

3. Results and Discussion

3.1. Physical Properties of Reclaimed Soil Treated with Different Improved Materials

After the application of different improved materials, the reclaimed soil physical properties of abandoned homestead changed significantly. Soil bulk density is influenced by soil particle composition, pore size, compactness, and other factors. The application of different improved materials makes soil bulk density decrease, which shows the order of TFO < TMO < TO < TF < TM < CK, with the lowest soil bulk density under the organic–inorganic combined treatment of TFO (

Table 2. Physical properties of reclaimed soil under the application of different modified materials.

Treatment	Bulk Density (g cm−3)	MWD (mm)	Water Content (%)	Particle Size Distribution (%)
				Clay	Silt	Sand
				(<0.002 mm)	(0.002–0.05 mm)	(0.05–2 mm)
CK	1.38 ± 0.02 a	0.32 ± 0.03 d	13.18 ± 0.32 d	10.00 ± 0.11 d	81.96 ± 0.13 d	8.04 ± 0.23 a
TO	1.22 ± 0.01 cd	0.45 ± 0.02 c	16.00 ± 0.10 b	11.58 ± 0.10 b	83.31 ± 0.32 b	5.11 ± 0.22 c
TF	1.25 ± 0.02 c	0.38 ± 0.02 c	14.99 ± 0.23 c	10.87 ± 0.09 c	82.39 ± 0.17 c	6.74 ± 0.09 b
TM	1.26 ± 0.01 b	0.32 ± 0.02 d	14.96 ± 0.13 c	10.08 ± 0.02 d	83.45 ± 0.27 b	6.47 ± 0.29 b
TFO	1.19 ± 0.00 d	0.80 ± 0.06 a	18.22 ± 0.15 a	12.66 ± 0.01 b	84.03 ± 0.15 a	3.31 ± 0.15 e
TMO	1.21 ± 0.01 d	0.53 ± 0.05 b	17.44 ± 0.66 a	11.56 ± 0.03 a	83.91 ± 0.11 a	4.53 ± 0.07 d

CK: no improved material; TO: organic fertilizer; TF: fly ash; TM: maturing agent (ferrous sulfate); TFO: fly ash + organic fertilizer; TMO: maturing agent + organic fertilizer; MWD: mean weight diameter. Different lowercase letters represent significant differences among different improved material treatments in the same index (p < 0.05).

). The results show that the application of different improved materials makes reclaimed soil loose, and the soil structure and permeability are gradually improved. With the long-term application of improved materials, the soil water content increased significantly (p < 0.05), especially under TFO and TMO treatments, under which the soil water content increased by 38.24% and 32.32%, respectively, compared with CK treatment, with the highest soil water content under TFO treatment. After more than five years of application of improved materials, the mean weight diameter (MWD) of reclaimed soil under organic–inorganic coupling TFO and TMO treatment is 2.50 times and 1.66 times that of the CK, respectively, and the MWD under TFO treatment increases to 0.80 mm. Organic fertilizer, fly ash, and other soil improvement materials can effectively improve the content of soil organic matter after being applied alone or in combination. The cementing materials formed in the process of transformation and decomposition contribute to the cementation and agglomeration of soil aggregates, which have significant influence on the aggregate size, distribution, and structural stability, and enhance the mutual adsorption and agglomeration ability of soil particles. Meanwhile, the application of the improved materials somewhat reduces the damage of soil structure caused by machinery and other artificial farming practices, and finally increases the agglomeration and stability of aggregate structure, thus promoting MWD value to increase [29,30]. Soil particle composition affects soil water and fertilizer status and is an important soil physical property. Long-term application of different improved materials reduces the content of soil sand, while increasing the content of clay and silt to a lesser extent. The reason for the improvement is as follows. The application of improved materials such as organic fertilizer, maturing agent, and fly ash could improve soil physical and chemical properties, increase soil enzyme activity, promote the formation of organic and inorganic colloids, stabilize the soil maturation environment, reduce the erosion of raindrops on the soil, and advance the growth of crop roots and the improvement of soil microbial activity. This increases the return amount of plant residues and roots into the soil, and the cementation substances such as polysaccharides and humus produced by decomposition, which enhance the adhesion, agglomeration, and cementation of soil particles, and increase the contents of clay and silt particles [11,31].

3.2. Chemical Properties of Reclaimed Soil Treated with Different Improved Materials

The application of different improved materials had a significant impact on the reclaimed soil chemical properties (

Table 3. Chemical properties of reclaimed soil under the application of different modified materials.

