The Addition of Exogenous Compost Humus Shortens the Composting Cycle of New Corn Stalks, Thereby Promoting Plant Growth

Abstract

The treatment of straw biomass has always been a global issue. Although composting processes a large amount of straw biomass as a low-cost technology, its low efficiency has resulted in redundant stores of straw biomass. However, the humus in compost not only has an impact on soil fertility, but also has important effects on the functioning of microbial ecosystems. Meanwhile, the colloidal properties of humus can enhance the water and fertilizer retention capacity of compost, creating a suitable living environment for microorganisms. This study aimed to address the problems of the long composting periods and low maturity efficiency of corn straw by exploring the regulatory effect of exogenous compost humus on the composting process of corn straw and its promoting effect on plant growth. Composting treatment groups were set up with the addition of exogenous humus to systematically monitor the temperature changes, dynamic formation of humus, and change in microbial community during composting. The effects of compost products on corn growth were also analyzed through pot experiments. The results showed that the intervention of exogenous humus can significantly accelerate the composting process of corn straw, extend the traditional composting heating period by 8 days, and increase the humus content by 27.98 g/kg. It also increased the content of organic functional groups in the humus and significantly promoted the growth of corn, increasing its nitrogen content by 5.11 g/kg and increasing plant height and leaf length by 5 cm. This provides a new technical path for the efficient utilization of composting for corn straw. The treatment of agricultural waste and the production of high-quality organic fertilizers will promote the development of green agriculture.

1. Introduction

As the main form of waste in agricultural production, the resource utilization of corn stover has always been a pressing topic in agricultural ecological research [1]. Traditionally, a large amount of corn stover is often burned outdoors due to the lack of efficient treatment methods, which not only wastes biomass resources but also generates a large amount of pollutants such as smoke, sulfur dioxide, and nitrogen oxides. These gases exacerbate air pollution and pose a serious threat to the ecological environment and human health [2]. The lignocellulosic biomass in corn stover is of great significance in resource utilization due to its sustainable characteristics. Corn stalks, as a form of agricultural waste, contain abundant organic matter and minerals, enhancing the sustainability of agricultural systems [3]. The humus formed after straw decomposition can improve soil fertility and enhance water and fertilizer retention capacity, especially for the remediation of compacted soil and saline alkali land. The question of how to utilize corn stover resources safely and effectively has become a key issue that urgently needs to be solved for the green development of agriculture. Composting, as a primary method to obtain corn stover resources, is widely used due to its low cost, easy operation, and environmentally friendly characteristics [4]. The composting of products can decompose organic waste through microorganisms, converting carbon and nitrogen elements into inorganic salt forms that plants can absorb, supplementing key elements such as nitrogen, phosphorus, and potassium [5]. Nitrogen promotes stem and leaf growth, phosphorus enhances root development, and potassium enhances disease and pest resistance. Increases in soil organic matter improve water retention and permeability, and accelerate soil nutrient cycling. This fertilizer effect is long-lasting and stable, supporting long-term plant growth by continuously releasing nutrients. However, due to its high C/N ratio and high content of lignocellulose, corn stover has a traditional composting cycle of 45–60 days and low maturation efficiency, limiting its large-scale application [6].

Humus is a type of macromolecular organic matter formed by the complex decomposition and re-synthesis of animal and plant residues under the action of microorganisms, and it exists widely in soil, water, compost, peat, and other environmental media [7]. As one of the largest organic carbon pools on earth, humus not only has an important impact on soil fertility and ecosystem functions, but also plays a key role in pollutant adsorption, carbon and nitrogen cycles, and climate change regulation [8]. Humus, as the final product of the composting process, plays an important role in regulating the microbial community and accelerating the degradation of organic matter [9]. Previous studies have shown that active functional groups (such as phenolic hydroxyl and carboxyl) in humus can be used as electron shuttles for microbial metabolism to promote the decomposition of refractory organic compounds [10]. At the same time, the colloidal properties of humus can improve the water and fertilizer retention ability of compost and create a suitable living environment for microorganisms [11,12].

