The experimental plot of reclamation soil improvement in Hollow Village is located in Fuping County, Weinan City, Shaanxi Province (34°42′N, 109°12′E) with the background of comprehensive land improvement of abandoned homestead in Hollow Village, mainly aiming at improving the reclaimed soil structure and fertility. The study area belongs to Weibei Loess Plateau, with an average annual temperature of 13.3°C, an average annual precipitation of 513.5 mm and an annual total solar radiation of 135.44 kcal/cm², which can meet the basic growth condition for crops. The experimental plot was completed in June 2015, and the reclaimed soil was from backfilling of the old wall soil of abandoned homestead in Hollow Village, with a backfill depth of 30 cm. After removal of the impurities such as rubble and stones in the old wall soil, the reclaimed soil was improved in structure and fertility by applying soil conditioners to enhance the food crops production capacity. The reclaimed soil was mainly developed from loess parent material. At the beginning of the experiment, the surface soil pH value (water-soil mass ratio of 1:2.5) was 8.5, and the soil texture was silty loam (USDA texture classification), in which clay (<0.002 mm) content was 10.15%, silt (0.05–0.002 mm) content 77.82%, sand (0.05–2 mm) content 12.65%, organic matter content 4.5 g kg^-1, soil total nitrogen content 0.16 g kg^-1, available phosphorus content 3.1 mg kg^-1, rapidly available potassium content 61.4 mg kg^-1, soil bulk density 1.40 g cm^-3, and the proportion of water-stable aggregates with particle size less than 0.25 mm 93.27%. The overall structure and fertility of reclaimed soil were poor.

The reclaimed soil remediation experiment began in June 2015. The amount of soil conditioners applied in this study was based on the conditioner properties and published reference research results, in which such amount is commonly used (Lei, et al., 2019; Gao, et al., 2021). The contents of environmental pollution indicators As, Hg, Cu, Pb, Zn and Cd in the fly ash were 13.59, 0.10, 91.6, 22.72, 57.81, and 0.06 mg kg^-1 respectively (Wang, et al., 2019), all of which met the Environmental Quality Standards for Soils (GB15618-2008, China). This is similar to the heavy metal contents in the fly ash studied by Sivapulliah et al., which indicated that the leachability of heavy metals is primarily influenced by soil liquid-solid ratio, original concentration of heavy metals, pH, and particle cementation (Sivapullaiah and Moghal, 2010). The experiment adopted a random block design with six treatments of soil conditioners, namely, control treatment (CK, without soil conditioners), maturing agent, ferrous sulfate (TM), fly ash (TF), organic fertilizer, chicken manure (TO), maturing agent + organic fertilizer (TMO) and fly ash + organic fertilizer (TFO), with three replicates for each treatment. In order to avoid mutual influence, an 80 cm-wide isolation belt was set between each treatment. The grain crops planting system was a 2-year triple cropping one with a winter wheat-summer maize rotation. The summer maize for the test was sown in early June with 65,000 plants per hectare and harvested in early October. The maize variety was Xianyu 335. Before planting crops, the soil conditioners of different treatments were evenly mixed into reclaimed raw soil, and the soil conditioners were applied for each treatment at one time, with 1,500 kg ha^-1 compound fertilizer applied. The concentrations of the compound fertilizer were nitrogen, phosphorus and potassium at 15%, 10% and 20% respectively, and the daily management measures such as irrigation, fertilization and pest control were the same for the six treatments. The detailed experiment treatment and soil conditioners application are shown in Table 1.

After the harvest of summer maize in early October of 2020, the surface physical and chemical samples of reclaimed soil at depth of 0–20 cm and undisturbed soil samples were collected according to the experiment treatments, and three replicates were randomly made for each treatment of soil samples to analyze the effects of different soil conditioners on the reclaimed soil physical and chemical properties and surface electrochemical properties. After the soil samples were naturally air-dried indoors, impurities such as plant residual roots and gravel were removed, and the samples were then ground and sifted through 0.25 mm and 2 mm sieves. The content of soil organic matter was measured by K₂Cr₂O₇ heat capacity method (Jenkinson and Kalembasa, 1973), the pH by electrode method (soil and water mass ratio of 1:2.5) (Liu Z et al., 2021), and the mean weight diameter (MWD), an index for soil water-stable aggregate distribution and structural stability, measured and calculated by wet sieving method (Six et al., 2004; Liu et al., 2022).

