Research on the Composting Technology of Cattle and Sheep Manure Based on Intelligent and Efficient Composting Equipment and the Evaluation Standard of Decomposition Degree

Abstract

The Inner Mongolia Autonomous Region is a crucial area in China with a significant advantage in animal husbandry, particularly in cattle and sheep farming. However, the disposal of the large quantities of manure produced during farming has severely impacted the industry’s healthy development. Proper treatment of the manure can convert it into organic fertilizer beneficial to farmland; otherwise, it will cause substantial environmental pollution. This study focuses on existing composting equipment and addresses the issues of cattle and sheep manure mixture ratios and compost maturity evaluation. Through experiments on the mixture of cattle and sheep manure, the optimal ratio for converting cattle and sheep manure into organic fertilizer was determined. Additionally, a fuzzy mathematical evaluation model was employed, along with experimental data, to establish a comprehensive evaluation system for aerobic compost maturity based on multiple indicators, revealing the variation patterns of maturity levels under different mixture ratios. The test results revealed that the composting equipment effectively controls the composting process, shortens the composting cycle, ensures the complete decomposition of organic matter, and meets national standards for livestock and poultry manure treatment. Regarding temperature and humidity, oxygen concentration, seed germination rate, pH value, electrical conductivity (EC), nitrogen, phosphorus, potassium content, and carbon-to-nitrogen ratio, the mixed compost of cattle and sheep manure in various ratios met the relevant standards for agricultural application. Various ratios of organic fertilizers containing cattle and sheep manure significantly promoted the growth of maize, wheat, and mung bean crops. Specifically, the compost decomposition cycle was shortest when sheep and cattle dung were mixed at a ratio of 2:1, while it was longest for all cattle dung. Finally, a fuzzy mathematical comprehensive evaluation model was established by selecting four indicators: water content, carbon-to-nitrogen ratio, apparent score, and germination index. The study demonstrates that the equipment and method offer significant advantages in efficiently treating cattle and sheep manure and producing organic fertilizer, thereby providing strong support for the sustainable development of animal husbandry.

1. Introduction

China has always had a large animal husbandry industry due to rapid population growth and increasing demand for meat products [1,2]. This has led to a transformation towards large-scale production [3]. For instance, in the Inner Mongolia region, there are 1.3 billion mu of natural grassland and 30 million mu of artificial grassland, along with around 95,000 large-scale cattle and sheep farms [4,5,6,7]. Over the past few decades, the cattle and sheep populations in Inner Mongolia have significantly increased; however, improper management of the resulting manure can lead to severe environmental pollution [8,9,10,11]. Furthermore, the excessive use of chemical fertilizers in China has resulted in irreversible damage to the soil [12,13,14]. Given that cattle and sheep manure is rich in nitrogen, phosphorus, potassium, and organic matter and other nutrients, its conversion into organic fertilizer has become one of the best ways to deal with this type of waste [15,16]. This transformation can not only effectively use livestock and poultry manure resources and reduce environmental pollution but also provide a high-quality organic fertilizer source for agricultural production, promoting the sustainable development of agriculture [2,17,18].

Despite its benefits, natural composting has limitations, such as factors like temperature, humidity, and ventilation affecting its effectiveness [19,20]. Organic fertilizers produced through natural composting are often unevenly ripened, and using them directly on the soil can have adverse effects, such as seedling burning [21,22,23]. Due to this, China has implemented policies to emphasize the quality of arable land, improve soil fertility, and emphasize the treatment of livestock and poultry manure from large-scale farms [24,25,26]. Consequently, there is a growing trend towards utilizing efficient composting equipment for fermenting cattle and sheep manure [27,28].

During the composting process, differences in conditions can lead to significant changes in compost materials [29,30]. The degree of decomposition of compost is a crucial indicator of its quality. Accurately evaluating compost maturity is necessary to ensure its safe use for agricultural purposes [31]. Thus, it is essential to establish a suitable compost maturity index system that takes into account various indicators like physical, chemical, and biological factors. Merely relying on a single evaluation index is not appropriate for assessing the degree of maturity in sheep and cattle manure composting. By considering multiple indicators, the maturity of the compost can be comprehensively reflected [32].

