The Effects of the Addition of Secondary Phyllosilicate Minerals on the Decomposition Process and Products of Maize Straw in Black Soil

Abstract

The interaction between secondary phyllosilicate minerals and straw is crucial for preserving soil organic carbon (SOC) and fertility. However, the specific mechanism through which these minerals affect straw decomposition and its products in northeast China’s black soil remains unclear. In this study, montmorillonite, illite, and vermiculite were mixed with quartz sand and maize straw, inoculated with microbes, and incubated to analyze the effects of different secondary phyllosilicate minerals on the degradation of organic components in maize straw and the formation of soil humus. The results showed that montmorillonite significantly facilitated the decomposition of maize straw hemicellulose and lignin, which decreased by 95.85% and 76.38%, respectively. Conversely, vermiculite decelerated hemicellulose and lignin degradation. Regarding soil organic acids, lactic acid and malic acid were predominant, with the highest content being found after the montmorillonite treatment. Montmorillonite was the most effective in enhancing extractable humic-like substances, which increased by 71.68%. Montmorillonite increased the content of G0 (water dispersion group), G1 (sodium ion dispersion group), and G2 (sodium grinding dispersion group) complexes. The addition of secondary phyllosilicate minerals increased the organic carbon (OC) content in the G0, G1, and G2 samples, with montmorillonite demonstrating the most pronounced effect. Secondary phyllosilicate minerals increased the abundance of fungi, particularly Ascomycota, with the highest abundance being found after the montmorillonite treatment. In conclusion, our results indicated that montmorillonite facilitated the decomposition of lignocellulose in maize straw, enhanced the accumulation of humus, and promoted the formation of organic–mineral complexes. These findings provide valuable insights into the interaction between secondary phyllosilicate minerals and maize straw and have important implications for improving the quality of black soil in northeast China.

1. Introduction

Secondary phyllosilicate minerals are essential soil components that play a vital role in determining soil’s physical and chemical properties, as well as the overall health of the ecosystem. Moreover, secondary phyllosilicate minerals’ unique crystal structure and large surface area greatly influence soil water retention, nutrient exchange, and organic matter adsorption. Thus, they are essential for preserving the fertility and structural stability of soil [1].

The high organic matter content and fertility of black soil in northeast China are primarily attributed to the interaction between secondary phyllosilicate minerals and organic matter. Organic matter integrates with the surface of secondary phyllosilicate minerals through adsorption or intercalation, forming stable organic–mineral complexes. These complexes play essential roles in the decomposition and stabilization of organic matter and are therefore vital in sustaining soil fertility [2]. The decomposition of straw enhances the organic matter in soil, and different secondary phyllosilicate minerals possess distinct surface areas, water retention capabilities, and cation exchange capacities, which exert varying influences on straw decomposition [3]. Previous studies have found that secondary phyllosilicate minerals positively affect the decomposition of organic matter [4,5,6]. Secondary phyllosilicate minerals can adsorb organic acids produced during the degradation of straw. These organic acids not only promote the dissolution of secondary phyllosilicate minerals, releasing more nutrients, but also serve as carbon and energy sources for microorganisms, thereby further promoting the degradation of straw [7]. Secondary phyllosilicate minerals can also affect the structural functional groups of straw. Straw contains a large amount of organic substances such as cellulose, hemicellulose, and lignin. Through adsorption and catalytic actions, secondary phyllosilicate minerals can alter the structural functional groups of these substances, thus regulating the degradation rate of straw [8]. In addition, secondary phyllosilicate minerals also affect the microbial community structure. Different secondary phyllosilicate minerals may provide distinct living environments and nutrients for microorganisms, thereby influencing the diversity and abundance of microbial species, and consequently altering the structure and function of the microbial community, leading to a heightened soil respiration rate and accelerated organic matter decomposition [9]. Finley et al. [10] found that minerals can provide specific microorganisms with different living environments and nutrients, thereby affecting the types and quantities of microorganisms, altering the structure and function of microbial communities, and consequently influencing the degradation of straw. The analysis of soluble organic matter in straw has revealed that secondary phyllosilicate minerals impact the decomposition process of straw and facilitate an increased degree of humification [8]. In addition, secondary phyllosilicate minerals provide a protective effect on the OC that is released during straw decomposition, which reduces the mineralization rate of OC and contributes to the sequestration of the soil carbon pool [11]. However, there are a limited number of studies that focus on the effects of various secondary phyllosilicate minerals on the organic components and decomposition products of straw.

Especially in recent years, the black soil region in northeast China has been confronted with issues such as soil erosion, reductions in organic matter content, and structural damage. These problems stem from long-term unreasonable tillage and the inadequate addition of organic matter, resulting in thinning of the black soil layer, which seriously affects its productivity and the regional ecological balance [12]. When the soil tillage layer thins, the parent material is gradually exposed, thereby amplifying the significance of secondary phyllosilicate minerals in facilitating plant residue decomposition and enhancing soil’s fertility. In deep soil fertilization, the interaction between organic matter and secondary phyllosilicate minerals is particularly noteworthy. Therefore, investigating the mechanisms underlying the interaction between secondary phyllosilicate minerals and straw decomposition is necessary.

In this study, a culture experiment was conducted to investigate the impact of secondary phyllosilicate minerals on the organic components of maize straw and the formation of humus, specifically montmorillonite, illite, and vermiculite, which constitute the primary components of such minerals in the black soil of Jilin Province. It was assumed that adding secondary phyllosilicate minerals would facilitate the degradation of organic components in the straw and subsequently promote the accumulation of soil humus. It is of great significance to enhance SOC storage capacity and promote the sustainable utilization of black soil resources.

2. Materials and Methods

2.1. Experimental Materials

Quartz sand (600–900 µm), quartz powder (<54 µm), montmorillonite (<54 µm), illite (<54 µm), and vermiculite (<54 µm) were purchased from Sinopod Group Chemical Reagent Co., Ltd (Hunan, China).

The maize straw for the incubation experiment was obtained from the experimental field at Jilin Agriculture University. It was dried at 55 °C for 72 h and passed through a 2 mm sieve. The resultant material had an OC content of 46.59%, a total N content of 0.79%, a hemicellulose content of 29.14%, a cellulose content of 33.99%, and a lignin content of 15.13%.