Composting refers to the process of microbial solid-state fermentation and humification [13]. By significantly increasing the input of exogenous humus into the compost materials in the early stages of the process, the degree of initial humification can be improved. In addition, this humus contains a large, stable carbon reserve, which microorganisms can use in the early stages of composting to promote microbial activity [14]. It can also serve as an adsorbent, reducing nitrous oxide emissions and achieving the synergistic control of multiple pollutants. However, the specific regulatory mechanism of exogenous humus on microbial community succession in the process of corn straw composting and its correlation with a shortened composting cycle has not been proven.

Therefore, this study used corn straw as a raw material and systematically analyzed the effects of exogenous humus on the temperature dynamics, organic matter degradation, and humus formation that occurs during corn straw composting through composting and pot experiments. We investigated how exogenous humus regulates microbial communities during composting and how improved compost products promote maize growth and nutrient absorption. The research results provide a theoretical basis and practical guidance for the development of highly efficient composting technology for corn straw.

2. Materials and Methods

2.1. Materials

The corn stalks used for composting were obtained from the experimental field of Jilin University in Changchun, Jilin Province. Using natural air drying to maintain the moisture content of the corn stover at 15%, stalks were crushed into a small round block of 2–3 cm for composting experiments [15]. The corn stover contained 34.23% carbon and 0.66% nitrogen. Animal feces were obtained from a cattle farm in Shuangyang District, Changchun City, Jilin Province. The cow manure contained 19.46% carbon and 0.97% nitrogen. The exogenous humus was obtained from a 30 d compost of corn stover and cow manure [16].

2.2. Compost Design

Solid-state composting experiments were conducted in a 53 cm long, 37 cm wide, and 29 cm high foam box. A vent was opened in the right side of the box, and connected to a small blower through a 3 cm small hole. A digital thermometer was used to monitor temperature. Two solid-state design composting experiments were performed in total, one consisting of ordinary compost made from corn stover and cow manure (CK) as raw materials, and the other using corn stover, cow manure, and exogenous humus (CH) produced from previous composting (Figure 1). The total mass of the material was 10 kg and the carbon-nitrogen ratio was set to 25:1 [17]. The composting takes 30 d and the samples are collected every 2 d.

2.3. Experimental Methods

A digital thermometer (WST-102/101, Sanchang, Changsha, China), pH meter (MH-P20, Minghui, Shanghai, China), moisture detector (MQ-SCTR, MQIAO, Shenzhen, China), and conductivity meter (B1040, DLT, Foshan, China) were used for the accurate measurement of reactor temperature, pH, moisture content and conductivity [18]. A total organic carbon analyzer (LJ-TOC10A, LANJING, Hangzhou, China) was used to assess the carbon content (TOC) of compost samples. The total organic nitrogen content (TON) was determined by the acid hydrolysis method [19], and humus components were determined using the Na₄P₂O₇-NaOH separation method with Na [20]. A TOC analyzer determined the carbon content of each component. Compost quality was assessed by Fourier transformation infrared spectrology (FTIR, Nolay-50, Norexinda, Tianjin, China), nuclear magnetic resonance spectroscopy (NMR, AVANCE NEO 500MHz, Bruker, Leipzig, Germany), and excitation emission matrix spectroscopy (EEM, F-4700, Techcomp, Osaka, Japan) [21]. A total of 20 carbon seeds were used for germination, with additional centrifuged CK and CH compost samples, and each treatment was repeated 3 times. The germination rate was monitored over 7 d [22].

Total DNA was extracted using the rapid DNA APIN extraction kit (MP Biomedical, Santa Ana, CA, USA) and stored at −80 °C. 16S rRNA was amplified via PCR for bacterial community analysis. The library was sequenced on the Illumina HiSeq platform [23].

2.4. Pot Experiment

Soil was derived from Jilin University Changchun Experimental Base for the potted plant experiments. The carbon content was 14 g/kg and the nitrogen content was 0.71 g/kg. We mixed a total of 5 kg of soil and 2 kg of compost products evenly, and adjusted the initial moisture content to 60%. The temperature in the artificial climate chamber was set to 25 °C, and the humidity was set to 50%.

2.5. Statistical Analysis

Origin Pro 2021 software was used for image visualization processing. Statistical analysis was conducted using IBM SPSS version 28.0 software, with a significance level of p < 0.05 [24].

