Role of Bioavailability in Compost Maturity During Aerobic Composting of Chicken Manure

Abstract

To evaluate the effects of the type and proportion of bulking agents on compost maturity, chicken manure feedstock (J) was selected as the main raw material for aerobic composting, and wood chips (M), straw (S), and cornmeal (Y) were used as bulking agents. The ratios of chicken manure feedstock to the three bulking agents were set at 1:3, 1:1, and 3:1, respectively. The compost mixture composed of wood chips (M) and feedstock (J) in a 1:1 ratio exhibited the highest temperature (75 °C). The treatment with a bulking-agent-to-feedstock ratio of 3:1 exhibited the lowest temperature (52 °C) and the longest high-temperature period (about 10 days). Moreover, the compost mixture composed of wood chips (M) and feedstock (J) in a 3:1 ratio exhibited the highest seed germination index (1.32), while the GI values for all cornmeal treatments did not meet the standard requirements (0.4). The predominant microorganisms in all three treatments included Firmicutes, Bacteroidetes, Actinobacteria, and Proteobacteria. The total carbon transformation-related microorganism abundance in MJ31, SJ31, and YJ31 was 1.65%, 10.69%, and 3%, respectively. Further analysis showed that the bioavailability of feedstock was strongly correlated with compost maturity. The treatment with a bulking-agent-to-feedstock ratio of 3:1, with the highest GI, also exhibited the highest bioavailability. These results can guide the selection of the appropriate bulking agent and the optimal bulking-agent-to-feedstock ratio, offering a new direction for the optimization of the composting process.

1. Introduction

Recently, the discharge of livestock manure has become rather large and is increasing rapidly, which is becoming a significant source of agricultural contamination. Statistical data indicate that the annual volume of livestock manure has increased to 3.8 billion tons [1,2]. These pollutants possess the dual nature of being both pollutants and potential resources. Among the various treatment methods available, aerobic composting is the most economical and straightforward approach. Through aerobic composting, manure can be transformed into organic fertilizer [3,4], offering a sustainable solution. Moreover, this process greatly enhances soil fertility and facilitates plant growth, owing to the abundant nitrogen, phosphorus, potassium, and other nutrients present in livestock manure. However, the existing aerobic composting process features several drawbacks, including low temperature, incomplete decomposition, and inconsistent effectiveness, all of which require further refinement and enhancement [5,6]. For example, conventional aerobic composting typically reaches a maximum temperature of around 60 °C. Under these conditions, insect eggs and weed seeds may not be entirely eradicated, leading to extended treatment periods. Moreover, the inadequate degradation of organic matter can result in the release of foul odors and excessive heat, potentially harming plant roots during the recycling process [7].

Several methods can be employed to increase the maximum temperature of compost, including the introduction of thermophilic bacteria, pre-treatment of raw materials at high temperatures, and utilization of electric field-assisted composting processes [8,9]. For example, pre-treating feedstock at 120 °C for 6 h enhances the degradation of lignin content, resulting in improved compost maturity [10]. The fundamental goal of these techniques is to improve the decomposition of organic matter. Therefore, ensuring a conducive aerobic environment is essential, and bulking agents are commonly employed as auxiliary materials to increase the porosity of the pile and allow air to penetrate into every corner of the material [11]. Bulking agents also aid in regulating moisture levels, supplementing organic matter, and adjusting pH values. For example, rice husk and electric-field assistance have been utilized to enhance oxygen utilization by 16%, resulting in temperature increases of over 8 °C and a 33% reduction in processing times [9]. Common bulking agent materials for composting include rice husk, wood chips, straw, and cornmeal [11]. Additionally, composting with these crop yields is an effective method of utilizing agricultural solid waste resources circularly. However, the selection of bulking agents typically relies on factors such as experience, price, and availability. Currently, there is a scarcity of research on the impact of various bulking agent materials on the elevation of compost temperature and maturity.