2.2. Preparation of Microbial Inoculum

The inoculum was prepared according to the method described by Pronk [13]. Specifically, 10 g soil was mixed with 90 mL of distilled water (suspension 1:9 soil to water) and stirred for 2 h. Subsequently, the suspension was centrifuged at 1000× g for 12 min. The obtained supernatant was centrifuged at 4000× g for 30 min. The precipitate from this second centrifugation was dissolved in sterile water. The ratio of g of soil to mL of inoculant produced was 1:15, and 60 mL of inoculant was added for each kg of artificial soil.

2.3. Preparation of Artificial Soils

Three single treatments, one mixed treatment (MIX), and one control treatment (CK) were established (Table S1). The single treatments were montmorillonite treatment (MT), illite treatment (IL), and vermiculite treatment (Verm). The single treatments were prepared by mixing quartz sand and quartz powder with montmorillonite, illite, or vermiculite. For the mixed treatment, the secondary phyllosilicate minerals were added in equal proportions (the properties of the secondary phyllosilicate minerals are shown in Table S2). The control treatment used quartz powder instead of secondary phyllosilicate minerals, which was mixed with quartz sand. Each treatment produced 2 kg of soil samples. Maize straw was added to each artificial soil at a rate of 21 g kg⁻¹.

All treatments were sterilized using moist autoclaving in triplicate for 2 h at 121 °C. An inorganic salt nutrient solution was added to adjust the C/N ratio to 25:1. The treatments were then inoculated with microorganisms from the topsoil layer (0–20 cm), which was sampled from Jilin Agriculture University and characterized as typical black soil. Finally, sterile water was added to adjust the soil moisture content to 20%.

2.4. Incubation Experiment

All treatment samples were thoroughly mixed and transferred into dry and sterilized plastic culture vessels (14 cm in diameter, 20 cm in height, with a vent hole on the bottle body), sealed with a plastic film with a vent hole. The soil samples were incubated at 25 °C for 360 d. Each treatment consisted of 2 kg of soil samples, with three repetitions. The artificial soils were moistened weekly to ensure a constant water content.

2.5. Sample Collection and Determination Index

Maize straw organic component content: The residue of maize straw was selected from soil samples at 0, 30, 60, 90, 180, 270, and 360 d. Lignin and cellulose were titrated using an ammonium ferrous sulfate solution. Hemicellulose was determined using copper iodide [14]. Briefly, for the determination of hemicellulose, a Ca (NO₃)₂ solution was added to the straw sample, and the mixture was centrifuged. HCl was subsequently added to the precipitate. The supernatant was then filtered into a volumetric flask and neutralized to an orange–red color with NaOH. A diluted portion of the solution was aspirated and mixed with an alkaline copper reagent, C₂H₂O₄-H₂SO₄ solution, and starch solution. Titration was performed using a Na₂S₂O₃ solution. For the determination of cellulose, a HNO₃-CH₃COOH solution was added to the straw sample, which was then boiled and centrifuged. The supernatant was discarded, and a H₂SO₄-K₂Cr₂O₇ solution was added to the precipitate. The mixture was placed in a boiling water bath for 10 min and titrated with an Fe (NH₄)₂·(SO₄)₂·6H₂O solution. For the determination of lignin, a CH₃COOH solution was added to the straw sample, and the mixture was centrifuged to remove the supernatant. The precipitate was washed with C₃H₆O, which was allowed to stand before the addition of distilled water for a boiling water bath treatment. A BaCl₂ solution was subsequently added, and the mixture was centrifuged again to remove the supernatant. A H₂SO₄-K₂Cr₂O₇ solution was added to the precipitate, which was then subjected to a boiling water bath for 15 min. Titration was carried out using an Fe (NH₄)₂·(SO₄)₂·6H₂O solution.

Organic acids: Soil samples were extracted after 30 d and 180 d. The contents of organic acids were measured by means of high-performance liquid chromatography (Agilent, Agilent 1260, Santa Clara, CA, USA) [15]. Ten organic acids were measured in the soils: oxalic acid, tartaric acid, formic acid, malic acid, lactic acid, acetic acid, maleic acid, citric acid, succinic acid, and propionic acid.

Maize straw functional group: A Fourier transform infrared (FTIR) spectrometer (Thermo Fisher, Nicolet 6700, Waltham, MA, USA) was utilized to analyze the decomposed maize straws after 180 d and 360 d. A spectral scanning range of 500–4000 cm⁻¹ was used, with a scanning speed of 0.2 cm s⁻¹ [16].

Thermal stability of straw: The maize straw was analyzed at 360 d using a thermogravimetric analyzer (Netzsch, STA 2500, Freistaat Bayern, Germany). The temperature was increased from 30 °C to 800 °C, with a heating rate of 10 °C min⁻¹ [17]. TG (thermogravimetric) and DTG curves were obtained based on the changes in temperature and sample mass, where DTG represents the derivative thermogravimetry.

Humic fractions: Extractable humic-like (HE) substances were extracted from 360 d old soil samples with Na₄P₂O₇·10H₂O and NaOH solutions, and humic acid-like (HA) and fulvic acid-like (FA) substances were separated using H₂SO₄ [18].

Organic–mineral complexes: Soil samples were selected after 360 d, and the colloidal dispersion fractionation method was used to extract the G0 (water dispersion group), G1 (sodium dispersion group), and G2 (sodium grinding dispersion group) complex fractions [19].

Soil microorganisms: Soil samples were selected after 360 d for an absolute-quantification 16S rRNA and ITS2 amplicon sequencing using Miseq [20].

2.6. Statistical Analysis

Microsoft Office Excel 2021 (Microsoft Windows, Redmond, DC, USA) was used to organize and summarize all the data. SPSS 21.0 (IBM, Armonk, NY, USA) was used to evaluate differences between samples (p < 0.05). The experimental graphs were created using Origin 2022 software (Origin Lab Inc., Northampton, MA, USA). One-way analysis of variance (ANOSIM) was used to compare the structure of the microbial community. The ggplot2 package in R (version 4.3.3) was utilized to generate the box plot [21]. Pearson correlation analysis evaluated the relationship between soil microorganisms and lignocellulose degradation rates. The heatmap was generated by Pheatmap [22]. The coordinate analysis plots (PCoA) were generated by R (version 3.2.2) [23]. Bacterial and fungal diversity (Chao1 and Shannon index) were estimated using the vegan R (4.3.3) package [24].