Treatment	SOM (g kg−1)	TN (g kg−1)	C/N	OP (mg kg−1)	AK (mg kg−1)	pH
CK	5.94 ± 0.17 c	0.52 ± 0.03 d	6.62 ± 0.31 c	14.29 ± 0.79 c	104.36 ± 8.38 c	8.66 ± 0.03 a
TO	11.36 ± 0.37 b	0.61 ± 0.01 b	10.81 ± 0.50 b	18.82 ± 1.49 a	114.72 ± 2.69 b	8.53 ± 0.05 b
TF	10.97 ± 0.56 b	0.57 ± 0.01 c	11.24 ± 0.68 b	16.43 ± 0.32 b	116.76 ± 2.24 b	8.52 ± 0.05 b
TM	6.64 ± 0.05 c	0.55 ± 0.02 cd	6.97 ± 0.29 c	15.44 ± 0.31 bc	111.30 ± 0.85 bc	8.38 ± 0.04 c
TFO	13.89 ± 0.78 a	0.74 ± 0.02 a	10.96 ± 0.76 b	18.70 ± 0.35 a	130.50 ± 2.62 a	8.27 ± 0.11 cd
TMO	13.08 ± 1.06 a	0.63 ± 0.02 b	12.36 ± 0.87 a	18.49 ± 0.22 a	123.99 ± 2.82 a	8.18 ± 0.07 d

CK: no improved material; TO: organic fertilize; TF: fly ash; TM: maturing agent (ferrous sulfate); TFO: fly ash + organic fertilizer; TMO: maturing agent + organic fertilizer; SOM: soil organic matter; TN: total nitrogen; AP: available phosphorus; AK: available potassium. Different lowercase letters represent significant differences among different improved material treatments in the same index (p < 0.05).

). The reclaimed soil was weakly alkaline, and with the application of different improved materials, the pH value of the reclaimed soil showed a significant decreasing trend (Table 3) (p < 0.05). The C, N, P, and K elements in soil are important nutrients that affected the crops normal growth and development, and played a key role in crop growth [32,33]. As the reclaimed soil of abandoned homestead is mainly the raw soil of loess parent material with poor fertility level, the application of different improved materials would gradually increase the soil nutrient content, and the soil organic matter (SOM), total nitrogen (TN), available phosphorus (AP), and available potassium (AK) contents in reclaimed soil showed a significant increasing trend (p < 0.05). Among them, the SOM content of reclaimed soil under TO, TF, TM, TFO, and TMO treatments increased by 91.2%, 84.7%, 11.8%, 132.3%, and 120.2%, respectively, compared with CK treatment, and the TN content increased by 17.3%, 9.6%, 5.8%, 42.3%, and 21.2%, respectively, compared with CK treatment, with the largest increase under TFO treatment. The content of AP and AK showed a similar trend to that of organic matter, with the highest content under TFO treatment and the lowest content under CK. Among them, organic–inorganic combined treatment of TFO and TMO had a better effect on improving the reclaimed soil nutrients. The results are consistent with those of Wei et al., who pointed out that the organic–inorganic combined treatments were more conducive to soil fertility and structure improvement [34,35]. The C/N in different improved materials, which is used to represent the balance of soil carbon and nitrogen nutrients, has an average value of 10.47, which is significantly higher than that in the control treatment. The application of improved materials increased the reclaimed soil C/N value.

3.3. Ion Exchange Equilibrium Results and Surface Electrochemical Properties of Reclaimed Soil Treated with Different Improved Materials

According to the principle of conjoint determination of material surface parameters put forward by Li et al. [10], the calculation results of equilibrium activity ($a_{\mathrm{Ca}}^0$, $a_{\mathrm{Na}}^0$), equilibrium concentration ($c_{\mathrm{Ca}}^0$, $c_{\mathrm{Na}}^0$), and corresponding k, m, I, and diffusion layer ions (N_Ca, N_Na) of soil treated with six different improved materials are shown in

Table 4. Calculation results of ion exchange equilibrium of reclaimed soil treated with different modified materials.

