A Physicochemical and Biological Composite Environment for the Humification of Crop Straw in Compost

Abstract

To examine the humification characteristics of crop residues, three types of crop residues—wheat, maize, and rice—were selected for experimentation. The latter two were subjected to steam explosion (SE) and nitrogen addition SE, respectively. Each of the three types of straw was inoculated in accordance with a specific microbial addition protocol. Furthermore, environmental controls, including the oxygen concentration and temperature type, were employed to enhance the variability of the samples. The results demonstrate statistically significant discrepancies in the concentrations of humic acid (HA) and fulvic acid (FA) between the three samples. Following a 96 h composting process, the highest concentrations of fulvic acid (FA) and humic acid (HA) in the samples were recorded at 10.4% and 15.8%, respectively. This result indicates the successful conversion of crop straw into a high-quality humic acid fertilizer product. Infrared spectroscopy data indicate that the breaking of carbon–carbon bonds and the ring opening, as well as the substitution of benzene rings, occurred during composting, and FA and HA substances were gradually formed. The results indicate that humus production is subject to raw materials, the pre-treatment of materials, the differentiation of strains, and methods of controlling environmental factors.

1. Introduction

China is the world’s leading food producer. As such, it produces 700 million tons of straw per year. Of these, more than 90% of the total annual crop straw production includes rice straw, wheat straw, and maize straw [1,2]. With the rapid increase in modern farming practices, straw use has become a focal point for resource and environmental issues [3].

Crop residues are biologically antagonistic, which is a self-protection mechanism of organisms, so it is difficult for a single chemical or biological method to break through the cell wall structure of plants to achieve various biotransformation and chemical uses [4,5,6]. The composting of crop residues is an effective conversion method, especially under certain physicochemical and biological conditions, and the active ingredients of crop residues can be converted into stable humus (HS), which can not only be used as agricultural fertilizer but also improve the ecological environment of the soil and realize the green and recycling development of agriculture [6,7,8,9]. The primary components of humus are humic acids, both fulvic and humic, although they are typically collectively referred to as humic acids when the two are combined [10].

However, humus production is influenced by a variety of physical, chemical, and biological factors, and the amount of humus produced by conventional composting methods is very low. Geochemical studies have shown that under natural conditions, it takes 300–500 years for the soil humus to build up to a thickness of 1 cm [11,12,13,14].

Lignite is rich in humic acid, and scientific research has shown that this is because the formation of humic acid requires a special mechanism [15,16]. Theories of HA (humic acid) production include lignin polyphenol theories, microbial synthesis theories, glycosaminoglycan synthesis theories, cellular auto fusion theories, etc. [17,18]. All of the above theories are strongly related to the physicochemical and biological environments that influence the lignocellulosic composition of crop residues [19,20,21].

The drivers of HA production in composting are therefore of interest to the industry [16,21,22]. As straw is a lignocellulosic material that is difficult to chemically degrade, many pre-treatments are required to modify the structure of the plant cell walls to enable microbial utilization of the material [13,18]. The SE (steam explosion) pre-treatment method is a promising approach to produce humic products through composting, as its process allows for the effective and rapid degradation of lignocellulosic components, as well as the complexation of amino groups through the addition of nitrogen. In addition, novel complexation can be accomplished through the residual carbon skeleton during the explosion process, which can facilitate the production of humic precursors [23].

Microorganisms and environmental factors are also important to effectively increase the humus content after composting crop straw. Related studies have shown that high or low oxygen concentrations can be detrimental to the production of humic acid, which is like the formation process of coal [15,24]. However, coal formation takes millions of years and is accompanied by the production and weathering of humic acid. We now hope to be able to study and simulate the production of humus using appropriate means, and we hope to shorten the time of this process.

To address scientific issues, this study employed physical methods such as ammonization and SE to pre-treat a variety of crop residues. The above samples were then inoculated with specific microorganisms, and the environmental factors were altered to enhance the comparability of the samples. The effects of these samples on the evolution of HA were analyzed through a comparison of the chemical indexes and physical shapes of the above samples in the composting process. Through this analysis, we aimed to obtain a scientific understanding of the impact of physical and chemical pre-treatments on the degradation of crop straw resources and the transformation of biological compost.

2. Materials and Methods

2.1. Raw Materials

Wheat, maize, and rice straws were collected from Zibo, Shandong Province, within a week of harvesting. They were then subjected to a pre-treatment process involving ammonia and SE, after which they were tested for their chemical compositions.