The C/N ratio reflects the proportional relationship between carbon and nitrogen in composting feedstock, a factor that significantly impacts microbial activity and, consequently, compost quality [12]. Maintaining a suitable C/N ratio is crucial for sustaining the normal metabolic activities of microorganisms. A too-high ratio may result in the volatilization of nitrogen in the form of ammonia (NH₃), while a too-low ratio could impede microbial growth and metabolic activities during composting [13,14]. To achieve optimal composting outcomes, a C/N ratio of 25–30 is recommended [15]. However, some studies have proposed a C/N ratio of 20 for the combined aerobic composting of vegetable waste, pig manure, and corn stover. Additionally, total organic carbon and total nitrogen have been commonly utilized as the basis for determining the C/N ratio in studies. Recently, Wang et al. (2024) discovered that carbon and nitrogen with varying degradation levels exerted a significant influence on compost maturity [16]. The authors concluded that easily degradable carbon was more readily utilized by microorganisms, thus enhancing compost maturation. Conversely, organic carbon with lower degradability is less likely to be utilized during composting. Moreover, the moisture content, kinds of bulking agent, and stacking density of raw materials significantly impact the heating process of composting [17,18]. The optimal values of pore space were reported as being between 30 and 36% in the composting process [19,20]. Nevertheless, these properties eventually make the material more viscous, which is detrimental to gas movement.

In the present study, chicken manure served as the primary feedstock for aerobic composting. We investigated three types of bulking agents (wood chips, straw, and cornmeal, in particular) and various bulking-agent-to-feedstock ratios (1:3, 1:1, 3:1) to assess their effects on compost maturity. The main objectives were as follows: (1) to determine the most effective types of bulking agents and the optimal ratios of raw materials to bulking agents for achieving compost maturation and (2) to examine the relationship between the bioavailability of raw materials and compost maturation. We aimed to establish precise bulking-agent-to-feedstock ratios for the aerobic composting of organic solid waste, such as livestock manure, to enhance compost quality and operational stability.

2. Materials and Methods

2.1. Composting Feedstocks and Reactor

The composting experiments were conducted in Danzhou City, Hainan, China (19°34′ N, 109°29′ E). Chicken manure was sourced from the experimental farm of the Chinese Academy of Tropical Agricultural Sciences in Danzhou City, Hainan Province. The physicochemical properties of chicken manure are shown in Table S1. Sawdust was procured from the Artisan Tang Furniture Factory in Qinghe County, Xingtai City, Hebei Province. Cornmeal was obtained from the BaoDaoXinCun Farmer’s Market in Danzhou City, Hainan Province. Rice straw and Chinese cabbage seeds were purchased from the JiaYao Agricultural Products Company in Yutai County, Jining City, Shandong Province.

In this study, a static bin composting process was employed, and a round reactor was constructed for this purpose. The compost reactor was made of polyvinyl chloride insulation material, measuring 5 cm in thickness, 75 cm in diameter, and 40 cm in height, with an effective volume of 0.17 m³.

2.2. Design of Experimental Treatment

Three types of bulking agents were utilized: wood chips, rice straw, and cornmeal. Bulking-agent-to-feedstock proportions of 1:3, 1:1, and 3:1 were adopted (Table S2). The sizes of these three bulking materials were kept comparatively constant. The raw materials were first cleaned with water and dried at 60 °C. Then, these materials were sieved during a sequential passage through two different apertures (2 cm and 1 cm, successively). The treatments were designated using abbreviations for the materials and proportions, with symbols “J” for chicken manure, “M” for wood chips, “S” for straw, and “Y” for cornmeal. For example, “MJ13” indicates a compost mixture composed of wood chips (M) and chicken manure (J) in a 1:3 ratio. In total, there were nine treatments, each with three duplicates. An intermittent blowing pattern, alternating one hour on and one hour off, was employed [9]. An online temperature sensor (Pt 100) was plated in the pile to monitor the temperature daily.

2.3. Material Characteristic Analysis

Relative density represents the ratio of the density of compost feedstock to the density of water. The saturated water content (SAWC) and relative water content (RWC) were determined through the oven drying method, as described in detail in a previous study [21].