Treatment	${\mathit{a}}_{\mathrm{Ca}}^{\mathbf{0}}$	${\mathit{a}}_{\mathbf{Na}}^{\mathbf{0}}$	${\mathit{c}}_{\mathbf{Ca}}^{\mathbf{0}}$	${\mathit{c}}_{\mathbf{Na}}^{\mathbf{0}}$	Κ Dm−1	m	I	NCa	NNa
	(mmol L−1)							(10−5 mol g−1)
CK	0.90 ± 0.11	6.29 ± 0.05	1.42 ± 0.19	7.04 ± 0.05	34,594,991	2.84 ± 0.07	0.011	5.33 ± 0.03	0.40 ± 0.03
TO	0.29 ± 0.04	6.03 ± 0.05	0.43 ± 0.05	6.63 ± 0.06	28,991,275	3.43 ± 0.06	0.008	6.18 ± 0.05	0.76 ± 0.05
TF	0.58 ± 0.01	6.32 ± 0.10	0.88 ± 0.02	7.02 ± 0.11	31,998,518	3.09 ± 0.01	0.010	5.79 ± 0.02	0.44 ± 0.06
TM	0.69 ± 0.02	6.32 ± 0.13	1.05 ± 0.03	7.03 ± 0.15	32,887,110	2.99 ± 0.02	0.010	5.64 ± 0.02	0.45 ± 0.09
TFO	0.38 ± 0.02	6.17 ± 0.05	0.57 ± 0.03	6.81 ± 0.06	30,071,527	3.30 ± 0.03	0.009	6.06 ± 0.03	0.60 ± 0.05
TMO	0.42 ± 0.02	6.22 ± 0.03	0.63 ± 0.03	6.88 ± 0.03	30,487,524	3.25 ± 0.02	0.009	6.01 ± 0.02	0.54 ± 0.03

CK: no improved material; TO: organic fertilize; TF: fly ash; TM: maturing agent (ferrous sulfate); TFO: fly ash + organic fertilizer; TMO: maturing agent + organic fertilizer; $a_{\mathrm{Ca}}^0$: Ca²⁺ equilibrium activity in solution; $a_{\mathrm{Na}}^0$: Na⁺ equilibrium activity in solution; N_Ca: the adsorption capacity of Ca²⁺; N_Na: the adsorption capacity of Na⁺.

. With the application of different improved materials, the adsorption capacity of Na⁺ and Ca²⁺ ions increased continuously, especially under TFO, TMO, and TO treatments, and the adsorption capacity of Ca²⁺ increased by 13.7%, 12.8%, and 15.9%, respectively, compared with the CK treatment (Table 4).

Surface potential (φ₀), surface charge density (σ₀), surface electric field strength (E₀), specific surface area (SSA), and surface charge number (SCN) are very important properties of soil colloid, which affect the physical, chemical, and biochemical processes in soil. Among them, the SCN is the key factor for crops to absorb nutrient elements, which determines the quantity of adsorbed ions in soil [36,37]. Compared with CK, different improved material treatments significantly increased the SCN (p < 0.05), and the SCN of reclaimed soil varied from 13.49 to 18.58 cmol kg⁻¹, among which those under TFO, TMO, and TO treatments increased by 28.9%, 25.2%, and 37.7%, respectively, compared with those under CK treatment (

Table 5. Soil surface electrochemical properties under the application of different modified materials.

Treatment	SCN (cmol kg−1)	SSA (m2 g−1)	σ0 (C m−2)	E0 (108 V m−1)	φ0 mV
CK	13.49 ± 0.12 f	10.22 ± 0.02 d	0.83 ± 0.03 d	11.67 ± 0.45 c	–122.35 ± 1.55 a
TO	18.58 ± 0.56 a	12.9 ± 1.94 bc	1.26 ± 0.06 b	17.80 ± 0.79 b	–142.04 ± 1.84 c
TF	15.36 ± 0.09 d	11.83 ± 0.28 bc	1.25 ± 0.03 b	17.66 ± 0.45 b	–139.79 ± 3.22 c
TM	14.54 ± 0.15 e	11.36 ± 0.31 cd	1.14 ± 0.07 c	16.35 ± 1.40 b	–134.23 ± 3.75 b
TFO	17.39 ± 0.26 b	14.05 ± 0.23 a	1.46 ± 0.05 a	20.94 ± 0.89 a	–141.21 ± 2.95 c
TMO	16.83 ± 0.20 c	13.25 ± 0.46 ab	1.44 ± 0.03 a	20.42 ± 0.42 a	–141.50 ± 2.83 c

CK: no improved material; TO: organic fertilize; TF: fly ash; TM: maturing agent (ferrous sulfate); TFO: fly ash + organic fertilizer; TMO: maturing agent + organic fertilizer; SCN: surface charge number; SSA: specific surface area; σ₀: surface charge density; E₀: surface electric field strength; φ₀: Surface potential. Different lowercase letters represent significant differences among different improved material treatments in the same index (p < 0.05).