2.2. Experimental Design

Three types of crop residues, namely wheat, maize, and rice, were employed as the base material. The latter two were subjected to SE and ammoniated by SE, respectively. The materials were then subjected to microbial inoculation in accordance with Table 1, with the strains used being Aspergillus Niger CICIMF 0410, Phanerochaete Chrysosporium AF 96007, and Candida CICC 319495. Additionally, the oxygen concentration and temperature were designated as environmental control factors, as outlined in Section 2.3.2. Each row of Table 1 represents a single experimental sample, and no crossover between factors was permitted. The experimental process is delineated in Figure 1.

2.3. Material Pre-Treatment and Microbial Cultivation

2.3.1. Amination

A total of 50 kg of each crop straw was collected, dried, and subjected to ammonia pre-treatment of rice straw in accordance with the experimental design scheme. The chemical used for ammonification was urea (Sinopharm Group Co., Ltd., Beijing, China), which was added in a 5% mass ratio of dry matter. The samples were then placed in a plastic container and sealed. Ammonification was carried out at an indoor temperature for seven days, followed by SE of the samples and maize straw and chemical composition testing of the materials.

2.3.2. SE Pre-Treatment

The methodology employed in this study is based on the SE technique described in reference [25]. The wheat straw, maize straw, and ammoniated rice straw were transferred to a high-pressure SE apparatus (qbs-80, Hebi Zhengdao Bioenergy Company, Hebi, China), whereupon superheated water vapor at a pressure of 2.0 MPa was injected to initiate the explosion.

The core chamber of the SE comprises two distinct components: an outer furnace-like shell that serves as a seal, and an inner core that can be moved up and down. The bottom of the core is sealed, while the top of the mouth can be opened and closed. This is for the purpose of facilitating the addition of material in a closed state. The side of the core body is a non-sealing cylindrical fence structure, which allows the piston rod below, driven by the air compressor, to send high-pressure air to push the core body in the upper position, thus facilitating the addition of material. Upon the completion of the charging process, the top cover of the core is closed, and a superheated water steam is passed through the steam control valve. The electronic control device regulates the unloading of air from the air compressor via a solenoid valve after a predetermined duration has elapsed, thereby enabling the piston rod to retract. Concurrently, the descent of the core body permits the lower section to interact with the external environment, resulting in a pressure differential between the upper and lower regions of the core body. This gravitational potential energy contributes to the expeditious opening of the SE chamber and the prompt release of the SE process.

The descent of the cylinder is facilitated by the spring positioned at the base of the cylinder, which serves to cushion the process and ensure a smooth and secure SE operation. The control system is primarily operated by the solenoid valve, which is in a non-electrified state when the compressed air is directed against the piston rod. This configuration allows the equipment to be in an upper feeding state, whereby the SE chamber and the atmospheric environment are connected. When the solenoid valve is in an open position, the ruptured core body is situated in the upper position, thus creating a sealed state.

The generation of superheated steam is facilitated by a water tank, a steam generator, and other analogous apparatus. A pressure gauge is situated at the front and rear of the steam control valve, allowing for the timely monitoring of the pressure within the SE chamber and the steam generator delivery pressure. This enables the prompt detection of any leakage or other issues. The steam generator is equipped with a pressure relief valve, which allows for the control of the maximum pressure at 4 MPa. The lower seal of the SE cylinder is constructed using a cone seal comprising a PTFE material. The upper seal is cylindrical in shape and made of silicone. All electrical control switches are integrated into a single control panel. The liquid resulting from the explosion is also collected and employed in the subsequent composting process.

The device’s operational mechanism is illustrated in Figure 2. Prior to operation, the top cover must be opened and the raw materials inserted. Subsequently, the lid is to be closed and superheated steam is to be injected. Following exposure of the material to elevated temperatures and pressures as per the experimental design for a designated period, the lower chamber is to be opened abruptly to induce rapid material ejection.

2.3.3. Microbial Culture

In this study, the culture methods and composition of three different microorganisms, Aspergillus Niger CICIMF 0410, Pseudomonas aeruginosa AF 96007, and Candida CICC 319495 (China General Microbiological Culture Collection Center), were consistent with those described in the studies by [26,27], respectively. The utilization of these organisms is predicated on their demonstrated capacity to facilitate the decomposition of cellulose and lignin, respectively. Aspergillus niger and Pseudomonas aeruginosa can rely on their own biological enzymes for this purpose. Furthermore, Candida aerobica, an aerobic fungus, can synthesize amino acids with high efficiency, which serve as precursors to humus formation. The concentration of the fungus solution is 1 × 10⁸ CFU mL⁻¹.