Carbon and nitrogen in the initial composting material were quantitatively separated into three degradable components through the acid hydrolysis method, as detailed in a previous study [22]. These components are referred to as LCP1, LCP2, RCP, LNP1, LNP2, and RNP [22]. The labile carbon proportion 1 (LCP1) and labile nitrogen proportion 1 (LNP1) components represent the portions of carbon or nitrogen components that are most easily decomposed, such as monosaccharides, non-cellulose carbohydrates, and monomeric amino acids [23,24]. The labile carbon proportion 2 (LCP2) and labile nitrogen proportion 2 (LNP2) components represent components with a moderate biodegradability of carbon and nitrogen, respectively. RCP encompasses a range of carbon-based compounds with substantial recalcitrant carbon structures, such as lignin, waxes, and humus [16,22]. RNP denotes macromolecular nitrogen-containing compounds that are highly resistant to biodegradation or mineralization over a short period. LCP is the sum of LCP1 and LCP2, while LNP is the sum of LNP1 and LNP2.

2.4. Sampling and Data Analysis

Several samples were collected on days 0, 3, 7, 12, 18, 26, 36, and 50, and approximately 300 g of solid material was obtained each time. These samples were then divided into three equal parts on average. The first part was utilized for basic water-soluble physicochemical factor analysis, the second part for microbial community analysis, and the final part was stored in a freezer at −80 °C as a backup.

The compost solid samples (5 g) were mixed with 50 mL of deionized water and thoroughly combined before being filtered through a filter paper with a pore size of 0.45 μm. Subsequently, pH and conductivity were measured using a pH meter (PHS-3C; Shanghai Electric Scientific Instrument Co., Ltd., Shanghai, China) and a conductivity meter (DDS-307; Shanghai Zhong-dianke Instrument Co., Ltd., Shanghai, China), respectively. NH₄⁺-N and NO₃⁻-N were extracted using 1 M KCl (1:10, w/v) and then determined using the indophenol blue colorimetric method and the ethylenediamine dihydrochloride colorimetric method, respectively [25]. Moisture content was determined by drying fresh samples in an oven (model 101-0A; Zhongxingweiye, Beijing, China) at 105 °C. The seed germination index (GI) was determined through a previously described method [26,27]. Three-dimensional fluorescence spectroscopy (3D-EEM) was conducted using a spectrophotometer (Carbon 60; Agilent Technologies, Santa Clara, CA, USA). The measurement covered emission (Em) and excitation (Ex) wavelengths ranging from 200 to 600 nm and 200 to 500 nm, respectively. The integrated area of the region was utilized for a qualitative analysis of specific substances, as recommended by EEM [28].

The MoBio PowerSoilTM DNA Isolation Kit (MoBio Laboratories, Carlsbad, CA, USA) was utilized to extract total genomic DNA. The bacterial 16S rRNA gene was amplified using primers 515F and 907R targeting the V4–V5 region. Subsequently, the DNA was sequenced by Shanghai Meiji Biotechnology Co., Ltd. (Shanghai, China) on an Illumina HiSeq 2500 platform (San Diego, CA, USA).

2.5. Statistical Analyses

The data are represented as the mean ± standard deviation (n = 3). Data were counted and processed using Microsoft Excel 2024 and Matlab R2023a, and graphs were plotted using Origin 8.5 software (OriginLab, Northampton, MA, USA). The Mantel test was conducted using the “linkET” package in R (version 4.0.5).

3. Results and Discussion

3.1. Analysis of Basic Properties of Raw Materials

In a previous study, we classified three types of organic carbon and nitrogen sources based on their levels of degradation caused by microorganisms [22]. In terms of ratio, the greater the bulking-agent-to-feedstock ratio, the higher the value of LCP (Figure 1A). The wood chip-added treatment had the lowest LCP, and the cornmeal-added treatment had the highest LCP (Figure 1A). For instance, MJ13 exhibited the lowest LCP (181.8 g/kg), while MJ31 had the highest LCP (223.6 g/kg). Within the LCP category, the lowest LCP1 values were observed in all three treatments with wood chips (MJ13: 77.4 g/kg; MJ11: 74.4 g/kg; MJ31: 71.4 g/kg), while YJ31 exhibited the highest value (272.9 g/kg) (Figure 1B). As for nitrogen composition (Figure 1C,D), the greater the bulking-agent-to-feedstock ratio, the lower the value of LNP. The wood chip-added treatment had the lowest LNP, and the cornmeal-added treatment had the highest LNP (Figure 1C). The lowest and highest LNP values were observed in MJ31 (6.2 g/kg) and YJ13 (18.8 g/kg), respectively. Moreover, both the straw and cornmeal treatments exhibited minimal RNP. Additionally, across all three treatments, the LNP1 value exceeded the LNP2 value, indicating that nitrogen was readily utilized by microorganisms and did not hinder microbial metabolism (Figure 1D). In summary, the addition of bulking agents led to an increase in carbon sources but a decrease in nitrogen sources. Specifically, the addition of wood chips resulted in a more pronounced decrease in nitrogen sources compared with other bulking agents. Conversely, cornmeal addition caused an increase in carbon sources and a decrease in nitrogen sources. These findings align with the inherent properties of the raw materials [29,30].