3. Results

3.1. Changes in Hemicellulose, Cellulose, and Lignin in Maize Straw

The contents of hemicellulose, cellulose, and lignin in the maize straw gradually decreased with the extension of the cultivation time, with a rapid decrease being observed in the early stage and a tendency toward stability in the later stage, as shown in Figure 1.

In the early stage of cultivation, the hemicellulose content of the maize straw was 29.14%, the cellulose content was 33.99%, and the lignin content was 15.14%. At the end of the cultivation, the hemicellulose and lignin contents following MT exhibited the most significant decreases, with reductions of 95.85% and 76.38%, respectively. In contrast, the hemicellulose and lignin contents in the Verm sample exhibited the smallest declines, with decreases of 79.65% and 62.53%, respectively. The contents of cellulose in the Mix sample decreased the most (86.70%), while the contents of cellulose in the CK sample decreased the least, by 77.91%.

At 360 d, compared with the CK sample, the hemicellulose contents in the MT, IL, and Mix samples were reduced by 77.43%, 15.80%, and 25.76%, respectively. The hemicellulose contents in the IL, Verm, and Mix samples were 3.73 times, 4.90 times, and 3.29 times that of the MT sample, respectively. The cellulose contents in the MT, IL, Verm, and Mix samples were lower than in the CK sample, with decreases of 24.35%, 8.03%, 4.47%, and 39.78%, respectively. The MT, IL, and Verm samples’ cellulose contents were 1.26 times, 1.53 times, and 1.59 times that of the Mix sample, respectively. Additionally, the lignin contents in the MT, IL, and Mix samples were reduced by 31.63%, 7.84%, and 19.16%, respectively. The lignin contents of the IL, Verm, and Mix samples were 1.35 times, 1.59 times, and 1.18 times that of the MT sample, respectively.

These results showed that adding montmorillonite was beneficial to the degradation of hemicellulose, cellulose, and lignin, with the most significant effects being observed in hemicellulose and lignin. The addition of mixed minerals significantly promoted the degradation of cellulose. While adding illite also facilitated the breakdown of these components, its effect was less pronounced than those of montmorillonite and mixed minerals. Conversely, vermiculite decelerated the degradation of hemicellulose and lignin.

Figure 2 depicts the decomposition rates of hemicellulose, cellulose, and lignin in maize straw. At 360 d, compared with the CK sample, the hemicellulose decomposition rates increased by 17.48%, 3.56%, and 5.81% in the MT, IL, and Mix samples; in the Verm sample, the rate decreased by 2.37%. The cellulose decomposition rates of the MT, IL, Verm, and Mix samples increased by 6.91%, 2.28%, 1.27%, and 11.28%, respectively. The lignin decomposition rates of the MT, IL, and Mix samples increased by 16.69%, 4.14%, and 10.11%, respectively, while the lignin decomposition rate of the Verm sample decreased by 4.48%. Montmorillonite significantly increased the decomposition rates of hemicellulose and lignin, while mixed minerals improved the cellulose decomposition rate.

3.2. Substance Changes During the Decomposition of Maize Straw

3.2.1. Low-Molecular-Weight Organic Acids

High-performance liquid chromatography technology was utilized to analyze the impact of secondary phyllosilicate minerals on low-molecular-weight organic acids in artificial soil. In this study, the contents of most low-molecular-weight organic acids were generally below the detection threshold of the instrument at 180 d; therefore, these data were not listed. Only the contents of low-molecular-weight organic acids in the soil at 30 d were listed (Table S3).

Except for the Mix sample, the lactic acid contents in the MT, IL, and Verm samples were higher than that of other organic acids. In the MT, IL, Verm, and Mix samples, the lactic acid contents were higher than in the CK sample, with the MT sample exhibiting the highest content at 1087.21 μg g⁻¹. Maleic acid was only detected in the MT, Mix, and CK samples, with the highest content observed in the MT sample, which was 1.68 μg g⁻¹. Furthermore, compared with the IL, Verm, and Mix samples, the oxalic acid content in the MT sample was relatively higher.

3.2.2. FTIR Analysis

FTIR analyses were performed to determine the changes in chemical structure in the process of maize straw degradation (Figure 3). At 0 d, the maize straw sample exhibited characteristic peaks near 1050 cm⁻¹, which could be linked to the stretching vibration of C-O in carbohydrates or polysaccharides [25]. The absorption peak at 1380 cm⁻¹ was due to the deformation vibration of -CH₃ and the asymmetric stretching vibration of -COO- in aliphatic compounds [25]. The absorption peak at 1506 cm⁻¹ was attributed to the C=O stretching of lignin and aromatic skeleton vibration, as well as the -CH- and -OH bending of carbohydrates [26]. The absorption peak at 1630 cm⁻¹ was due to the C=O stretching vibration, which is connected to the aromatic ring in lignin [27]. The absorption peak at 2920 cm⁻¹ was due to the C-H bond antisymmetric stretching vibration of the -CH₂ group in the aliphatic and alicyclic groups [27]. The characteristic peaks near 3400 cm⁻¹ were attributed to the hydrogen bond hydroxyl stretching vibration and the N-H stretching vibration in the amino acid [25].

At 180 d, the positions of some absorption peaks had changed. The FTIR spectra showed that the absorption peak of MT at 1050 cm⁻¹ exhibited the largest amplitude, moving from 1050 cm⁻¹ to 1087 cm⁻¹. The absorption peak at 1506 cm⁻¹ exhibited the largest amplitude in the MT and IL samples, shifting from 1506 cm⁻¹ to 1548 cm⁻¹. In the Mix sample, the absorption peak amplitude at 1380 cm⁻¹ was the largest, moving from 1380 cm⁻¹ to 1400 cm⁻¹.

From 180 d to 360 d, the intensity of the absorption peak at 1050 cm⁻¹ in the Mix sample was significantly weakened. In the MT sample, the absorption peak at 1548 cm⁻¹ exhibited the largest shift, moving from 1548 cm⁻¹ to 1512 cm⁻¹. Additionally, the MT sample showed the largest changes in lignin-related characteristic peaks. At 180 d, the absorption peaks at 3415 cm⁻¹, 2926 cm⁻¹, and 1645 cm⁻¹ shifted to 3420 cm⁻¹, 2928 cm⁻¹, and 1649 cm⁻¹, respectively, after 360 d. These changes indicated that the structural changes in lignin were more evident in the MT sample.