2.4. Environmentally Controlled Composting

The composting process occurs within a self-made cylindrical receptacle with a bottom diameter of 1.5 m and a height of 1.5 m. The box was placed in a chamber where the temperature could be controlled, the container was continuously aerated using an air pump, and the microbial suspension was to be inoculated at a concentration of 1 cm³ kg⁻¹. During the composting process, the requisite moisture was incorporated to maintain a humidity level of 60%.

The oxygen concentration and temperature were set as environmental control factors. Oxygen was supplied by an oxygenating pump through a conduit with a high oxygen supply of 1 L/h, while variable oxygen was used by alternating between 1 L/h and 0.1 L/h every 1 h. Temperature control was achieved in a constant temperature chamber, with a constant temperature control of 28 °C. Variable temperature control was employed to simulate the four phases of the composting process, with the temperature gradually increasing to 65 °C within 10 days. This temperature was then maintained for 7 days. Thereafter, the temperature was gradually decreased to ambient temperature within 10 days for a total duration of 90 days.

2.5. Characterization Methods

2.5.1. Chemical Composition

The lignocellulose fraction (comprising the polysaccharides lignin, cellulose, and hemicellulose) was quantified by the method of Van Soest, as described in reference [27].

The extraction of HA and FA was achieved by utilizing a combination of reagents, including sodium hydroxide, sodium pyrophosphate, and potassium dichromate. The concentration of these acids was subsequently determined using ferrous sulfate in accordance with a method described in detail in the NY/T 1867 [28,29]. The reducing sugars and amino acids present during the humification process were quantified using a UV spectrophotometer (759S Shanghai Jing Hua Technology Instrument Co., Ltd., Shanghai, China). The experimental reagent employed was dinitro salicylic acid (Sinopharm Group Co., Ltd., Beijing, China). The solution was boiled, cooled, and diluted with deionized water to within the range of the standard solution. The absorbance of the reducing sugar concentration was measured at 540 nm. The amino acid concentration was obtained from the absorbance measurement at 280 nm [30].

2.5.2. Physical Characteristics

The identification of straw morphology is conducted through the utilization of scanning electron microscopy (ZEISS EVO MA 10, ZEISS Co., Ltd., Oberkochen, Germany). The resolution was 100 nm@20 KV (secondary electronics). The magnification was 15–10,000. The acceleration voltage or landing voltage range was 7 kV~30 kV, 10 V step, and continuously adjustable. The probe current had a maximum current of ≤100 nA. It can accommodate a maximum sample diameter of 200 mm and a maximum sample height of 100 mm. The five-axis central motor drives the stage, with the following maximum travel range indicators: X = 80 mm, Y = 100 mm, Z = 35 mm, a tilt range of 0°–90°, and rotation ≥360° (continuous rotation). The instrument utilized for the measurement of infrared spectroscopy is the IS10 (Nicolet Bankshares Inc., Madison, WI, USA), which possesses a spectral coverage range of 7800–350 cm⁻¹, with spectral accuracy of <0.4 cm⁻¹ and wavenumber accuracy of <0.01 cm⁻¹. Prior to scanning, the samples were subjected to drying at a temperature of 105 °C for a duration of 2 h. The instrument automatically scans each sample 16 times in succession, and subsequently, the material base is manually rotated 90° to facilitate continued scanning. Following a single rotation, the software provided by the instrument calculates the mean of the four curves as the final result.

2.5.3. Statistical Analysis

All of the experiments were performed in three replicates, and the results are expressed as the mean ± standard error. The least significant difference (LSD) test was analyzed using SPSS 22.0 (IBM Inc., Amonk, New York, NY, USA) software.

3. Results and Discussion

3.1. Change in Chemical Properties of Samples

In combination with Table 1 and Table 2, the initial hemicellulose content of wheat straw was slightly lower than that of maize straw and rice straw, and after composting, the hemicellulose of the latter two was fully utilized. There was a significant difference in the cellulose content of maize straw after SE compared to the other two samples. This may be due to the nature of the maize straw itself. Combined with the mass loss of dry matter after composting, it was found that the difference in cellulose utilization between S_M-SE and S_R-SE-N after composting was no longer significant.