The soil surface electrochemical properties, such as specific surface area (SSA, m² g^-1), surface charge density (σ₀, C m^-2) and cation exchange capacity (CEC, cmol kg^-1), were determined and calculated by the conjoint determination method of material surface properties established by Li et al. (2011). The specific steps are as follows. Firstly, six kinds of 0.25 mm 200 g soil samples with different treatments were respectively weighed and put into a 1 L beaker, and 0.6 L 0.5 mol L⁻¹ hydrochloric acid solution was added, the mixture of which was repeatedly stirred until the calcium carbonate in the soil samples was completely removed. Then, 0.6 L of 0.1 mol L^-1 HCl solution was added and stirred for 5 h, with the supernatant removed by centrifugation. The above shaking and centrifugation were repeated for 3 times, and the soil samples were dried at 65°C and ground and sifted through a 2 mm sieve to obtain hydrogen-saturated soil samples with different treatments. Third, 5–10 g of hydrogen-saturated sample was weighed and put into a 100 mL centrifuge tube, 40 mL of mixed solution of NaOH and Ca(OH)₂ of 0.01 mol L^-1 was added, the pH of the mixed solution was adjusted to about 7.0 with 1 mol L^-1 HCl after being shaken for 24 h, and the concentration of Na⁺ and Ca²⁺ in the mixed solution was measured with atomic absorption spectrometer, with the measurement of each treated soil sample repeated for three times. Finally, The surface electrochemical properties of reclaimed soil were calculated by following Eqs (1)–(5) with the above experimental measured data (Li et al., 2011; Liu et al., 2021a). φ 0 = 2 R T 2 β C α − β N α F ln a C α 0 N C α a C α 0 N C α (1) σ 0 = s g n φ 0 ε R T 2 π a N a 0 e β N a F φ 0 R T − 1 + a C a 0 e 2 β C a F φ 0 R T − 1 + a N a 0 + 2 a C a 0 e F φ 0 R T − 1 (2) S S A = N N α k m a N α 0 e β N α F φ 0 2 R T = N C α k m a C α 0 e 2 β C α F φ 0 R T × 10 − 2 (3) C E C = 10 5 S S A σ 0 F (4) m = 0.5259 ⁡ ln c N α 0 + c H 0 c C α 0 + 1.992 (5)

Where φ ₀ (mV) is the surface potential; σ ₀ (C m⁻²) is the surface charge density; SSA (m² g⁻¹) is the specific surface area; SCN (cmolkg⁻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 cation valence; 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; N _i (mol g⁻¹) is the total number of the cation (i = Ca²⁺, Na⁺) adsorbed on the soil particle surface; κ (dm⁻¹) is the Debye–Hückel parameter; I (mol L⁻¹) is the ionic strength; к = (4πF ²∑𝑍_𝑖 ²α_𝑖 ⁰/εRT)^1/2; and c N a 0 (mol L⁻¹), c c a 0 (mol L⁻¹) and c H 0 (mol L⁻¹) are equilibrium Na⁺, Ca²⁺, and H⁺ concentrations in the bulk solution, respectively.