3.2.3. Thermogravimetric Analysis

To further investigate the thermal stability characteristics of the maize straw, a thermogravimetric analysis was conducted on the samples (Figure 4). The weight loss between 200 and 300 °C was due to the decomposition of hemicellulose to produce volatile substances. In the range of 300–400 °C, the decomposition of cellulose occurs [28]. At 360 d, the mass loss percentages of the MT sample at 200–300 °C were the lowest, while those of the Verm sample were the highest. At the 300–400 °C stage, the mass loss percentages of the MT, IL, Verm, and Mix samples were lower than that of the CK sample, and the mass loss percentage of the Mix sample was the smallest. These results indicated that the content of hemicellulose in the MT sample was low, that montmorillonite was beneficial to the degradation of hemicellulose, and that mixed minerals promoted the decomposition of cellulose. At the same time, vermiculite decelerated the degradation of hemicellulose.

After reaching 400 °C, the mass loss percentages of the maize straw in all treatments tended to slow down. The weight loss in the 400–600 °C stage was mainly caused by the decomposition and volatilization of refractory substances [28]. Within the temperature range of 400–600 °C, the MT sample exhibited the lowest mass loss percentages, while the Verm sample showed the highest, which indicated that the addition of montmorillonite facilitated the degradation of lignin.

3.3. Straw Decomposition Products and Soil Organic–Mineral Complexes

3.3.1. Effects of Secondary Phyllosilicate Minerals on HE, HA, and FA

As illustrated in Figure 5, slight increases in the contents of HE were observed in the MT, IL, Verm, and Mix samples at 360 d. The MT sample exhibited the most notable increase of 71.68%, whereas the Verm sample showed the smallest increase of 2.33%. The Mix sample exhibited a significant increase in the content of FA, by 22.68%. Furthermore, the contents of HA increased in the MT, IL, and Verm samples, with the MT sample recording the most substantial increase. These results indicated that adding secondary phyllosilicate minerals could increase the contents of HE. The mixed minerals were most conducive to the accumulation of FA, while the addition of montmorillonite significantly affected the accumulation of HA.

The ΔlgK value of FA was the largest in the Mix sample and the smallest in the MT sample. Compared with the CK sample, the ΔlgK values of FA in the Verm and Mix samples increased by 8.50% and 57.07%, respectively. In contrast, the ΔlgK values of the MT and IL samples decreased by 5.18% and 4.44%, respectively. The ΔlgK values of HA exhibited a downward trend. The ΔlgK values of HA in the Mix sample decreased the least (0.17%), while the ΔlgK values of HA in the IL sample decreased the most (30.76%) (Figure 6a). At 360 d, the PQ value in the MT sample was the highest, with an increase of 63.76%, while the Mix sample showed a decrease of 8.46% (Figure 6b).

3.3.2. Effects of Secondary Phyllosilicate Minerals on Organic–Mineral Complexes

As shown in

Table 1. Effects of secondary phyllosilicate mineral addition on the composition of soil’s organic–mineral complexes. Lowercase letters indicate significant differences among different treatments.

Treatment	Complex Composition (%)			Complex Content (g kg−1)
	G0	G1	G2	G0	G1	G2	G1 + G2	G0 + G1 + G2
CK	89.51 ± 0.91 a	6.05 ± 0.70 e	4.44 ±0.53 c	110.67 ± 1.59 b	7.50 ± 1.02 d	5.50 ± 0.74 d	13.00 ± 1.41 d	123.67 ± 2.97 c
MT	61.42 ± 0.24 d	23.84 ± 0.13 a	14.74 ± 0.11 a	119.75 ± 2.04 a	46.50 ± 1.22 a	28.75 ± 0.82 a	75.25 ± 2.04 a	195.00 ± 4.08 a
IL	71.03 ± 0.94 c	17.34 ± 0.29 c	11.63 ± 0.80 b	77.75 ± 1.74 c	19.00 ± 0.89 c	12.75 ± 1.14 b	31.75 ± 1.87 c	109.50 ± 3.36 d
Verm	76.29 ± 3.41 b	12.28 ± 1.64 d	11.42 ± 1.83 b	44.50 ± 1.02 d	7.25 ± 1.43 d	6.75 ± 1.47 d	14.00 ± 2.88 d	58.50 ± 3.89 e
Mix	73.93 ± 0.79 bc	21.27 ± 0.07 b	4.80 ± 0.77c	119.00 ± 1.62 a	34.25 ± 0.82 b	7.75 ± 1.43 d	42.00 ± 2.25 b	161.00 ± 3.81 b

, at 360 d, the contents of organic–mineral complexes in the MT, IL, Verm, Mix, and CK samples were G0 > G1 > G2. The contents of organic–mineral complexes in the G0 (water dispersion) group were ordered MT > Mix > CK > IL > Verm. The content of the G0 complex in the MT sample increased the most (8.21%). The contents of organic–mineral complexes in the G1 (sodium ion dispersion) group were ordered MT > Mix > IL > CK > Verm. The contents of the G1 complex in the MT, IL, and Mix samples increased by 5.20 times, 1.53 times, and 3.57 times, respectively. In the MT, IL, Verm, and Mix samples, the contents of organic–mineral complexes in the G2 (sodium grinding dispersion) group were all higher than that in the CK sample, with the most notable increase being observed in the MT sample. The contents of total complexes (G0 + G1 + G2) and water-stable complexes (G1 + G2) in the MT sample were the highest, at 195.00 g kg⁻¹ and 75.25 g kg⁻¹, respectively.

As shown in Figure 7, the MT and Mix samples exhibited significantly increased OC contents in the G0 complex, by 85.13% and 74.25%, respectively. The addition of secondary phyllosilicate minerals increased the OC contents in the G1 and G2 complexes, with the MT sample exhibiting the most prominent increase. Furthermore, the MT, IL, and Mix samples exhibited significantly increased OC contents within their organic–mineral complexes, with the MT sample exhibiting the greatest amplification (3.74 times). In the MT sample, the OC contents in the organic–mineral complexes were 3.40 times, 6.99 times, and 1.87 times those of the IL, Verm, and Mix samples, respectively.