The water adsorption capacity of the three types of bulking agents was tested. As illustrated in Figure 1E, the RWC of cornmeal (YJ13: 0.92, YJ11: 0.98, YJ31: 1.06) was higher than that of straw and wood chips, while the SAWC of cornmeal (YJ13: 0.65, YJ11: 0.61, YJ31: 0.56) was lower than that of straw and wood chips. This result indicated that cornmeal could absorb more water. However, this characteristic ultimately results in the increased viscosity of the material, which is detrimental to gas flow.

Relative density is a crucial parameter for composting raw materials as it considerably influences pore structure. Lower density corresponds to a larger pore structure in compost feedstock. Generally speaking, a porosity between 30% and 36% is regarded as appropriate [19,20]. Among the treatments in this study, straw exhibited the lowest relative density (0.3), while YJ31 exhibited the highest relative density (0.8) (Figure 1F). This suggests that the pore structure was the smallest in the presence of cornmeal and largest in the presence of straw.

3.2. Physicochemical Properties of Compost

Temperature is widely recognized as one of the key factors influencing organic matter degradation [31]. As illustrated in Figure 2A–C, the maximum temperature reached 70 °C when wood chips were used as the bulking agent (MJ31). The SJ11 treatment exhibited the highest temperature (75 °C) (Figure 2B). In contrast, SJ13 and SJ31 exhibited lower temperatures than the treatment with the wood chip bulking agent. The cornmeal treatments exhibited lower (YJ13: 52 °C, YJ11: 67 °C, YJ31: 57 °C) temperatures than the treatments with the other two bulking agents. The heating and cooling rates were higher under the 1:3 and 1:1 ratios than those under the 3:1 ratio. However, the duration of the high-temperature period was the longest in the 3:1 treatment (Figure 2C), attributable to the presence of easily degradable organic matter and the pore structure. Cornmeal, mainly composed of starch rich in glucose, is easily decomposed by microorganisms [32,33]. Wood chips and straw are rich in lignin and cellulose, making them the most difficult for microbes to break down. Conversely, absorbent cornmeal increases feedstock viscosity, which can hinder air flow.

The moisture content in all treatments exhibited a similar decreasing trend, showing a slight rise on day 10 (Figure 2D–F). This increase is attributable to water vapor condensation during the temperature drop. In the cornmeal treatments (YJ13, YJ11, YJ31), the moisture decrease rate was the highest, and the moisture reached the lowest levels, attributable to the strong hygroscopicity of cornmeal. Conversely, the water content was the highest in the treatments with wood chips (MJ13, MJ11), as wood chips had a weaker water absorption capacity than the other two bulking agents. Temperature and water content affected the degradation of organic carbon. Traditionally, the total-carbon-to-total-nitrogen ratio (C/N) should be 20–30 to ensure the rapid biodegradation of organic matter [34,35]. However, the initial C/N ratio in all treatments was below 10 (Figure 2G–I), indicating a higher nitrogen content when chicken manure was used as the composting material.

In the early stages of composting, ammonia is produced, owing to the decomposition of readily decomposable organic matter, such as proteins and carbohydrates. As composting proceeds, many metabolites are decomposed and released into the material, thus increasing the EC of the material. The changes in pH and EC followed the general expected pattern. The NH₄⁺ concentration showed a downward trend in all treatments during the composting process. Treatments with straw exhibited the highest degradation rate, attributable to the higher protein content in feedstock. The NO₃⁻ concentration gradually increased, with the highest levels occurring in the wood chip treatments (MJ13, MJ11, MJ31). This trend aligned with the changes in temperature, suggesting more microbial metabolic activity in these treatments.