3. Results and Discussions

3.1. Physicochemical Properties of Compost

Temperature is the core regulatory factor in the composting process, and it directly affects microbial activity, organic matter decomposition efficiency, environmental safety, and the decomposition process [25]. The dynamic analysis of composting is helpful in observing this process. The temperature in the overall composting process shows a trend of first increasing and then decreasing (Figure 2a). The CK rose to the highest temperature of 59.8 °C on the 4th day and maintained this until the 9th day before starting to decline. On the 17th day, the temperature dropped to 31.6 °C and the compost entered a maturing stage. On the 30th day, it dropped to 24.1 °C. The CH reached a high temperature of 57.6 °C on the second day and a maximum temperature of 62.8 °C on the eighth day. This process lasted for 10 d, and the temperature dropped to 43.5 °C on the 12th day, marking the beginning of the cooling period. On the 20th day, the temperature dropped to 30.1 °C, entering the maturing period again, and on the 30th day, it dropped to 21.8 °C, indicating that composting had ended [26].

pH is an extremely critical parameter in the composting process, playing a decisive role in efficiency, microbial activity, nutrient retention, final product quality, and the potential environmental impacts of the whole composting process [27]. Dynamic monitoring of pH throughout the composting process revealed a trend of first decreasing, then increasing, and then decreasing again (Figure 2b). The final compost heap attains a neutral pH [28]. The CK continued to rise after dropping to 5.5 on the third day and rose to 7.32 on the 14th day. Finally, it remained at 7.02 on the 30th day. The CH decreased to 4.86 on the 6th day and increased to 8.69 on the 16th day. Finally, on the 30th day, it remained at 7.13. The moisture content of compost is a core parameter that affects the efficiency and quality of composting. It is important in regulating microbial activity, controlling nutrient retention and loss, and advancing fermentation processes. Except for the increase in moisture content during watering, it continued decreasing throughout the entire composting process. In terms of speed, the CH decreased faster than the CK (Figure 2c). Thus, the addition of exogenous humus can effectively promote the fermentation process and accelerate the rapid rise and fall of pH. It also promotes water scattering in the stack [29].

Conductivity is slightly higher in compost, which provides specific advantages [30]. Moderately increased conductivity can enhance ion diffusion efficiency in the compost stack, provide more abundant soluble minerals for microorganisms, and accelerate the mineralization process of organic matter [31]. Throughout the composting process, the conductivity of the CH remained higher than that of the CK, but both groups maintained the same rate of increase (Figure 2d). The CK increased from 2.24 ms/cm to 3.32 ms/cm, and the CH increased from 2.67 ms/cm to 3.77 ms/cm. This difference is caused by exogenous humus [32].

3.2. Compost Humus

The formation of humus in compost is a hallmark of compost maturity [33]. As an organic colloid, humus bridges the mineral parts of compost through hydrogen bonds and cations, forming stable aggregates [34]. In addition, humus has a huge specific surface area and negative charge, which means it can absorb some base ions in the compost heap while also buffering acid–base fluctuations. It also makes significant contributions to plant growth and microbial activation. The dynamic observation of compost humus shows that regardless of the treatment, the content of humus shows an overall upward trend [35] (Figure 3a). CK increased from 13.52 g/kg to 58.26 g/kg, and CH increased from 36.58 g/kg to 86.24 g/kg. Between them, CH showed a downward trend on the 5th day, dropping to 30.11 g/kg. Humus is organic matter in which simple substances can be quickly utilized and decomposed by microorganisms. The addition of exogenous humus promotes the rapid activation of microorganisms, leading to the accelerated decomposition of organic matter and a decrease in the humus content in the CH. However, in the subsequent maturation stage, the humus content of the CH is much higher than that of the CK, leading to a significant increase in its content [36].