The reclaimed soil particle interaction forces include electrostatic repulsive force, van der Waals attractive force and hydration repulsive force, and the net resultant force (P_net) of soil internal forces is the sum of electrostatic repulsive force pressure (P_E), hydration repulsive force (P_hyd) and van der Waals attractive force (P_vdW). The reclaimed soil particle interaction force and net resultant force were calculated according to Eqs (6)–(10) based on the calculation results of parameters indicating electrochemical properties such as soil specific surface area and surface charge density (Li et al., 2013; Liu et al., 2021a; Hu et al., 2021). P E = 2 101 R T c 0 cosh Z i F φ d / 2 R T − 1 (6) P h y d = 3.33 × 10 4 e x p − 5.76 × 10 9 d (7) θ m = − ρ w S S A 3 A e f f 6 π ρ w g ψ × 1000 (8) P v d W = − A e f f 0.6 π 10 d − 3 (9) P n e t = P E + P h y d + P v d W (10)

Where A _eff (J) is an effective Hamaker constant which was estimated by analyzing the dry end of the soil water characteristic curves with a dew point potentiometer. θ _m is the gravimetric water content (kg kg⁻¹), ρ_w is the density of water (kg m⁻³), g is acceleration due to gravity (m s⁻²), ψ is the matric potential head (m H₂O), Z_i is cation valence, d (dm) is the distance between two adjacent particles; φ(d/2) (V) is the potential at the middle of the overlap of the electric double layers of two adjacent particles.

Data statistics and analysis were performed using Microsoft Excel 2010 and SPSS25.0. Figure generation was performed using Origin software. Differences among different treatments were evaluated using one-way analysis of variance (ANOVA), and the least significant range (LSD) method was used for testing the signifcance in soil surface properties of reclaimed soil (p < 0.05).

After 5 years of application of different soil conditioners, the reclaimed soil properties in abandoned homestead had changed obviously. As can be seen from Table 2, the reclaimed soil was generally weakly alkaline, and a decreasing trend could be seen in the pH value after 5 years of application. With the use of different soil conditioners, the organic matter content of reclaimed soil showed a significant increase (p < 0.05), and the organic matter content increased from 5.94 g kg⁻¹ to 13.89 g kg⁻¹. The organic-inorganic treatments of TFO and TMO had a better improvement effect on soil organic matter. Similar to the organic matter change, the specific surface area (SSA), surface charge density (σ₀), CEC and MWD also increased continuously. The specific surface area of reclaimed soil under TO, TF, TM, TFO, and TMO treatments increased by 26.2%, 15.8%, 11.2%, 37.5%, and 29.6% respectively compared with that under the CK, and the CEC value by 21.7%, 10.6%, 3.2%, 34.3%, and 25.2% respectively compared with that under the CK, indicating that the soil absorption of ions was on the rise, among which the TFO treatment recorded the largest increase. With the increase of soil organic matter content, the MWD under TFO and TMO treatments increased by 150.3% and 65.6% respectively compared with that under the CK, demonstrating that the reclaimed soil particle aggregation and structural stability were significantly improved.

The distribution of electrostatic repulsive force between reclaimed soil particles under the treatments of different soil conditioners is shown in Figure 1, according to which, the electrostatic repulsive force between soil particles decreased sharply with the increase of the distance between two particles, and increased continuously with the decrease of the soil electrolyte solution concentration. The electrolyte concentration of 10⁻² mol L^-1 was the critical concentration for the drastic change of electrostatic repulsive force between reclaimed soil particles. When the concentration of soil electrolyte decreased from 1 mol L^-1 to 10⁻² mol L^-1, the electrostatic repulsive force between soil particles increased sharply. When the concentration of soil electrolyte was less than 10⁻² mol L^-1, the electrostatic repulsive force between soil particles changed relatively slowly and gradually tended to be stable. Under a certain concentration of electrolyte solution and distance between particles, the overall trend of electrostatic repulsive force between particles in reclaimed soil treated with different soil conditioners showed the order of TFO > TMO > TO > TF > TM > CK.