3.4. The Composition of Microbial Communities in Soil

The absolute abundance of bacteria and fungi under different treatments at 360 d of maize straw decomposition is illustrated in Figure 8. The total abundance of bacteria in the IL sample was the highest (1.88 × 10⁸), while that in the MT sample was the lowest (6.96 × 10⁷). The total abundance of bacteria in the MT, Verm, and Mix samples decreased by 52.48%, 30.43%, and 26.65%, respectively, whereas that in the IL sample increased by 28.29%. The absolute abundances of Proteobacteria and Bacteroidota were higher than those of other bacteria groups, making them the dominant bacteria in all samples. Among them, the IL sample had the highest content of Proteobacteria (1.01 × 10⁸). The number of Bacteroidetes was the highest in the CK sample (3.86 × 10⁷). Actinobacteria exhibited greater abundances in the MT and IL samples. The Mix sample showed higher abundances of Verrucomicrobiota, Chloroflexi, and Gemmatimonadota compared with the other treatments (Figure 8a).

The addition of secondary phyllosilicate minerals increased the abundance of fungi. Among all treatments, Ascomycota and Basidiomycota were the predominant phyla. The absolute abundance of Ascomycota was the highest in the MT sample (2.21 × 10⁷), followed by the Verm sample, with an absolute abundance of 1.88 × 10⁷. In contrast, the CK sample exhibited the lowest absolute abundance of Ascomycota. For Basidiomycota, the IL sample had the highest absolute abundance (8.65 × 10⁵), while the Mix sample had the lowest (Figure 8b).

Pearson correlation analysis revealed associations between the microbial abundance and hemicellulose, cellulose, and lignin degradation rates in maize straw. As shown in Figure S1, Actinobacteriota correlated positively with hemicellulose degradation, while Verrucomicrobiota, Chloroflexi, and Gemmatimonadota were positively associated with cellulose and lignin. The fungi Ascomycota was significantly positively correlated with hemicellulose and cellulose degradation, as was Basidiomycota with lignin degradation.

Bacteria and fungi responded differently to the phyllosilicate minerals in the five treatments, resulting in communities with different α diversity indexes (Figure S2). The bacterial diversities in the MT, IL, Verm, and Mix samples increased, among which the Verm had the highest Shannon index. The IL sample showed the lowest Shannon value, resulting in the most significant loss of bacterial diversity (Figure S2a). For the fungal community, the Shannon index of the CK sample was significantly higher than those of the MT, IL, Verm, and Mix samples. Among them, the fungal diversity in the Verm sample was the highest, while that of the Mix sample was the lowest (Figure S2b).

A one-way analysis of variance (ANOSIM) test of variance was employed to compare the microbial community structures under the MT, IL, Verm, Mix, and CK treatments. The ANOSIM analysis indicated significant differences in the compositions of bacterial and fungal communities among the different treatments (Figure S3).

As shown in Figure S4, the bacterial community of the CK sample was significantly separated from those of the MT and Verm samples by PC2 (21.04%), and it was separated from the Mix sample by PC1 (28.18%). The IL and CK samples were also relatively far apart (Figure S4a). The fungal community in the CK sample was significantly separated from those in the IL, Mix, and Verm samples along PC2 (19%). The MT and CK samples were also relatively distant (Figure S4b). Furthermore, the MT, IL, Verm, and Mix samples were also distinctly separated in terms of their bacterial and fungal communities, which indicated that the bacterial and fungal community composition in the CK sample was distinct from those in the MT, IL, Verm, and Mix samples. There were also significant differences between the treatments containing secondary phyllosilicate minerals.

4. Discussion

4.1. The Influence of Adding Secondary Phyllosilicate Minerals on the Organic Components of Maize Straw

This study demonstrated that montmorillonite, illite, and mixed minerals were beneficial to the degradation of lignocellulose in maize straw. Montmorillonite had the most significant effect on the degradation of hemicellulose and lignin, while vermiculite was found to decelerate the decomposition process of maize straw.

The rapid degradation of the hemicellulose, cellulose, and lignin in maize straw following the addition of montmorillonite is due to the relatively small morphological unit of montmorillonite, which enhances the contact with both straw and microorganisms [29]. Due to its inherent characteristics, the intermolecular force between the adjacent crystal layers of montmorillonite is weak, resulting in a huge inner surface area within these layers [30]. This larger interlayer spacing provides more space for the microbial degradation of lignocellulose. Simultaneously, montmorillonite has the catalytic properties of promoting the conversion of lignocellulose [31], and it exhibits a superior buffering capacity and a heightened adsorption capacity for toxic substances, thereby promoting microbial activity in soil [32]. In addition, water is an important factor in the decomposition of maize straw. The significant water capacity retention of montmorillonite is a key factor contributing to its influence on the rapid degradation of hemicellulose, cellulose, and lignin in maize straw [31]. Numerous studies have demonstrated that montmorillonite positively affects organic matter decomposition. Wei et al. demonstrated that a higher montmorillonite content promoted organic matter decomposition, probably by increasing the substrate availability and altering the microbial biomass [33]. Li et al. found that montmorillonite exhibited strong water-swelling performance, which is conducive to straw degradation [34]. Additionally, Zhang et al. demonstrated that montmorillonite enhances microbial activity due to its buffering capacity, thereby primarily promoting the decomposition of organic matter [35].

In the present study, vermiculite demonstrated a capacity to decelerate the decomposition of maize straw due to the rich Si-O bonds in vermiculite, which enhance its adsorption capacity. The strong adsorption characteristics of vermiculite hinder the activities of microorganisms, thus inhibiting the degradation of organic matter [36]. This result is consistent with those reported by Mikutta et al. [37]. Baldock et al. demonstrated that secondary phyllosilicate minerals can reduce soil oxygen levels and substrate accessibility [38], thus inhibiting biological degradation by soil microorganisms and decelerating the decomposition process [3]. Although montmorillonite also possesses strong adsorption capabilities, its adsorption of organic matter is mainly concentrated on the surface, and it does not provide as many adsorption sites as the layered structure of vermiculite [39]. Due to the isomorphic substitution, vermiculite possesses a high charge density and strong binding site. This means that once organic matter is adsorbed, it becomes difficult for microorganisms to access it easily. However, montmorillonite has a much lower isomorphic substitution and surface charge density. Organic matter adsorbed between montmorillonite layers can be partially degraded by microbial secretion of organic acids or iron reduction [40]. Additionally, due to its relatively loose structure, montmorillonite is more easily disrupted by microorganisms, leading to the release of organic matter. In contrast, the stable interlayer structure of vermiculite hinders the degradation of organic matter to a certain extent [41].