Humus is formed from humic acid and fulvic acid, with humic acid being a large molecular condensate containing aromatic rings that can enhance soil aggregate structure and nutrient retention capacity [37]. Fulvic acid has a smaller molecular weight and contains more carboxyl groups, which can promote mineral dissolution and release trace elements [38]. Humic acid shows an overall upward trend similar to that of humic substances (Figure 3b). The CK increased from 8.1 g/kg to 45.25 g/kg, and the CH increased from 20.44 g/kg to 71.96 g/kg. The CH also showed a decreasing trend on the 5th day, dropping to 18.36 g/kg. Both humic and fulvic acid are influenced by the amount of humus formed (Figure 3c). The CK increased from 8.1 g/kg to 6.36 g/kg, and the CH increased from 2.45 to 5.36 g/kg. The CH group also decreased to 2.15 g/kg on the fifth day. This indirectly proves the formation of new humus. In addition, in compost, fulvic acid can be converted to humic acid, and the more frequently this occurs, the higher the degree of humification. The humification index represents this process through the ratio of humic acid to fulvic acid, and clearly expresses the degree of humification (Figure 3d) [39]. The CK increased from 4.01 to 7.11, while the CH increased from 8.31 to 13.43.

3.3. Characteristics of Compost Structure

Composting products can improve soil’s structure, physical properties, soil fertility and nutrient supply. Its structural features are also important. Fourier-transform infrared spectroscopy is one of the methods for identifying functional groups in compost samples (Figure 4a). Clear peaks were observed at 3440 cm⁻¹, 1623 cm⁻¹, and 1070 cm⁻¹. The two types of compost are rich in most aromatic and aliphatic structures [40]. The -OH vibration observed at 3440 cm⁻¹ was attributed to the formation of humus with a lignin structure after the depolymerization and recombination of lignin. The peak intensity of CH compost at this time appears to be slightly higher than that of CK compost. The addition of exogenous humus in CH increased its overall strength compared to CK. The C-O stretching led most of the aromatic groups in lignin to continue to vibrate, and aromatics in humus at the 1070 cm⁻¹ peak led to a CH compost rich in exogenous humus and thus stronger than CK [41]. At 1623 cm⁻¹, this is caused by the vibration of the aromatic skeleton of humus, indicating that humus has dissociated. And the carbonyl group of the acid also vibrates here, causing a decrease in pH. Accurate information on the transformation of functional groups and most organic components during the composting process can be obtained using 13C NMR (Figure 4b). The spectrum of compost products is divided into five basic functional groups, namely alkyl C (0–50 ppm), alkoxy C (50–112 ppm), aromatic C (112–163 ppm), amide C (163–190 ppm), and carbon C (190–215 ppm) [42]. There is a peak at 30 ppm, which is emitted by the -CH group, due to the combined actions of methyl, methylene, and aliphatic methylene groups. The strong signal at 56 ppm in the spectrum indicates the presence of the -OCH3 methoxy group. In addition, at 130 ppm, there is a signal emitted by a similar structure of lignin [43]. Here, the signal of CH is significantly higher than that of CK, indicating the CH compost’s higher degree of humification. Its aromatic C has a stronger binding affinity with other molecules. EEM can usually be used to determine the strength and quality of CK (Figure 4c) and CH (Figure 4d) compost products. The CH compost products have a wider range of fluorescent regions and an excitation wavelength range of 500–600 nm. It forms new humus using the benzene rings produced by lignin and exogenous humus. In addition, exogenous humus activated microorganisms for rapid transformation to enhance the unsaturated structure in composting, thus forming a more aromatic and compact structure in CH.