). From the adsorption amount of Na⁺ and Ca²⁺ ions in Table 4, the adsorption amount of ions increased with the application of different improved materials. The huge specific surface area of soil colloid is an important place for soil adsorption reaction and ion exchange, which is closely related to soil’s ability to maintain and supply nutrients and water necessary for crops [38,39]. After the application of different improved materials, the SSA of reclaimed soil increased significantly (Table 5), showing an order of TFO > TMO > TO > TF > TM > CK, with the specific surface area range increasing to 11.36–14.05 m² g⁻¹, and the largest SSA under the organic–inorganic TFO treatment. The reason may be that the application of different improved materials increased the organic matter and silt and clay particle contents in the soil, and the increase in the content of organic and inorganic colloids in the reclaimed soil further increased the specific surface area and SCN of the reclaimed soil. These results are similar to those of Yu et al., who found through indoor culture experiments that the number of inceptisols surface charge and specific surface area after straw application increased by 18.75% and 14.64%, respectively, compared with CK treatment [15].

The surface charge density of soil particles refers to the number of charges per unit area of soil particles, which affects the ion adsorption strength. The greater the charge density, the greater the ion adsorption capacity [40]. With the application of organic fertilizer, fly ash, and other improved materials, the σ₀ of reclaimed soil increased significantly (p < 0.05), ranging from 1.14 to 1.76 C m⁻², among which TO, TF, TM, TFO, and TMO treatments increased by 51.8%, 50.6%, 37.3%, 75.9%, and 73.4%, respectively, compared with CK treatment, with the highest σ₀ under TFO treatment. This shows that with the application of organic fertilizer, fly ash, and other improved materials, the amount of charges per unit area and the firmness of soil to retain nutrient ions increased. In addition, soil colloids with low surface charge density more easily form aggregates than those with high surface charge density, which implies that the surface charge properties of soil colloids play an important role in the formation process and structural stability of soil aggregates. After different improved materials are applied, the cementation and agglomeration ability of reclaimed soil will be changed to a certain extent with the change of surface charge density. As the effect of soil electric field measured by traditional methods is very small, the more accurate methods found that the surface electric field strength of reclaimed soil was as high as 10⁸ orders of magnitude (Table 5). Under the treatment of returning different improved materials to the field, the E₀ of reclaimed soil showed a significant increasing trend, with the order of TFO > TMO > TO > TF > TM > CK, and the highest under the organic–inorganic TFO treatment. Such a strong surface electric field will inevitably affect the soil interface reaction, as well as the micro-process and macro-phenomenon in the soil [41,42].

3.4. Correlation Analysis between Surface Electrochemical Properties and Basic Physicochemical Properties of Reclaimed Soil

In order to further find out the correlation among the parameters of reclaimed soil treated with different improved materials, the correlation analysis of the relationship between soil physicochemical properties and surface electrochemical properties was carried out in this paper (Figure 2). The contents of soil organic matter, clay particles, silt particles, C/N, and available phosphorus showed significantly positive correlation with the surface electrochemical properties (Figure 2) (p < 0.01). Soil pH value had no significant correlation with specific surface area and surface charge quantity but had significant negative correlation with surface charge density and surface electric field strength (p < 0.05). Soil bulk density and sand content showed significantly negative correlation with surface electrochemical properties (p < 0.05), while total nitrogen and rapidly available potassium contents showed significantly positive correlation with surface charge density and specific surface area but had no significant relationship with surface charge quantity and surface electric field strength.

The redundancy analysis (RDA) was made on the basic physical and chemical properties and the surface charge properties of soil treated with different organic improved materials (Figure 3). In Figure 3, the first axis and the second axis account for 88.59% and 7.72% of the total variation, respectively. The contents of organic matter and silt particles have strong correlation with the first sorting axis. There are differences in soil physical and chemical properties under different improved materials. The soil distribution under CK treatment is in the first quadrant, that under TFO and TO treatment is in the second quadrant, that under TMO and TF treatment is in the third quadrant, and that under TM treatment is in the fourth quadrant. Soil organic matter, silt, C/N and clay content are significantly positively correlated with surface charge quantity, specific surface area, surface electric field strength, and surface charge density, while pH value and sand content are negatively correlated with surface electrochemical properties. Soil organic matter and silt content (F = 72.4, p = 0.002; F = 8.7, p = 0.01) are the main factors that affect the reclaimed soil surface electrochemical properties, accounting for 69.4% and 8.3%, respectively (

Table 6. Redundancy analysis of the soil surface electrochemical properties and physicochemical properties.