The performance of the three samples for lignin was statistically significantly different both for the initial samples and after composting. Regarding the reducing sugar (RS) content, the highest of the three initial samples was the S_M-SE sample, followed by S_R-SE-N and S_W. This shows that not only does the material determine the RS content, but also the SE pre-treatment significantly increases the reducing sugar content of the material. After composting, there was no longer a significant difference between the reducing sugar contents of the two groups of samples pre-treated with SE. Moreover, they were closer to the values of S_W. Although the significant performance of AA before and after composting of the three samples was consistent, we believe that the dominant factor of this variability is the result of the dual effect of the composition of the material and SE.

No FA and HA were detected in the wheat straw of the original sample variety, and only trace amounts of FA and HA were present in the S_M-SE samples, which showed macroscopic values after composting, especially the latter, which was very significant. The S_R-SE-N sample contained some FA immediately before composting, but only trace amounts of HA. After composting, it had the highest FA and HA values of the experiment. The significance of these results is attributed to the combined effects of microbial inoculation programs, environmental control factors such as variable oxygen and temperature, and physical pre-treatment methods.

3.2. SEM

Electron microscopy is an important method for observing the physical morphology of wood fibers [28,31,32].

Figure 3a–c show the initial micromorphology of wheat, maize, and rice straw without composting and without any pre-treatment, respectively. As can be seen from the figure, all three crop straws show dense physical structures at the scale of 1 μm, and there are no significant differences between them.

The SEM morphology of the three samples, S_W, S_M-SE, and S_R-SE-N, after 30 days of composting is shown in Figure 3d–f. A comparison of the images reveals the following information. The original flat and bundled fiber morphology of sample S_W became granular, and the underlying plate-like structure was largely unobservable. Sample S_M-SE also showed many punctate structures, but the original fiber morphology could still be observed. Sample S_R-SE-N, on the other hand, showed a bulbous follicle-like structure, with the previous groove shape and hard structure boundaries completely disappearing.

The formation of the initial two phenomena can be attributed to the efficient digestion of hemicellulose and cellulose by Aspergillus Niger and Phanerochaete chrysosporium, which leave behind chain-like lignin structural bodies. The mixed polymicrobial species decomposed the samples more thoroughly, as shown by the morphology of the S_R-SE-N samples after composting. A comparison of the microstructural morphology of sample S_W and the other two samples shows that although SE promotes initial lignocellulosic fiber disintegration, complete lignocellulosic fiber degradation and HA formation may still depend mainly on the action of microorganisms.

3.3. FTIR

The scanned infrared spectra of the three samples after 30, 60, and 90 days of composting are shown in Figure 4.

The main functional groups of the S_W samples after 30 days of composting were as follows: 3447 cm⁻¹, representing O-H stretching vibrations; 2936 cm⁻¹ and 2083 cm⁻¹, representing the stretching vibrations of unsaturated C=C or C≡C or C-H stretching vibrations such as CH₃ and CH₂; 1639 cm⁻¹, representing C=C stretching vibrations; 1042 cm⁻¹, representing C-N bonds such as RNH₂, R₂NH, and R₃N, representing the compositions of proteins, nucleic acids, or amino acids; 1109 cm⁻¹, representing C-O stretching vibration; and 615 cm⁻¹, representing a substituent of the benzene ring, showing that the substitution of the benzene ring is taking place [33,34,35,36].

Subsequent to a 60-day period of composting, the alterations in the infrared spectrum of S_W samples became predominantly evident in the following manner: there was a C-H bending vibration at 1381 cm⁻¹; there was a benzene ring C-O ether bond and stretching vibrations of carbon bonds at 1248 cm⁻¹; and 771 cm⁻¹, 532 cm⁻¹, and 469 cm⁻¹ were benzene ring substituents, with a higher number of corresponding substituent groups [34].

The decrease in transmission of 3445 cm⁻¹ after 90 days of S_W composting was further reduced, and the content of the O-H stretching vibration increased, which may be the hydroxyl group of HAS. The 1406 cm⁻¹ group almost disappeared, indicating that the conversion of its amino component to HA was higher after 90 d [34]. The composted compost had a relatively lower concentration of benzene ring substituents, indicating that benzene ring activation was relatively reduced.

Compared to the S_W sample, the S_M-SE sample composted for 30 days had the following major changes in the infrared spectrum functional groups: absorption features at 2083 cm⁻¹ were not observed; a new 1391 cm⁻¹ group appeared, representing C-H bending vibration; and the 897 cm⁻¹ group appeared, representing a benzene ring substitution reaction. Additionally, the benzene ring substitution reaction at 602 cm⁻¹ was higher than that of contemporaneous sample S_W [35].