3.4. Microbial Characteristics

Microorganisms are the core driving force in the composting process, leading to the decomposition and transformation of organic matter as well as driving humus synthesis and stabilization [44]. We performed microbial dynamic analysis on the compost. For the analysis of PCA diversity at the phylum level (Figure 5a), it was found that there were significant differences in microbial diversity among the low-temperature, high-temperature, and cooling periods. During the low-temperature period, there was not much difference in microbial diversity between the two compost treatments, but during the high-temperature period, the difference between the two treatments was significant. By contrast, during the cooling period, microorganisms were very close to each other. The amount of exogenous humus added affects the type of microorganisms that are present during the high-temperature period [45]. We conducted a specific analysis on the horizontal types of microbial phylum. In terms of microbial species richness (Figure 5b) and cluster analysis (Figure 5c), it was found that Proteobacteri, Firmicutes, and Bacteroidota have a significant advantage [46]. During the low-temperature period, Firmicutes and Bacteroidota in the CK were higher than those in the CH, while Proteobacteria values were lower than those in the CH. This indicates that exogenous humus can promote Proteobacteri in the early stage, enabling it to utilize organic matter in the matrix for metabolic activities and the promotion of rapid compost heating. During the high-temperature period, Proteobacteri and Bacteroidota rapidly expanded, and at this time, the level of CH was significantly higher than that of CK. Proteobacteri significantly shortened the composting cycle by exhibiting a strong decomposition ability during the high-temperature stages and rapidly decomposing large organic molecules such as cellulose and lignin by secreting various enzymes [47]. Bacteroidota can efficiently degrade complex organic matter in compost, breaking down large-molecule organic matter into simple substances and accelerating the composting process. Proteobacteria can reduce the resistance of lignocellulose to degradation through oxidative attack, which creates favorable conditions for subsequent extracellular enzymatic hydrolysis processes. Bacteroidota can produce beneficial metabolites such as short chain fatty acids during the degradation of lignocellulose, which provide a foundation for subsequent biotransformation. This makes the CH process last longer during the high-temperature period. During the final cooling period, the abundance of Proteobacteri and Firmicutes was almost equal between the two groups, but Bacteroidota still had a high level in the CH. This shortened the maturation period of the CH, allowing it to convert more humus [48].

3.5. GI and Plant Growth

The application of compost products to plants is an important standard to test the success of composting. In terms of the seed germination rate (GI, Figure 6a), both groups reached 80% on the fifth day. The germination rate of CH was always higher than that of CK, reaching 93% germination by the end of the experiment. In a 20 d pot experiment (Figure 6b), the nitrogen content of the CK was 8.51 g/kg, with a plant height of 26 cm and a leaf length of 17 cm. The nitrogen content of the CH was 13.62 g/kg, with a plant height of 31 cm and a leaf length of 22 cm, which was higher than CK. These results indicate that compost products from the CH have a better promoting effect on plants.

3.6. Discussion

Other traditional composting methods use naturally selected microbial communities and have lower diversity [49]. The composting method of adding exogenous humus directly forms stable humus, promoting the rapid transformation of lignocellulose by microorganisms and enriching specific functional bacteria. Exogenous humus provides high-quality carbon and energy sources for composting microorganisms. Humus contains a large amount of easily degradable substances such as small-molecule organic acids, polysaccharides, and amino acids, which can be rapidly utilized by microorganisms, thereby stimulating the proliferation and metabolic activities of the microbial community. Firstly, the fast heating rate is related to the rapid activation of thermophilic bacteria growth via humic substances. Secondly, the colloidal properties of humus improve the physicochemical environment of the heap. Humus, as an organic colloid, has strong water and fertilizer retention and ion exchange abilities, which can maintain the appropriate moisture content and pH value of the heap and create a stable environment for microbial activity. This guarantees that microorganisms will perform their best functions. Finally, exogenous humus regulated the microbial community structure of compost and significantly increased the abundance of Proteobacteri, Firmicutes, and Bacteroidota; the synergistic effect of these functional bacterial communities accelerated the degradation of lignocellulose and the formation of humus. However, the decomposition of organic matter and humus promotes an increase in carboxylic groups, leading to a decrease in pH in the initial stage. The addition of humus may inhibit the activity of acidic bacteria (such as lactic acid bacteria) while promoting the proliferation of ammonia-producing microorganisms (such as Bacillus), accelerating the release of ammonia nitrogen and making the heap more alkaline. During the decomposition of humus, soluble salts such as K⁺ and Na⁺ are released, and the decomposition of organic matter is accelerated, generating small-molecule organic acids and inorganic salts [50]. Directly increasing the ion concentration and conductivity of the compost solution promotes the electronic conductivity of the heap, indirectly promoting microorganisms.

The promoting effect of compost modified with exogenous humus on maize growth is the result of multiple mechanisms working together. Firstly, the rich humus in the improved compost products directly affects the development of plant roots. Humus can stimulate the secretion of auxin via the plant roots and induce the formation of lateral roots and root hairs, thereby expanding the absorption area of roots. Secondly, humus indirectly promotes plant growth by improving soil structure and nutrient availability. As a binder of the soil aggregate structure, humus can combine single soil particles into aggregates, increasing soil porosity and aeration. Meanwhile, the complexation and ion exchange of humus enhance the availability of nutrients in the soil [51].