Soil Physicochemical Properties	SOM (g kg−1)	Silt	C/N	Clay	AP (mg kg−1)	AK (mg kg−1)	TN (g kg−1)	pH	Sand
Interpretation rate (%)	69.4	8.3	5.8	4.7	3.8	1.8	1.1	0.9	0.1
Contribution (%)	72.4	8.7	6.0	4.9	4.0	1.8	1.2	0.9	0.1
F	36.3	5.6	11.4	3.7	3.6	1.8	1.2	1.8	0.2
p	0.002	0.01	0.004	0.018	0.068	0.18	0.328	0.176	0.824

SOM: soil organic matter; AP: available phosphorus; AK: available potassium; TN: total nitrogen.

). Soil organic matter, silt, C/N, clay, available phosphorus, rapidly available potassium, and total nitrogen jointly account for 99% of the variation in surface electrochemical properties of the compound soil. The basic physical and chemical properties of reclaimed soil affect the surface electrochemical properties of soil in the order of soil organic matter, silt, C/N, clay, available phosphorus, rapidly available potassium, total nitrogen, pH, and sand.

Correlation analysis (Table 6) and RDA analysis (Figure 3) show that organic matter content and soil texture are the main factors affecting the surface electrochemical properties of reclaimed soil from abandoned homestead. As an important part of colloidal soil particles, organic matter can promote the cementation and aggregation of particles, enhance the stability of soil structure, change the soil colloidal state, and improve soil particle adsorption capacity [43]. Soil organic matter is generally negatively charged, and every 10 g kg⁻¹ increase in organic matter content can increase the amount of negative charge by 1 cmol kg⁻¹. In organic matter composition, the specific surface area of humus is about 800–900 m² g⁻¹, which is nearly 10 times that of common inorganic clay minerals. With the increase of organic matter content, the soil charge amount and specific surface area also gradually increase [44,45]. Therefore, the application of organic fertilizer, fly ash, and other exogenous improved materials increases the organic matter content and structural stability of reclaimed soil in the hollow village, promotes the formation of organic–inorganic complex in soil, and improves the ion adsorption capacity, thus increasing the surface charge quantity and specific surface area of soil particles. This is consistent with the research results of Yu et al., who showed that organic matter content has a significant positive correlation with soil specific surface area and cation exchange capacity. With the addition of straw, the organic matter content in inceptisols increased, and the soil surface charge quantity and specific surface area increased by 18.75% and 14.64%, respectively [36,46]. Clay and silt particles in soil are small and have a huge surface area, and their components are mainly layered aluminosilicate clay minerals and clay oxides. The isomorphic replacement of clay minerals and the dissociation of hydroxyl groups on the surface of clay oxides make the soil negatively charged, thus affecting the quantity and density of soil charges [47]. Through correlation analysis and RDA analysis, it was found that the content of clay particles in reclaimed soil has a highly significant positive correlation with the surface electrochemical properties. As fly ash is rich in clay particles and Al₂O₃, Fe₂O₃, and other oxides, it can significantly improve the mutual adsorption and agglomeration ability of soil particles. The humus and other cementing substances produced by the decomposition of organic fertilizer have high specific surface area and multi-level pores, which promote the adhesion, cementation, and agglomeration of soil particles. The improvement in clay and silt content directly increases the specific surface area and surface charge quantity of reclaimed soil [27,48]. This is consistent with the results of Hepper et al., who found that soil silt content has a significant positive correlation with specific surface area and cation exchange capacity, with silt content accounting for 70.0% of the variability of specific surface area and clay content accounting for 9.1% [49]. Therefore, soil clay and silt particles are closely related to the properties of soil surface charges. Higher content of soil clay and silt particles means larger specific surface area, more adsorption and exchange sites of particles, more negative soil charges, and greater amount of surface charges.

4. Conclusions

The application of different improved materials has a significant effect on the basic physical and chemical properties and surface electrochemical properties of reclaimed soil. Compared with the control treatment, the organic matter, total nitrogen, available phosphorus, rapidly available potassium, clay and silt particles, aggregate structural stability, and water content of reclaimed soil in hollow village increased after five years of application of different improved materials, while soil bulk density and pH value decreased. The application of different improved materials had a positive effect on the reclaimed soil surface electrochemical properties, such as specific surface area, surface charge quantity, and surface electric field strength. The results showed that the organic–inorganic combined treatment of TFO had the most significant effect on soil physical and chemical properties and surface electrochemical properties. Soil organic matter and silt particles contents showed a highly significant positive correlation with the surface electrochemical properties and were the main contributors to the variation in surface electrochemical properties of reclaimed soil, such as soil specific surface area and surface charge quantity, accounting for 69.4% and 8.3%, respectively. These findings provide important information for improving the reclaimed soil fertility and surface electrochemical properties in hollow village.