From a microbial perspective, the layered structure of montmorillonite can provide a relatively stable environment for microorganisms, protecting them from the adverse effects of environmental changes, thereby enhancing the activity of microorganisms in degrading lignocellulose [42]. Moreover, montmorillonite has good ion exchange properties, enabling it to adsorb and release various ions. During the process of microbial degradation of lignocellulose, montmorillonite gradually releases these ions according to the needs of microorganisms, providing them with essential nutrients and thus enhancing their ability to degrade lignocellulose [43]. Zhao also demonstrated that montmorillonite can adsorb and release nutrient ions in the soil, such as Ca²⁺ and Na⁺, promoting the growth and metabolism of microorganisms [42]. Montmorillonite has an adsorption and enrichment effect on intermediate products and enzymes in the lignocellulose degradation process. It can adsorb small organic acids, alcohols, and other substances produced during the degradation of lignocellulose, which can serve as nutrient sources for microorganisms, promoting their growth and metabolism [7]. At the same time, montmorillonite can also adsorb degradative enzymes secreted by microorganisms, forming a higher local concentration on its surface or interlayers, thereby improving the catalytic efficiency of enzymes on lignocellulose [44]. In addition, microorganisms can disrupt the interlayer structure of montmorillonite by secreting organic acids or carrying out specific metabolic activities, thereby releasing lignocellulose or its degradation intermediates adsorbed in the interlayers of montmorillonite, making these substances more easily degradable by microorganisms [45]. In this study, the MT sample exhibited the highest abundance of Ascomycota, which is capable of secreting a wide range of enzymes for cellulose and hemicellulose degradation [46]. Fukasawa et al. demonstrated that the hyphae of Ascomycota can form a network structure within the straw, increasing the contact area with it and thereby more effectively absorbing and decomposing the nutrients in the straw, thus promoting its decomposition. Additionally, the hyphal network can improve the physical structure of the soil, creating a more favorable environment for the growth and activity of microorganisms, further accelerating the decomposition rate of the straw [47]. Ma et al. also demonstrated that Ascomycetes were involved in every stage of straw decomposition and constituted one of the critical microbial groups in this process [48]. There were more Actinobacteria in the MT sample, which are the primary sources of bacterial lignocellulose that decompose enzymes and play a key role in lignocellulose biodegradation [49]. Yu et al. found that Actinobacteria play an important role in the degradation of plant residues, especially in the degradation of complex organic matter (such as lignin and resin), enabling straw to be more thoroughly decomposed [50]. Wilhelm et al. demonstrated that Actinobacteria can interact synergistically with other microorganisms. The intermediate products produced by the degradation of Actinobacteria can be further decomposed and utilized by other microorganisms, thereby accelerating the decomposition rate of straw [51]. Simultaneously, the higher content of fungi in the MT sample also promoted the degradation of straw, with fungi playing a dominant role in the later stages of straw degradation, especially in breaking down lignin and other refractory components [52]. Consequently, these microorganisms all contributed to the degradation of lignocellulose in the MT sample. Furthermore, Chloroflexi, which represents straw hemicellulose and cellulose-degrading bacteria [53], and Gemmatimonadota, which plays a crucial role in cellulose degradation, were found in significant quantities in the Mix sample [54]. Consequently, their presence affected this sample’s decomposition of hemicellulose and cellulose.

Montmorillonite can significantly alter the bacterial community structure in soil. Zhao et al. demonstrated that the diversity of bacterial communities in soil treated with montmorillonite decreased, and the relative abundance of certain bacteria changed. This change may be due to the physicochemical properties of montmorillonite, which can inhibit the growth and reproduction of certain bacteria, thereby leading to a reduction in bacterial abundance [42]. The chemical properties of montmorillonite can also affect the growth of bacteria. The main cations in montmorillonite are Al³⁺ and Mg²⁺, and the forms and concentrations of these ions in the soil can influence the growth environment of bacteria [55]. Qin et al. found that Mg²⁺ in montmorillonite may affect the pH value of the soil and the content of water-soluble organic carbon (WSOC), both of which can have an impact on the growth and metabolism of bacteria [56]. Although the bacterial abundance in montmorillonite is relatively low, these bacteria possess highly efficient degradation capabilities in specific environments. In this study, the abundance of Actinobacteria was high in MT sample, and Actinobacteria are effective degraders of maize straw. Additionally, the high abundance of fungi, particularly the highest abundance of Ascomycota, also accelerated the degradation of maize straw. The microbial community in the Verm sample exhibited high diversity but lacked efficient degraders. Acidobacteriota was predominant in terms of diversity in the Verm sample; however, many strains of this phylum are relatively inefficient in degrading complex organic matter, which in turn can impede the degradation of maize straw [57].

These findings are consistent with the results obtained from the thermal stability and FTIR analyses of maize straw. After the heat treatment, the MT sample exhibited the lowest weight loss percentages in the 200–400 °C and 400–600 °C stages, whereas the Verm sample displayed the highest. Yang et al. [28] demonstrated that the interval between 200 and 400 °C corresponds to the primary decomposition phase for hemicellulose, cellulose, and lignin. The weight loss that was observed in the 400–600 °C range was attributed to the decomposition and volatilization of lignin, which possesses a benzene ring structure and is thermally stable. Yang et al. [58] also discovered a linear relationship between the weight loss during the heat treatment process and the proportion of hemicellulose or cellulose, which further confirmed the heat treatment causing damage to the structure of these components. The positions of the absorption peaks in the FTIR analysis corresponded to the vibrational absorption characteristics of specific chemical bonds or functional groups. In this study, the absorption peak in the MT sample shifted from 1050 cm⁻¹ to 1087 cm⁻¹, which was associated with the stretching vibration of C-O. Wang et al. [59] indicated that this was due to the disruption of the C-O bond stretching vibrations in soluble sugars, hemicellulose, and cellulose during the decomposition of straw. The absorption peak at 1506 cm⁻¹ moved to 1548 cm⁻¹, indicating that the vibration mode of the aromatic structure or C=O bond in the lignin changed [26]. Zhang et al. [60] also demonstrated that the vibration of the absorption peak at 1506 cm⁻¹ was related to the partial decomposition of lignin.