In order to further clarify the influence of different soil conditioners on the change of electrostatic repulsive force, we drew the relationship between electrostatic repulsive force and electrolyte concentration at the distance of 1.5 and 2 nm between reclaimed soil particles under different treatments (Figure 2). When the concentration of electrolyte solution was 1 mol L⁻¹, the electrostatic repulsive force at the distance of 2 nm between reclaimed soil particles under TFO, TMO, TFO, TF, TM and CK treatments was 1.59, 1.54, 1.52, 1.51, 1.47, and 1.36 atm, respectively. With the incorporation of soil conditioners, the reclaimed soil electrostatic repulsive force increased slightly. For reclaimed soil samples treated with six different soil conditioners, the decreases of the distance between soil particles and electrolyte solution concentration would lead to the increase of electrostatic repulsive force. Similarly, when the electrolyte concentration decreased from 1 mol L^-1 to 10⁻² mol L^-1, the electrostatic repulsive force between adjacent particles in reclaimed soil would increase sharply with the decrease of electrolyte concentration, and when the electrolyte concentration decreased from 10⁻² mol L^-1 to 10⁻⁵ mol L^-1, the electrostatic repulsive force tended to be stable with the decrease of electrolyte concentration. This finding is similar to the research results of Hu et al., who depicted that the electrostatic repulsive pressure at 2 nm between the Loess soil particles increased with biochar application, which is mainly due to the increase of soil surface charge density and surface electric field after biochar incorporation (Xu et al., 2020; Hu, et al., 2021). Meanwhile, the electrolyte solution concentration of 10⁻² mol L^-1 is the key solution concentration that affects electrostatic repulsive pressure, and the electrostatic repulsion between particles increases with the decrease of bulk solution electrolyte concentration, which was also confirmed by previous literature research results (Hu, et al., 2018b; Ding, et al., 2019; Hu, et al., 2021). According to theoretical analysis, it should be that the greater the electrostatic repulsive force between soil particles is, the worse the stability of soil aggregates becomes. However, the stability of soil aggregates is not determined by electrostatic repulsive force alone, but the resultant force of internal forces. Therefore, we will further discuss the van der Waals attractive force and hydration repulsive force next.

To quantitatively analyze the van der Waals (vdW) attractive force among reclaimed soil particles, the fitting curve of the moisture content in the reclaimed soil weight and the matric potential under different treatments (Figure 3) was first obtained according to the method of Tuller et al. (Tuller and Or, 2005), and then the Hamaker constants (A _eff ) under the treatments of TFO, TMO, TO, TF, TM and CK were calculated to be 9.53 × 10⁻²⁰, 5.75 × 10⁻²⁰, 4.62 × 10⁻²⁰, 3.48 × 10⁻²⁰, 3.40 × 10⁻²⁰ and 2.45 × 10⁻²⁰ J, respectively. In order to evaluate the influence of organic matter content on the Hamaker constant of soil, by comparison with the organic matter content shown in Table 1, we found that after the incorporation of different soil conditioners, the A_eff of reclaimed soil increased significantly while the soil organic matter (SOM) content increased.

After determination of the A_eff of reclaimed soil, the van der Waals attractive force and hydration repulsive force of reclaimed soil under different treatments when the concentration of electrolyte solution were 1 mol L⁻¹ (Figure 4) were calculated according to Eqs 8, 9. The positive value in the figure represents a repulsive force between reclaimed soil, and the negative value represents an attractive force. With the application of different soil conditioners, the van der Waals attractive force among the reclaimed soil particles was on the rise, and the organic-inorganic coupling treatments of TFO and TMO had a better effect on increasing the van der Waals attractive force of soil (Figure 4). When the distance between adjacent soil particles was 2 nm, the van der Waals attractive forces of reclaimed soil under the treatments of TFO, TMO, TO, TF, TM, and CK were −6.32, −3.82, −3.07, −2.31, −2.25, and −1.62 atm, respectively, and the absolute values were 290.1%, 135.8%, 89.5%, 42.6%, and 38.9% higher than that under the treatment of CK, respectively. The van der Waals attractive force under the treatment of TFO was the largest, indicating that the soil structure was more stable. With the increase of the distance between soil particles, the hydration repulsive force decreased exponentially, with the hydration repulsive force at 2 nm of 0.33 atm. In addition, when the distance between soil particles was or greater than 1.6 nm, van der Waals attractive force was larger than hydration repulsive force at the same distance, and an attractive force showed among soil particles; while the distance between soil particles was smaller than 1.6 nm, the difference between van der Waals attractive force and hydration repulsive force would increase sharply, and a repulsive force showed among soil particles. Therefore, when the dry soil suddenly went wet, the hydration repulsive force could always overcome the van der Waals attractive force within a certain range of soil particles, making the soil swell and break to some extent. By comparison of the van der Waals attractive forces under different treatments of soil conditioners, it was found that the van der Waals attractive forces under the six treatments showed the order as follows: TFO > TMO > TO > TF > TM > CK, indicating that the van der Waals attractive force of reclaimed soil increased with the rise of organic matter content after the application of soil conditioners. In particular, the capacity of organic soil conditioners to enhance the van der Waals attractive force among soil particles was better than that of inorganic soil conditioners, and the organic-inorganic coupling treatment had a better effect on improving the van der Waals attractive force of reclaimed soil.