Many scholars, both domestically and internationally, utilize composting equipment to convert manure into organic fertilizer. Commonly used composting combinations include cattle dung with wood chips, livestock manure with straw, pig manure with straw, cattle dung with corn stover, sheep manure with biochar, chicken manure with sheep manure, and more [33,34,35,36,37,38,39,40,41,42,43]. These combinations were subjected to systematic evaluation and experimental study, focusing on key indicators like carbon and nitrogen losses, greenhouse gas emissions, and seed germination index during the treatment process. The research findings revealed significant differences in these indicators among the different composting materials, providing a scientific basis for optimizing the composting process and improving the quality of organic fertilizers. On the other hand, based on current research findings on compost decay evaluation indexes from both domestic and international sources, most scholars share a similar understanding of these evaluation indexes. They generally employ mathematical methods like the Gray correlation method and fuzzy mathematical method to integrate single evaluation indexes, enhancing the comprehensiveness, scientific nature, and practicality of the evaluation results [44,45,46,47,48].

Therefore, based on the existing small-scale aerated heating composting equipment, this paper investigates the optimal mixture ratio of cattle and sheep manure. Using experimental indicators, a novel decay evaluation model is established to reveal the variation patterns of maturity levels under different mixture ratios, providing a scientific basis for accurately assessing the degree of compost decay.

2. Materials and Methods

2.1. Intelligent Composting Equipment

Figure 1 displays both a three-dimensional simulation and a physical drawing of small-scale aeration-heating composting equipment [49,50]. This equipment primarily consists of five parts: the control system, aeration heating system, compost fermentation system, discharging system, and rack. It features efficient mixing and intelligent real-time monitoring and adjustment capabilities, allowing it to effectively complete the composting and fermentation process of livestock and poultry manure. The fermenter’s capacity is 120 L, with a maximum loading capacity of 64 kg.

The control system of the composting equipment is highly intelligent, allowing real-time monitoring of air and compost data inside the fermentation tank via the display screen in the control cabinet. When the oxygen concentration or compost temperature deviates from the set range, the control system automatically activates the aeration and heating systems, taking appropriate measures to maintain optimal fermentation conditions.

2.2. Composting Test Equipment

The test equipment for composting is shown in Figure 2. Activated carbon was used to adsorb malodorous gases emitted from the composting equipment to reduce environmental pollution. Various tools, including seedling pots, spraying pots, petri dishes, centrifuge tubes, and qualitative filter paper, are employed to determine the germination rate of seeds and evaluate the seed germination index. Electronic scales accurately weigh the composting materials to ensure the correct proportion of ingredients. A soil multi-element tester detects key indicators such as pH value, temperature, humidity, and the content of nitrogen, phosphorus, and potassium. Centrifuges and filter paper are used to separate solids and liquids in suspensions, preparing the filtrate for the seed germination index test. An elemental analyzer measures the total carbon and nitrogen content in the compost. An electric blast drying oven measures the moisture content of the raw material, exploring the optimal moisture adjustment ratio. A horizontal shaker ensures the uniform mixing of samples with water.

2.3. Composting Test Materials

The test materials for composting are shown in Figure 3. Cattle dung, sheep dung, and corn stalks were selected for this study. The sheep manure was collected from a fertilizer processing plant in Salazi Town, Tumet Right Banner, Baotou City, Inner Mongolia Autonomous Region. The cattle manure came from Helin Modern Dairy Breeding Ranch. The maize stover was sourced from the Agricultural Machinery Laboratory of the College of Mechanical and Electrical Engineering, Inner Mongolia Agricultural University, and was then crushed and processed to 2–3 mm to ensure the smoothness of the mixed composting process.

2.4. Test Method for Composting Cattle and Sheep Manure

In the livestock manure composting process, the ideal initial moisture content of the pile should be between 55 and 60 percent [49]. However, the original moisture content of cattle and sheep manure is usually lower than this, and therefore the moisture content needs to be increased through a pre-treatment step to meet the composting requirements. According to previous research, mixing the compost with water at a mass ratio of 1:2 was found to best regulate the initial moisture content. In addition, the addition of crop residues can effectively increase the content of key nutrients, such as nitrogen, phosphorus and potassium, in the final organic fertilizer product in the compost [50]. In particular, mixing cattle dung, sheep dung and maize stover at a ratio of about 20 to 25:1 can achieve the desired carbon-to-nitrogen ratio [7].