4.2. The Influence of Secondary Phyllosilicate Minerals on the Formation of Low-Molecular-Weight Organic Acids During Maize Straw Decomposition

Low-molecular-weight organic acids play a significant role in promoting the degradation of lignocellulose by interacting with microorganisms and minerals. During the degradation of maize straw, microorganisms secrete organic acids to initially break down lignocellulose. These low-molecular-weight organic acids can form stable complexes with minerals. These complexes have unique structures and properties that enable them to more effectively act on lignocellulose [7]. Sun et al. [61] demonstrated that when low-molecular-weight organic acids bind to minerals, their molecular structures change, enhancing their binding ability with lignocellulose, thereby facilitating the degradation of lignocellulose more effectively. Moreover, these complexes can serve as a controlled-release system, slowly releasing low-molecular-weight organic acids to continuously provide the driving force for the degradation of lignocellulose. Additionally, the interaction between low-molecular-weight organic acids and minerals can regulate the physicochemical properties of the environment, creating more favorable conditions for the growth of microorganisms and the degradation of lignocellulose. Zhang et al. [49] found that certain microorganisms are more efficient in degrading lignocellulose under acidic conditions, and the release of low-molecular-weight organic acids can lower the environmental pH, creating a more favorable acidic condition for these microorganisms, thereby increasing the degradation rate of lignocellulose. Through these synergistic effects, low-molecular-weight organic acids, microorganisms, and minerals jointly promote the efficient degradation of lignocellulose.

The content of organic acids in the soil environment is influenced by biological and abiotic factors [62]. The background and environmental conditions that were selected in this experiment were relatively consistent, and the differences in the soil’s organic acid content and composition came from the species differences in the treatments of different phyllosilicate minerals. In this study, the microbial community composition of each treatment was different, which also affected the content and composition of organic acids. Peng et al. [63] also showed that the content of organic acids changes with the changes in microbial communities, and the organic acids that are produced are also utilized by microorganisms. Organic acids are readily adsorbed by the soil’s solid phase after release [64]. The adsorption of the soil’s solid phase by low-molecular-weight organic acids can reduce their biodegradation rate, and the stability of low-molecular-weight organic acids will also be improved due to the adsorption of minerals [65]. The structural characteristics of different secondary phyllosilicate minerals, such as their interlayer spacing, specific surface area, and charge density, significantly influence their adsorption capacity of organic acids, thereby affecting the contents of those acids [66].

In the early stages of straw decomposition, the content of low-molecular-weight organic acids is usually high. This is because straw is rapidly decomposed under the action of microorganisms, producing a large amount of low-molecular-weight organic acids. In the later stages of decomposition, the recalcitrant components in straw are difficult for microorganisms to rapidly decompose, leading to a decrease in the production of low-molecular-weight organic acids [67]. When microbial activity decreases, their metabolic activities are weakened, and the direct secretion of low-molecular-weight organic acids also correspondingly decreases [68]. Moreover, under aerobic conditions, low-molecular-weight organic acids are easily decomposed by microorganisms into CO₂, CH₄, and other simple organic compounds, which are released into the atmosphere and disappear [69]. Especially in the later stages of decomposition, the contents of some organic acids fall below the detection limit of the instrument. Consequently, in this study, the concentrations of most low-molecular-weight organic acids were below the detection limit at 180 d.

In this study, oxalic, lactic, and acetic acids were high in the MT sample. The decomposition of organic residues is an essential source of organic acids. Saber et al. [70] demonstrated that straw lignocellulose can be converted into formic and oxalic acids by means of biodegradation. Casella et al. [71] confirmed that acetyl hydrolysis in hemicellulose can produce acetic acid. Chatgasem et al. [72] showed that cellulose-decomposing bacteria can convert lignocellulose into lactic acid. The high contents of these acids in the MT sample also suggested that straw lignocellulose had been degraded to some extent. Simultaneously, organic acids can facilitate the decomposition process through the action of microorganisms. In this study, the lactic acid content was higher than the contents of other organic acids, and the content of this acid was the highest in the MT sample. Liu et al. [73] found that lactic acid has a strong ionization ability for hydrogen ions, which enables lactic acid to selectively destroy the glycosidic bonds in hemicellulose, thereby promoting the depolymerization and separation of hemicellulose. Jin et al. [74] indicated that lactic acid can improve cellulase’s accessibility to cellulose, meaning that cellulase can act more effectively on cellulose. In addition, the MT sample also exhibited higher contents of maleic and oxalic acids compared with the other treatments. Lee et al. [75] showed that oxalic acid has a strong ability to catalyze hemicellulose hydrolysis. Wang et al. [76] also proved that oxalic acid has a more substantial effect on the decomposition of maize straw than propionic acid. Maleic acid is a dicarboxylic acid with a good catalytic effect, which can effectively improve the biodegradability of lignocellulose by releasing H⁺ to depolymerize hemicellulose and cellulose [77]. These organic acids were conducive to the degradation of maize straw lignocellulose in the MT sample.

4.3. The Influence of Secondary Phyllosilicate Minerals on the Decomposition Products of Maize Straw

The hemicellulose, cellulose, and lignin in maize straw are decomposed by microorganisms, which can be converted into low-molecular-weight compounds and then condensed and polymerized to form humus [78]. This experiment showed that adding secondary phyllosilicate minerals could increase the content of HE. Among them, the content of HE was the highest in the MT sample, and the MT was beneficial to the formation of HA.

Secondary phyllosilicate minerals can combine soil OC through various mechanisms such as ligand exchange, electrostatic attraction, and π bonds, making it easier to resist microbial mineralization [79] and thereby producing more stable OC, which is conducive to the synthesis of humus. Therefore, the addition of secondary phyllosilicate minerals in this study increased the content of HE, which is consistent with previous research results [80]. Miura et al. [81] demonstrated that secondary phyllosilicate minerals can shorten the time required for dark matter formation and increase the number of humic acid polymers that are extracted by alkali. Fukuchi et al. [82] also found that secondary phyllosilicate minerals can significantly promote the condensation reaction of humus precursors.

In this study, montmorillonite significantly increased the content of HE compared with other secondary phyllosilicate minerals due to its distinct binding mechanisms with organic matter. The organic matter in montmorillonite is rich in more stable aromatic compounds [83]. This binding mechanism is conducive to the formation and stability of humus. At the same time, the surface charge of montmorillonite is dominated by the permanent negative charge that is generated by isomorphous substitution [84], which allows it to adsorb more OC and thereby promotes the formation of humus. We obtained similar results to those of Li et al. [85]. When different minerals are mixed, different effects are produced. The mixed minerals provide different surface properties and more active sites [86]. Therefore, the amount of extractable humic-like substances that are produced will also be different.