Sample S_M-SE was subjected to a 60-day composting process. The following functional groups merit particular attention: 2091 cm⁻¹, representing stretching vibrations of unsaturated C=C or C≡C, or C-H stretching vibrations, which have similar performance as S_W samples after 30 days of composting; 851 cm⁻¹, 755 cm⁻¹, 697 cm⁻¹, and 610 cm⁻¹, which are the substituents of the benzene ring and are more diverse and abundant [33].

After 90 days of composting, the 3442 cm⁻¹ group, representing O-H stretching vibration, and the 1640 cm⁻¹ group, representing C=C stretching vibration, can represent the HA component of the further synthesized macromolecule; the 470 cm⁻¹ group represents C-O stretching vibration. Although benzene ring substitution gives a small variety, the content is a little high [3].

Composting S_R-SE-N for 30 days showed the following main functional groups: 3437 cm⁻¹, representing O-H stretching vibration with a low content; 2923 cm⁻¹, representing C-H stretching vibration; 1641 cm⁻¹, representing C=C stretching vibration; 1051 cm⁻¹, representing C-H bond; and substituent groups of 560 cm⁻¹ and 472 cm⁻¹ representing benzene rings [34].

The following are the primary alterations observed on the infrared spectrum of S_R-SE-N after 60 days of composting: 3438 cm⁻¹, showing that the O-H stretching vibration content is higher than the same period value of S_M-SE; 2923 cm⁻¹, in which the C-H stretching vibration decreases. New peaks appear at 1380 cm⁻¹, 1325 cm⁻¹, 1247 cm⁻¹, 1054 cm⁻¹, 603 cm⁻¹, etc., where 1504⁻¹ is the typical absorption region of humic acid [35,36].

When S_R-SE-N was composted for 90 days, the main noteworthy functional groups were as follows: 3447 cm⁻¹, representing O-H stretching vibration, which decreased in content; 2931 cm⁻¹, representing C-H stretching vibration, which decreased; 1098 cm⁻¹, representing C-O stretching vibration; and 790 cm⁻¹ and 469 cm⁻¹, which are benzene ring substitutions [34,36]. Infrared data show more C=C bonding of lignocellulose in the early stages of composting, the emergence of HA components in the later stages, and then polymerization to the more polymerized HA components.

3.4. Statistical Analysis Results and Limitations

In Table 2, the chemical compositions of samples S_W, S_M-SE, and S_R-SE-N following 90 days of composting are presented. The analysis of variance demonstrated that the sum of FA and HA yields of the three samples after 90 days of composting was statistically significant (p < 0.05). At present, we can confirm that the different pre-treatment methods of the three samples caused this difference, as illustrated by the LSD test in Table 2.

The present study corroborates the hypothesis concerning lignin evolution at the origins of HA. Concurrent with the results, evidence suggests that the recently synthesized humic acid substances may be subject to a dynamic evolutionary process, wherein their generation and decomposition may occur in a concurrent manner. The results of the intergroup comparisons show that material, physical environment, and microbiological factors also have a strong influence on HA production.

However, given the inherent limitations of the experimental design, it is not possible to ascertain the extent to which the formation factors of this effect can be influenced by various factors, such as ammonification and steam explosion physical pre-treatments, and environmental factors, particularly the combined effect of the two pre-treatments under composting biological action on the sample.

In consideration of the complexities inherent in reaction theory concerning the genesis of humic acid, subsequent research endeavors may entail the refinement and decomposition of the experiments. Such refinement may encompass the study of lignocellulose degradation kinetics and the enzymatic activity of lignocellulose. Additionally, research may be conducted on total nitrogen, with a particular focus on the ${\mathit{NH}}_4^{+}$ form. From a biochemical standpoint, this biosynthesis of metabolic products from crop straw will be useful to explore the correlation between lignocellulose degradation and humus formation.

The authors’ previous research demonstrated that steam explosion pre-treatment exerts a positive effect on the direct degradation of lignocellulose in rice straw [30]. This study posits that it can also promote the production of high-quality humic acid in the subsequent composting process. Although this operation method is energy-consuming at the experimental level, it can maximize energy recovery in industrial implementation, such as hot water reuse. Therefore, the findings of this study are of considerable significance for the realm of industrial green recycling of agricultural waste resources [37,38].

4. Conclusions

The findings of this study suggest that it is challenging to efficiently augment the humus content following the composting of crop straw through microbial action alone. The application of ammonization, SE, and their combination as pre-treatment methods has been shown to enhance the humus content of crop straw in compost. Furthermore, environmental control factors should be considered in the process to improve the conversion rate of humic acid. In this experiment, the sample with the highest sum of HA and FA contents had a value of 26.2%, which already makes it a good quality humic fertilizer. This study not only indicates the direction for the comprehensive utilization of crop straw but also lays a research foundation for green recycling agriculture and ecological environmental engineering.