Since the classical DLVO force is hard to explain the result that dry Na⁺ saturated aggregates can still break and disperse in different concentrations of NaCl solution when the van der Waals attractive force is significantly greater than the electrostatic repulsive force, it is necessary to consider the influence of hydration repulsive force on the basis of the classical DLVO force between aggregate particles. In order to accurately assess the variation of net resultant force among soil particles, we have drawn the distribution diagram of the sum (net resultant force) of electrostatic repulsive force, van der Waals attractive force and hydration repulsive force among reclaimed soil particles (Figure 5), in which positive value represents repulsive force and negative value represents attractive force. In general, with the application of soil conditioners, the net resultant force between reclaimed soil particles is gradually decreasing, showing an attractive force. The results are consistent with that by Hu, et al. (2021); Yu et al. (2020); Calero et al. (2017), which showed that the incorporation of soil conditioners such as biochar, organic fertilizer and straw could largely enhance the van der Waals attractive force between soil particles, reduce the net pressure and weaken the adverse effect of electrostatic repulsive force on soil aggregates, thus increasing the attractive force between soil particles and improving soil structural stability (Calero, et al., 2017; Yu, et al., 2020; Hu, et al., 2021.). When the concentration of electrolyte solution was 10⁰ mol L⁻¹, the net resultant forces between soil particles at the distance of 2 nm under the treatments of TFO, TMO, TO, TF and TM all showed an attractive force except under the CK, which were −4.46, −2.02, −1.22, −0.42, and −0.39 atm, respectively, so the soil structure under this condition was relatively stable, in which the net resultant force under the organic-inorganic treatment of TFO was the smallest, and the net attractive force between soil particles was the largest. When the electrolyte concentration was less than 1 mol L⁻¹, the net resultant force between the reclaimed soil particles was gradually showing a repulsive force, and the net repulsive force increased with the decrease of the electrolyte concentration. When the electrolyte concentration was or less than 10^–2 mol L⁻¹, the net resultant force curves of reclaimed soil under different treatments were similar, showing a net repulsive force overall. The above results indicate that the electrolyte concentration of 10^–2 mol L⁻¹ is the key concentration for the variation of net resultant force, at which the structural stability of soil is easy to be destroyed. Liu Z et al. (2021) and Hu et al. (2021) also came to similar conclusions, showing that solution electrolyte concentration was closely related to soil net resultant force. With an increase in solution electrolyte concentration and the incorporation of soil conditioner, the net repulsive resultant force between soil particles decreased, and the electrolyte solution concentration of ≤10^–2 mol L⁻¹ is the critical concentration where the net resultant force tends to be stable and overlap (Liu, et al., 2021b.; Hu, et al., 2021). In addition, at the same distance, when the electrolyte concentration was or greater than 10^–2 mol L⁻¹, the net repulsive force between particles increases significantly with the decrease of the electrolyte solution concentration. When the distance between soil particles was less than 1.8 nm, there was a strong net repulsive force at any electrolyte concentration due to the hydration repulsive force, indicating that soil particles would swell to some extent at a very close distance.