In addition, except for the Mix sample, the PQ values of all treatments were higher than that of the CK sample. These higher PQ values suggested a higher degree of humification and a more complex molecular structure [87]. Thus, our results indicated that adding secondary phyllosilicate minerals increased the degree of soil humification. The ΔlgK values of HA in the MT, IL, Verm, and Mix samples decreased, and the ΔlgK values of FA in the Verm and Mix samples were higher than in the CK sample, indicating that the addition of secondary phyllosilicate minerals increased the degree of oxidation and aromatization of HA, and that the structure was more complex. The FA in the vermiculite and mixed components had more aliphatic structures and simpler molecular structures, which promoted the renewal of soil humus.

4.4. The Effect of Secondary Phyllosilicate Minerals on the Formation of Organic–Mineral Complexes During Maize Straw Decomposition

This study indicated that the addition of secondary phyllosilicate minerals contributed to an increase in the content of organic–mineral complexes, with montmorillonite leading to the most significant increase in the content of organic complexes. The high specific surface area of secondary phyllosilicate minerals provides numerous contact points, which increases the likelihood of contact between organic molecules and the secondary phyllosilicate minerals’ surfaces [88]. This characteristic dramatically enhances the ability of secondary phyllosilicate minerals to adsorb organic matter, which is conducive to forming organic–mineral complexes. Additionally, the negative charge on the surface of secondary phyllosilicate minerals adsorbs positively charged organic molecules through electrostatic interactions, and this cation exchange capacity is also a key mechanism in forming organic–mineral complexes [7]. Therefore, the addition of secondary phyllosilicate minerals increased the content of organic–mineral complexes.

Montmorillonite and vermiculite have high surface areas and cation exchange capacities, giving them strong adsorption capacities for organic substances. However, vermiculite has a lower basal spacing and a lower tendency to expand than montmorillonite [89]. Montmorillonite exhibits considerable expansion properties, allowing its interlayer spaces to absorb organic molecules [90]. Organic materials are intercalated into the interlayers of montmorillonite by means of cation exchange, forming more organic–mineral complexes. The specific surface area and adsorption capacity of illite are relatively low [89], and the low specific surface area limits the contact area between the illite and organic matter, thus affecting the formation of organic–mineral complexes. Bronick et al. [91] showed that the specific surface area, cation exchange capacity, and expansibility of clay minerals influence the formation of soil aggregates. Fernandez-Ugalde [90] also found that different clay minerals have different abilities to form soil aggregates, with montmorillonite being more capable of complexing with organic matter to form micro-aggregates than illite. Montmorillonite forms stable chemical bonds with organic matter through cation bridges (Ca²⁺, Mg²⁺). This chemical bonding not only enhances the adsorption capacity of organic matter but also significantly improves the stability of organic–mineral complexes [92]. Gao’s [93] research also confirmed this viewpoint. Illite forms robust chemical bonds with organic matter through hydrogen bonds, which enhances the stability of the combination of organic minerals [94]. Although vermiculite has a large specific surface area and high cation exchange capacity, the interactions between vermiculite and organic matter are primarily achieved through physical adsorption and ion exchange, rather than the formation of stable chemical bonds to fix organic matter [95]. Consequently, vermiculite struggles to form stable organic–mineral complexes in the soil, which are more susceptible to disruption by environmental factors. This makes vermiculite less effective than montmorillonite and illite in the formation and stability of soil organic–mineral complexes.

The humic substance content can influence the formation of organic–mineral complexes in soil. Consequently, in this study, the MT sample, which had an elevated extractable humic-like substance content, also exhibited a higher total content of organic–mineral complexes on account of the extensive siloxane surface of montmorillonite, which is prone to wrinkling when exposed to a certain level of soil moisture, conducive to the adsorption of small, hydrophobic humus particles [96]. This increased the content of organic–mineral complexes in the soil. Secondary phyllosilicate minerals can increase the content of organic–inorganic complexes in soil, which is essential for improving the quality of black soil and enhancing its fertility.

Our findings indicated that incorporating secondary phyllosilicate minerals was advantageous for enhancing the OC content within the G0 complex, which may be because minerals can induce changes in the microbial community structure and soil matrix content, thereby affecting the extracellular enzyme activity and increasing the OC content [97]. In the G2 group, the OC content was highest in the MT sample. This can be explained by the presence of Al³⁺ ions in the aluminum–oxygen octahedron layer of the montmorillonite crystal structure, which primarily replace Mg²⁺ ions. To maintain electrical neutrality, cations with larger radii, including iron ions, are adsorbed between the crystal layers [31]. Montmorillonite has relatively high contents of iron and aluminum ions, which give it a strong surface-bonding capability. Aluminum ions exhibit a significant bonding effect on fresh organic matter. Since most of the organic matter that is derived from straw decomposition is considered fresh, the bonding effect of aluminum ions becomes particularly prominent. Consequently, the OC content in the G2 group of the MT sample was the highest.

5. Conclusions

Montmorillonite, illite, and mixed minerals were beneficial to the degradation of maize straw lignocellulose. Among them, montmorillonite significantly promoted the degradation of hemicellulose and lignin. In contrast, vermiculite decelerated the degradation of hemicellulose and lignin. During the degradation of the maize straw, lactic acid and malic acid were the primary organic acids in the soil, and the contents of lactic acid and malic acid were the highest in the MT sample. Montmorillonite, illite, vermiculite, and mixed minerals increased the soil’s extractable humic-like substances, with montmorillonite showing the most significant increase. Montmorillonite was beneficial to the formation of HA, while mixed minerals notably elevated the content of FA. Humus can form a stable organic–mineral complex by interacting with mineral particles, in which montmorillonite increased the contents of the G0 water, G1 sodium ion, and G2 sodium grinding dispersion groups and the G0 + G1 + G2 total complex. The addition of secondary phyllosilicate minerals enhanced the OC content within the complexes of the G0, G1, and G2 groups, with montmorillonite leading to the most significant increase in OC content. Overall, this study enriches the research on interactions between organic matter and secondary phyllosilicate minerals, providing a significant reference for improving the quality of black soil in northeast China.