Compost containing exogenous humus promotes plant growth through various pathways. Composting products contain active functional groups such as carboxyl and phenolic hydroxyl groups, which chelate nutrients such as potassium, calcium, and magnesium to reduce loss, slowly release key nutrients such as nitrogen and phosphorus to avoid the nutrient waste caused by the excessive release of traditional fertilizers, and adjust the soil’s pH value to alleviate the stress of acidic or alkaline soil on plants [52]. Composting products, such as natural biostimulants, directly promote cell division and elongation. A particularly important aspect of this is the ability to enrich specific microorganisms, promote microbial plant interactions, and accelerate nutrient cycling.

In terms of agricultural waste treatment, this technology can shorten the composting cycle of corn straw by nearly half, significantly improve treatment efficiency, and is also suitable for promotion and application in areas with large straw production. Combining straw granulation technology can further enhance composting efficiency, increase the contact area with humus and microorganisms, and is expected to further shorten the composting cycle. The external humus composting scheme can still be carried out for different crop residues. The microbial community activated by this method can accelerate the transformation of recalcitrant components and significantly improve the degradation efficiency of lignocellulose in lignocellulosic residues. In addition, humus can balance the carbon nitrogen ratio and promote microbial activity. It can quickly process residual waste in high carbon to nitrogen ratio residues. In terms of organic fertilizer production, exogenous humus-modified compost has higher humic acid content and nutrient effectiveness and can be used as high-quality organic fertilizer for economic crops such as vegetables and fruits, improving the quality of agricultural products.

4. Conclusions

This study systematically investigated the effects of adding exogenous composting humus on the composting process of corn stover and plant growth. The composting cycle of corn stover was significantly shortened, increasing composting efficiency. This treatment extends the duration of high temperature in compost to 8 d and accelerates the composting process by regulating the microbial community structure. The addition of humus also significantly increased the abundance of Proteobacteri, Firmicutes, and Bacteroidota; promoted the degradation of lignocellulose and the synthesis of humus; and increased the humus content in compost products by 27.98 g/kg compared to the control. In addition, exogenous humus significantly promoted the growth and development of corn. This treatment resulted in an increase of 5 cm in maize plant height and leaf length compared to the control group, while the nitrogen content increased by 5.11 g/kg. Future research should focus on low-cost exogenous humus composting technology and long-term field effect evaluations to promote the practical application of this technology. These innovative technological methods for the resource utilization of corn stover can be applied to agricultural waste treatment and high-quality organic fertilizer production to promote green agricultural development and ecological construction.

At the economic level, if low-cost humus such as industrial by-products or agricultural waste compost is used, the input cost can be controlled, and the benefits brought by shortened composting cycles, faster site turnover, and premium high-quality composting can form positive benefits. Agricultural waste can be converted into humus through composting and input into other compost as a source of humus. It can meet the requirements of a short fermentation cycle and high degree of humification of products. In terms of scalability, the addition of specific functional strains through biological reinforcement technology can further accelerate the degradation of lignocellulose. By using physical field-assisted technology, microbial metabolic activity can be promoted, resulting in an increase in humus and humic acid content. At the same time, the synergistic regulation of multiple pollutants is achieved through the passivation ability of humic-acid-bound heavy metals. In large-scale production, the benefits of improved efficiency are more obvious, and the economy is more prominent. Overall, the rational utilization of exogenous humus can improve composting efficiency while achieving economic feasibility through cost optimization. In the future, the precise regulation of composting is needed. The properties of corn stover (such as lignin content) and environmental conditions (such as temperature and humidity) vary greatly in different regions. Therefore, it is necessary to ascertain the optimal amounts of external humus needed as well as composting process models based on local conditions. The interaction between humus and other substances in the heap, such as heavy metals and antibiotics, should also be explored at the level of action mechanisms. In addition, further research is needed to investigate the long-term application of exogenous humus to improve compost to determine its effects on the biochemical reactions of soil ecosystems to ensure the sustainability of this technology.