To investigate the optimal ratio of mixed compost of cattle and sheep manure, five different combinations were set up: pure sheep manure, sheep manure and cattle manure at a ratio of 2:1, sheep manure and cattle manure at a ratio of 1:1, sheep manure and cattle manure at a ratio of 1:2, and pure cattle manure. The initial composting material ratio was chosen to be a mixture of corn stover and cattle and sheep manure at a mass ratio of 1:20. To ensure the accuracy of the ratios and reproducibility of the experimental results, plastic buckets were used to carry the raw materials, which were divided into six stages for feeding. Before each feeding, the materials were accurately weighed using an electronic scale, and the data of each weighing were recorded in detail.

After all the heap materials were added, graphs of heap fermentation temperature change, seed germination rate, oxygen concentration change in the tank, heap humidity change, and seed germination index were obtained through the MCGS data report function and experimental test data. These graphs were analyzed to compare the effects of different cattle and sheep manure mixing ratios.

During the composting experiment, data were recorded using a touch screen with a recording frequency of every three hours until the end of the composting cycle. According to previous studies, after three cycles of aeration and heating treatment, the compost can decompose within a few days. Therefore, the five groups of mixed-ratio composts in this experiment were all subjected to three cycles of aerated heating treatment [7]. When the compost was prepared and placed in the fermenter, samples were taken daily to ensure the accuracy and completeness of the data. The content of the above test process is shown in Figure 4.

The collected sample device bucket was divided into four parts. The first part (as shown in Figure 5) involved mixing the heap samples with a certain amount of farmland soil. This mixed organic fertilizer soil was evenly filled into nursery pots. Mung bean seeds, wheat seeds, and maize seeds were then spread in different nursery pots, and a spray can was used to regularly spray the right amount of water to simulate actual planting conditions. The germination rates of the three seed varieties were observed and measured, enhancing the organic fertilizer’s effectiveness.

The second part (shown in Figure 6) involved pouring the compost samples into buckets and using a soil integrated sensor to detect the compost and count the corresponding index data.

The third part (shown in Figure 7) was used to determine the germination index. Fresh compost samples were stored at 4 °C to maintain freshness. The sample treatment process followed these steps: samples were mixed with ultrapure water at a ratio of 1:10 (w/v) and shaken in a horizontal shaker for 2 h to extract organic components. The mixture was then transferred to a centrifuge under established conditions (10 min, set speed). After centrifugation, the supernatant was filtered through filter paper to remove fine suspension. Five milliliters of filtrate were accurately measured and added to sterile petri dishes pre-lined with filter paper. Twenty uniform and healthy cabbage seeds were placed in each petri dish to ensure consistency. The petri dishes were incubated at a constant temperature and protected from light to allow germination for 4 to 7 days. Ultrapure water was used as a control sample to assess the effect of the heap extract on seed germination. A minimum of four biological replicates were carried out for each sample.

In the last part (shown in Figure 8), the compost samples underwent pre-treatments for subsequent measurements of total carbon and total nitrogen content. The preliminary treatments involved air-drying the samples to a constant weight to eliminate moisture effects. The dried samples were crushed to a fine powder using mechanical milling methods, screened through a 60-mesh sieve for consistency, and stored in airtight containers. Prior to elemental analysis, each sample was accurately weighed, and its initial weight after air drying was recorded. The samples were then placed in a crucible dedicated to the elemental analyzer. The total carbon and nitrogen contents were determined, and the carbon-to-nitrogen ratios were calculated.

3. Results

During the initial analysis of the continuous test data, it was observed that the measured parameters did not show significant fluctuations before and after short intervals. Based on this observation, and to increase efficiency during the data processing and analysis phase, three specific time points—7 am, 3 pm, and 11 pm each day—were selected for graphical analysis. In the subsequent charts, each day’s three time points were represented so that the primary scale of the horizontal coordinate denoted the 11 pm data collection, while the two secondary scales preceding it represented the 7 am and 3 pm data collections, respectively. For instance, “1” on the primary scale refers to data collected at 11 pm on the first day, while the preceding secondary scales correspond to data collected at 7 am and 3 pm on that day.