In order to further compare and analyze the impact of different soil conditioners on the variation of net resultant force among reclaimed soil particles, we presented the relationship between net resultant force and electrolyte concentration at a distance of 2 nm among reclaimed soil particles under different treatments (Table 3). The overall trend of net resultant force under different treatments was as follows: TFO < TMO < TO < TF < TM < CK, and the negative value represents a net attractive force, which means that the application of soil conditioners enhanced the agglomeration and structural stability of soil particles, among which the organic-inorganic treatments of TFO and TMO had a better effect on improvement of the structural stability of reclaimed soil. With the decrease of electrolyte concentration from 1 mol L⁻¹ to 10^–2 mol L⁻¹, the net resultant forces of reclaimed soil under the six treatments all showed an increasing trend, indicating that the net resultant force gradually exhibited a tendency towards repulsive force, and the structural stability of aggregates would keep decreasing. When the electrolyte concentration was or less than 10^–2 mol L⁻¹, the net resultant force of reclaimed soil tended to be stable, and the structural stability of aggregates did not vary greatly at large. Furthermore, when the electrolyte concentration was 1 mol L⁻¹, the net resultant forces under the treatments of TFO, TMO, TO, TF, and TM were all negative. Compared with the net resultant force of 0.06 atm under the CK treatment, the application of different soil conditioners significantly enhanced the net resultant force and structural stability of reclaimed soil particles under the same conditions.

The net resultant force between soil particles on a micro scale played a crucial role in the formation and stability of soil aggregates, and the structural stability of soil aggregates could also reflect the changes in the properties of soil surface charges and the interaction forces, which significantly influenced the soil fertility and environmental issues (Hu et al., 2021; Liu Z et al., 2021). The results of our study showed that the net resultant force among reclaimed soil particles was significantly negatively correlated with soil MWD and CEC (MWD, R ² = 0.7611, p < 0.0001; CEC, R ² = 0.8360, p < 0.01 Figure 6), which revealed that with the application of soil conditioners, especially the application of TFO and TMO, the net resultant force between reclaimed soil particles decreased continuously, and the negative value represented the net attractive force between particles; the MWD value and CEC content of reclaimed soil aggregates increased constantly, and the stability and agglomeration of soil structures were improved continuously.

The application of soil conditioners has significantly ameliorated the basic properties of reclaimed soil in abandoned homestead, and can effectively enhance the structural stability and fertility of soil (Chivenge, et al., 2011; Gao, et al., 2021). Our findings showed that the long-term incorporation of soil conditioners slightly reduced the pH of reclaimed soil, which may be due to the fact that the decomposition of soil conditioners enhances the production of acid substances and the buffering capacity of the pH of reclaimed soil. That is similar to the previous research results of Lentz and Ippolito, (2012) and Choudhary et al. (2011). The returning incorporation of different soil conditioners to the field is an important source of improving soil organic matter. Soil conditioners such as organic fertilizer and fly ash can significantly increase organic matter content in the reclaimed soil of abandoned homestead after application alone or in combination, which is also confirmed in Table 2. The long-term application of soil conditioners furthers the rise of crop biomass and enhances the return of plant residues and roots into the soil. With the increase of organic matter content, the cementing substances help enhance the mutual adsorption and agglomeration among soil particles; Meanwhile, the incorporation of soil conditioners weakens the destructive effect of tillage patterns on soil structure to some extent, ultimately promoting the formation and stability of reclaimed soil aggregates (Singh, et al., 2011; Zhang, et al., 2012; Dong, et al., 2022), and thereby the MWD, an index for structural stability of reclaimed soil, increases significantly.