Author Contributions

X.Z.: Methodology, Formal Analysis, Data Curation, and Writing—Original Draft. B.L.: Writing—Review and Editing and Funding Acquisition. C.Z.: Conceptualization, Resources, and Funding Acquisition. F.A.C.: Methodology and Investigation. All authors have read and agreed to the published version of the manuscript.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The raw data supporting the conclusions of this article will be made available by the authors on request.

Conflicts of Interest

The authors declare no competing interests.

Footnotes

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Figures and Tables

Figure 1. Schematic diagram of experimental process and composting equipment.

Figure 2. Schematic diagram of steam explosion device operation.

Figure 3. Scanning electron microscope images of the samples (SEM). (a) Initial micromorphology of sample SW under scanning electron microscope; (b) Initial micromorphology of sample SM-SE under scanning electron microscope; (c) Initial micromorphology of sample SR-SE-N under scanning electron microscope; (d) Micromorphology of sample Sw under scanning electron microscope 30 days after composting; (e) Micromorphology of sample SM-SE under scanning electron microscope 30 days after composting; (f) Micromorphology of sample SR-SE-N under scanning electron microscope 30 days after composting.

Figure 4. Infrared spectra of different composting durations for SW (wheat straw sample), SM-SE (steam explosion pre-treated maize straw sample), and SR-SE-N (ammonization and steam explosion pre-treated rice straw sample).

Table 1

Experimental design.

Name	Straw Type	SE	N Addition	Strains	Oxygen	Temperature
SW	Wheat	−	−	P. C.	Constant	Constant
SM-SE	Maize	+	−	A. C.	Variation	Variation
SR-SE-N	Rice	+	+	A. P. C.	Variation	Variation

Note: S_W, S_M-SE, and S_R-SE-N are wheat straw, steam-exploded maize straw, and steam-exploded rice straw with added nitrogen. A. P. C. is represented by the three species Aspergillus niger CICIMF 0410, Phanerochaete chrysosporium AF 96007, and Candida CICC 319495.

Table 2

Chemical compositions of samples (%).

Chemical compositions of initial samples
Samples	HC	CL	LIG	RS	AA	FA	HA
SW	15.2 ± 0.03 a	39.7 ± 0.11 a	14.3 ± 0.18 a	0.72 ± 0.03 c	0.98 ± 0.16 b	- c	- b
SM-SE	19.2 ± 0.05 a	26.3 ± 0.10 b	6.12 ± 0.13 c	14.5 ± 0.05 a	3.89 ± 0.16 a	Trace b	Trace a
SR-SE-N	19.1 ± 0.07 a	38.9 ± 0.13 a	9.50 ± 0.16 b	9.52 ± 0.04 b	4.12 ± 0.02 a	0.02 ± 0.01 a	Trace a
Chemical compositions of samples after 90 days of composting
Samples	HC	CL	LIG	RS	AA	FA	HA	Mass loss
SW	3.20 ± 1.37 a	23.5 ± 3.88 a	12.6 ± 2.02 a	3.19 ± 0.51 a	2.67 ± 0.62 b	0.11 ± 0.05 c	0.17 ± 0.09 c	20.2 ± 2.25 a
SM-SE	1.32 ± 0.2 b	13.7 ± 3.65 b	4.50 ± 1.58 c	2.01 ± 0.33 b	6.39 ± 0.55 a	6.20 ± 1.26 b	10.3 ± 2.13 b	18.7 ± 2.38 a
SR-SE-N	1.39 ± 0.15 b	12.1 ± 3.43 b	7.21 ± 1.96 b	2.62 ± 0.29 b	6.36 ± 0.51 a	10.4 ± 2.11 a	15.8 ± 3.27 a	18.8 ± 2.50 a

Note: S_W, S_M-SE, and S_R-SE-N are wheat straw, steam-exploded maize straw, and steam-exploded rice straw with added nitrogen. HC, hemicellulose; CL, cellulose; LIG, lignin; RS, reducing Sugar; AA, amino acid; FA, fulvic acid; HA, humic acid. Trace refers to a content below 0.01%, and “-” means the content cannot be detected. The data were statistically analyzed using the LSD method. The letter in the upper right corner of the number indicates the level of significance, with different letters denoting significant differences.

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