3.3. Analysis of Compost Maturity

The technique of 3D-EEM spectroscopy is often used to study humification during composting. It reveals the composition and structure of and changes in organic matter by measuring the fluorescence intensity of a sample at different wavelengths [36]. Additionally, 3D-EEM can differentiate between various types of organic matter, such as proteins, amino acids, and humic substances [28,36,37,38]. The 3D-EEM spectra featured two distinct peaks at Ex/Em wavelengths of 240–250/400–420 nm (peak A) and 320–330/400–420 nm (peak B) (Figure 3A–I). These two peaks indicate the formation of FA-like (fulvic acid) and HA-like (humin) substances, respectively [39]. The peaks of FA- and HA-like substances were the most pronounced in the MJ11 and MJ31 treatments (Figure 3D,G), suggesting that the addition of wood chips was more conducive to humification. In contrast, these peaks were not as prominent in the YJ11 and YJ31 treatments, indicating that the addition of cornmeal as a bulking agent was less favorable for humification (Figure 3F,I). These results are consistent with the observed temperature changes.

The GI serves as an indicator for assessing the impact of compost on seed growth. The presence of incompletely decomposed hazardous substances or pathogens in compost may hinder seed germination, leading to a lower GI value [6,40]. A GI value greater than 0.8 indicates a significant enhancement in seed germination caused by compost; a GI value between 0.6 and 0.8 represents the toxicity threshold of compost products; and a GI value below 0.6 is considered unfavorable for plant growth or demonstrates an inhibitory effect [41,42]. As illustrated in Figure 3J–L, treatments with wood chips exhibited higher GI values. Specifically, the GI of MJ31 exceeded 0.8 and reached 1.4, indicating that the addition of wood chips effectively promoted seed growth. Conversely, treatments with cornmeal added exhibited lower GI values, with the GI value decreasing with increasing cornmeal content (YJ31 approaching 0) (Figure 3L). The GI of YJ11 was also below 0.4 (Figure 3K), indicating that cornmeal addition was not favorable for plant growth. The main reason for this may be that cornmeal’s absorbent qualities allowed it to absorb a lot of water when combined with compost materials. There are two primary repercussions from this effect. The scarcity of water prevented bacteria from growing. However, after absorbing water, cornmeal became extremely sticky, which prevented oxygen from flowing and made compost anaerobic. As a result, it was difficult for microbes to break down organic molecules, and the final level of decomposition was low. According to the temperature, moisture, and density data, cornmeal could absorb a lot of moisture, and this could lead to the formation of a viscous state that hinders airflow and impedes organic matter decomposition.

3.4. Bacterial Dynamics During Composting

Aerobic composting is a solid fermentation process driven by microbes, resulting in the conversion of macromolecular organic matter into stable, phytotoxic humus precursor material. The main microorganisms involved in this process include bacteria, fungi, and other microbes. As illustrated in Figure S3, the microbial community diversity of the straw and wood chip treatments was significantly higher than that of the cornmeal-added treatment at the end of composting. The predominant microorganisms in all three treatments included Firmicutes, Bacteroidetes, Actinobacteria, and Proteobacteria, consistent with previous research findings [43,44]. Biological diversity gradually decreased as the cornmeal content increased. These findings align with previous analyses, further confirming the very low microbial activity in YJ11 and YJ31.

Furthermore, microbial populations involved in carbon transformation were analyzed (Figure 4A–C). Carbon transformation-related microorganisms were sparse in treatments with higher proportions of bulking agents (Figure 4C), particularly in treatments with a bulking-agent-to-chicken-manure ratio of 3:1. The total abundance of carbon transformation-related microorganisms in MJ31, SJ31, and YJ31 was 1.65%, 10.69%, and 3%, respectively, at the end of composting. In the YJ13 treatment, the main microorganisms were Luteimonas (1.35%), Corynebacterium (0.56%), Thermobifida (3.59%), and Nocardiopsis (12.04%) by the end of composting. In the YJ11 treatment, Brevibacterium was predominant, accounting for 2.41%. Straw addition led to an enrichment in carbon-related microorganisms, with the highest abundance occurring among these treatments (SJ13: 17.1%; SJ11: 16.76%; SJ31: 10.69%). MJ13, SJ13, MJ11, and SJ11 exhibited almost the same microbial abundance and structure. These microorganisms included Thermobifida, Nocardiopsis, and Luteimonas. Thermobifida is a typical class of thermophilic microorganisms naturally capable of utilizing lignocellulosic biomass [45], indicating that lower amounts of bulking agent (in ratios of 1:3 and 1:1) were conducive to the growth of thermophilic microorganisms. Luteimonas has the function of denitrification [46]. Corynebacterium belongs to heterotrophic nitrifying bacteria and has the function of denitrification [47]. These findings align with the observed temperature patterns.