Among them, the improvement effect of the organic-inorganic treatment of TFO showed the best, followed by the TMO and TO treatment, and that of the inorganic treatment (TM) showed less. The reason is that: due to the difference in the properties and structure of organic fertilizer, maturing agent and fly ash and the discrepancy in the improvement of the physical and chemical properties of reclaimed soil, and the fact that the fly ash itself has high specific surface area and multi-level pores and is rich in clay particles and oxides such as Al₂O₃ and Fe₂O₃, its application to the reclaimed soil can significantly enhance the mutual adsorption, agglomeration and cementation among soil particles; Besides, since the exogenous organic fertilizer itself is rich in organic matter and multiple nutrient elements, the cementing substances such as polysaccharide and humus produced by decomposition can further promote the agglomeration of soil particles and the structural stability of aggregates, as well as physically protect SOM from rapid decomposition (Meng, et al., 2014; Parab, et al., 2015). Therefore, the combined incorporation of organic and inorganic fertilizer (TFO) has the most significant effect on the improvement of organic matter and soil structure, thus it is a good combination to improve the structural stability and fertility of reclaimed soil. The findings in this research are similar to those of Ren et al. (2012). The latter showed that the capacity of organic cement to promote the formation and stability of aggregates is better than that of inorganic cement, and the organic-inorganic coupling treatment is more conducive to the improvement of soil structure and fertility (Ren, et al., 2012; Wei, et al., 2016).

Previous studies showed that the addition of organic and inorganic soil conditioners had a significant influence on the soil surface properties (Gruba and Mulder, 2015; Liu, et al., 2020; Dong, et al., 2022). In the studies on the degraded loess in Northwest China, Liu et al. (2020) found that with the extension of the period of vegetation restoration and the increase of organic matter content, a growth could be seen correspondingly in the surface charge density and specific surface area of the loess (Liu, et al., 2020). And the studies of Dong et al. (2022); El-Naggar et al. (2019) showed that the specific surface area and cation exchange capacity of soil particles could be increased with the use of fly ash and biochar soil conditioners (El-Naggar et al., 2019; Dong et al., 2022). Furthermore, it was manifested in our findings that the application of exogenous soil conditioners such as organic fertilizer and fly ash increased not only the content of organic matter and structural stability of reclaimed soil, but also the specific surface area and surface charge of soil particles, thus improving the capacities of ion adsorption and the agglomeration and cementation of clay particles. Among them, the organic-inorganic treatment (TFO) had a better effect on improving the soil surface electrochemical properties (Table 1).

Particle interaction forces are mainly affected by internal and external factors such as specific surface area (SSA), charge density (σ ₀), cation exchange capacity (CEC), solution concentration and pH value (Rengasamy, et al., 2016; Yu, et al., 2016). This study found that with the incorporation of soil conditioners, there was a slightly increase in electrostatic repulsive force (ERF) of reclaimed soil, and the electrostatic repulsive force between soil particles increased constantly with the decrease of the distance between particles and the concentration of electrolyte solution (Figures 1, 2). The results were similar to those of previous studies, mainly due to the increase of particle surface charge density and electric field intensity after the application of different soil conditioners (Ding, et al., 2019; Xu, et al., 2020). Meanwhile, according to the theory of electric double layers, the increasing solution electrolyte concentration would compress the electric double layers of soil and decrease the thickness of diffusion layer, and there would be more ions in adsorption layer, thus reducing the electrostatic repulsive force between particles (Liang, et al., 2007). In this study, van der Waals (vdW) attractive force increased gradually with the addition of soil conditioners such as organic fertilizer and fly ash, indicating that the addition of different soil conditioners could to a certain extent enhance the attractive force between soil particles. According to Eq. 9, molecular attractive force was only related to the distance between particles and A_eff, and the order of size in the A_eff under different treatments showed as follows: TFO > TMO > TO > TF > TM > CK (Figure 3). Therefore, the organic-inorganic treatments of TFO and TMO could significantly improve the van der Waals (vdW) attractive force between reclaimed soil particles (Figure 4), thus increasing the structural stability of reclaimed soil, which was also consistent with the high MWD value of aggregates under TFO and TMO treatments (Table 1).