The microbial communities involved in nitrogen transformation during composting were assessed (Figure S4). Nitrogen-related microbes showed a general decrease in abundance, except in the case of YJ31 at the end of composting (Figure S4A–C). Thermobifida, a thermophilic bacterium associated with nitrogen metabolism, was the predominant nitrogen-related microbe in these treatments, because most nitrogen-metabolizing microorganisms are susceptible to destruction at high temperatures [48,49]. The abundance of Staphylococcus in YJ31 increased from less than 2% to over 50% over time. Staphylococcus could break down organic matter in compost, such as proteins, fats, and carbohydrates [50,51]. The addition of excessive amounts resulted in low degradation rates. Following an extended period of temperature pre-treatment, Staphylococcus began to accelerate the degradation of organic matter at a later stage.

3.5. Co-Occurrence Networks of Microbial and Environmental Factors

To further investigate the inherent reasons for the difference in compost maturity, the relationships between feedstock and compost maturity were assessed using Pearson’s correlation coefficient (r) and Mantel’s test (p). Density showed no significant correlation with temperature, considering individual treatments alone or comparing them overall (p > 0.05), while it showed a positive correlation with the GI as more bulking agent was added (Figure 5A–C). And the r-value was 0.41 (p < 0.01) when the bulking-agent-to-chicken-manure ratio was 3:1 (Figure 5C). Density was also significantly and positively correlated with Brachybacterium, Brevibacterium, Corynebacterium, Luteimonas, Thermobifida, and Thermomonospora (p < 0.01), suggesting that suitable densities favor the growth of thermophilic and carbon-metabolizing microorganisms, which is consistent with the latest findings [52]. RWC was significantly and positively correlated with temperature (p < 0.05), which showed no significant correlation with the GI considering individual treatments alone. It was shown that moisture has a significant effect on warming. However, moisture was positively correlated with the GI when comparing all the treatments together (p < 0.05) (Figure 5D). RWC was positively correlated with Brachybacterium and Corynebacterium in all treatments (p < 0.01).

LCP was significantly and positively correlated with the GI (p < 0.01), and the r-value increased when a bulking agent was added (0.22, 0.49, and 0.71 for bulking-agent-to-chicken-manure ratio of 1:3, 1:1, and 3:1, especially). LCP was positively correlated with Brachybacterium, Brevibacterium, and Corynebacterium (p < 0.05). LNP was significantly and positively correlated with temperature (p < 0.05), considering individual treatments alone. All treatments considered, LNP was not related to temperature but positively correlated with the GI (Figure 5D). LCP was not correlated with the majority of microorganisms, implying that nitrogen sources were not a limiting factor for microorganisms during composting.

Therefore, a bioavailability index was defined to evaluate the effects of raw material characteristics on compost maturity in terms of temperature, the GI, and the growth of microorganisms. The main ingredients of this index included density, moisture, and carbon sources. The ideal values provided were 0.2–0.4 for relative density, <80% for RWC, and >15% for LCP, summarizing the recent progress made in the latest research [11,53].

4. Conclusions

The effects of bulking agents and bulking-agent-to-feedstock ratios on compost maturity during the composting of chicken manure were investigated. The results revealed that excessive cornmeal addition did not increase temperature or promote compost maturation, mainly because cornmeal absorbed more water than wood chip and straw, which hindered gas flow, resulting in low organic carbon degradation and weak compost maturation. The addition of wood chips increased the pore space within the material, thereby raising the heap temperature and promoting maturation. Straw addition increased the available carbon sources, enhancing compost maturation. These results suggest that the bioavailability of compost materials (encompassing density, moisture, and carbon and nitrogen sources) can be used as an indicator for evaluating compost maturation and guiding production. This study provides insights into the intrinsic factors of raw materials affecting compost maturity, presenting a straightforward and practical novel index for compost production.