In our study, when the distance between reclaimed soil particles was less than 1.6 nm, the difference between van der Waals (vdW) attractive force and hydration repulsive force increased sharply, and the hydration repulsion force would be greater than van der Waals force, resulting in net repulsion force between soil particles (Figure 4). This was similar to the research results of Hu et al. (2015)., who found in the study on purple soil when the distance between particles was less than about 1.4 nm, the hydration repulsive force was significantly greater than the van der Waals attractive force, largely influencing the swelling and dispersion of dry soil under aqueous conditions (Hu, et al., 2015). And our results revealed that the order of size in the net resultant force of reclaimed soil particles showed as follows: TFO < TMO < TO < TF < TM < CK (Figure 5), indicating that the addition of soil conditioners reduced the net resultant force between particles and increased the attractive force between soil particles. In particular, the organic-inorganic treatments of TFO and TMO well improved the net resultant force of reclaimed soil structure, and the structural stability of reclaimed soil was greatly bettered. The research results were consistent with those of Yu et al. (2020) and Calero et al. (2017), which showed that the addition of soil conditioners such as organic fertilizer and straw could largely enhance the van der Waals attractive force between soil particles while increasing organic matter, weaken the adverse effect of electrostatic repulsive force on soil aggregates, and thus improve the structural stability and fertility of soil (Calero, et al., 2017; Yu, et al., 2020).

Soil organic matter was a vital soil cementing agent, and 0.1 cmol kg^-1 negative charge could be increased with an increase of 1 g kg^-1 organic matter content, and the surface charge density also grew with the increase of organic matter content (Hu et al., 2015). The specific surface area of humus in organic matter was about 800–900 m² g^-1, nearly 10 times that of inorganic clay minerals. With the growth of organic matter content, the specific surface area of soil particles gradually increased (Zhao et al., 2019). Therefore, in this study, the application of exogenous soil conditioners such as organic fertilizer, fly ash and maturing agent notably increased the content of organic matter in the reclaimed soil of Hollow Village, promoted the formation of organic-inorganic complexes in soil, improved the ion adsorption, cementation and agglomeration, and significantly reduced the net resultant force between particles, thereby showing an attractive force, thus increasing the cation exchange capacity and structural stability of soil particles (Figure 6). Yu et al. (2017). also confirmed the above-mentioned, showing that organic matter content was closely related to soil structural stability and the net resultant force between particles. With the growth of soil organic matter content, the van der Waals (vdW) attractive force between soil particles increased, and electrostatic repulsive force and hydration repulsive force were weakened. The higher the organic matter content was, the smaller the net resultant force between particles was and the greater the stability of soil aggregates became (Xu, et al., 2015; Yu, et al., 2020).

Our results manifested that the electrochemical properties of soil surface and the interaction forces between particles had important effects on the stability of soil aggregates. Since the cementing substances such as humus produced by the decomposition of organic fertilizers had high specific surface area and multi-level pores, fly ash was rich in clay particles and oxides such as Al₂O₃ and Fe₂O_3, and maturing agent ferrous sulfate could adjust soil pH and improve soil structure (Yukselen-Aksoy and Kaya, 2010; Gao, et al., 2021), the organic-inorganic treatments of TFO and TMO significantly enhanced the amount of charge between particles and cation exchange capacity, increased van der Waals attractive force, and generally weakened the influence of repulsive forces between particles, thereby promoting the mutual adsorption and agglomeration among reclaimed soil particles, and improving the structure and fertility of reclaimed soil. This study explored the internal mechanism of the employment of organic matter in improving the stability of reclaimed soil aggregates during the application of soil conditioners, and provided new ideas for further study of the effect of soil conditioners on the stability of soil aggregates and the prevention of soil erosion. The composition of organic matter includes particulate organic matter and mineral-associated organic matter. In the future, we will further explore which component profoundly affects the surface properties and interaction forces of soil particles, which will play an important role in improving soil quality and ecological environment.