3.1. Temperature and Humidity Variations in the Composting Process

Compost temperature is a key factor in determining its quality. Maintaining the right high-temperature range is essential for effectively killing disease-causing microorganisms, ensuring the safety and quality of the compost product. Figure 9 illustrates the temperature variations in compost with different mixing ratios of cattle and sheep manure.

The overall duration of the experiment was about 12 to 13 days. The high-temperature phase of the composting process lasted approximately 7 days in an artificially controlled environment with ventilation and heating. After this phase, the heap temperature was reduced to below 40 °C within about 1 day, marking the end of the high-temperature fermentation, even compared to natural fermentation processes. Thus, the total duration of the high-temperature phase under artificially heated conditions was around 8 days. In this experiment, the interval between each artificial ventilation and warming session was about 48 h, indicating that the fermenter used has a holding capacity of about 48 h.

According to the national standard Technical Specification for Harmless Treatment of Livestock and Poultry Manure (GB/T 36195-2018), the heap temperature needs to be maintained above 50 °C for at least 7 days or above 45 °C for at least 14 days [50]. In this study, the composting test with five different proportions of cow and sheep manure mixes showed that the heap temperature remained above 50 °C for about 7 to 8 days, satisfying the national standard. This demonstrates that the optimized small-scale ventilated heated composting equipment used in this study is well-designed and can effectively support the environmentally sound treatment of livestock and poultry manure.

By analyzing Figure 9a,b,e, it was observed that the minor temperature increase in the later stages of compost maturation is caused by the interaction of biological and physicochemical processes. At these later stages, most readily decomposable components, such as proteins, carbohydrates, and fats, have been consumed by the microbial community, leading the pile into a relatively stable cooling period. However, the decomposition of substances such as cellulose and lignin may still be ongoing. Due to the dense polymeric structure and high chemical stability of these substances, their decomposition is slow and typically requires specific microorganisms that secrete enzymes like cellulases and lignin peroxidases. This decomposition may generate a limited amount of heat, and if this heat dissipates more slowly than it is generated, the internal temperature of the pile may experience a slight increase.

The control of humidity is critical in the compost fermentation process and is an important parameter for assessing the quality of organic fertilizer. Maintaining the desired level of humidity is essential to keep the microbial community active; these microorganisms are responsible for decomposing and transforming organic matter, ultimately determining the nutritional status and suitability of the product. Excessively high or low humidity can adversely affect the composting process; excessive humidity may lead to anaerobic conditions and inhibit microbial activity, while excessively low humidity may slow down the rate of organic matter decomposition and affect the maturity and quality of the compost. The variation in humidity during the composting process is shown in Figure 10.

As the composting process progressed in this experiment, the moisture content of the compost showed a continuous decreasing trend. During the high-temperature stage of composting, the rate of moisture evaporation significantly accelerated because the high temperature made it easier for moisture to evaporate from the compost. Additionally, the heat generated by microorganisms decomposing the organic matter further promoted moisture evaporation. Subsequently, during the decreasing temperature stage, the evaporation rate of water slowed due to the lowering of the external temperature, resulting in a gradual decrease and final stabilization of the compost’s humidity. The experimental results showed that the humidity of the five different ratios of mixed cow and sheep manure compost stabilized at about 9% on average by the end of the fermentation process. Specifically, the pure sheep manure compost showed lower humidity levels at the end of composting, down to 8.1%, while the pure cow manure showed relatively higher humidity levels, up to 10.3%. This difference can be partly attributed to the marked differences in the physical properties and chemical composition of sheep and cow manure. Sheep manure has a lower moisture content, higher density, and more insoluble solids, making it more susceptible to water loss at high temperatures. Due to its more compact structure, the rate of water evaporation is likely higher. In contrast, cattle manure typically contains higher moisture and fewer insoluble solids, which explains its higher water retention rate during composting. Additionally, differences in microbial degradation pathways and rates between sheep and cattle manure may also contribute to the moisture differences. Sheep manure, being nutrient-rich, may attract a larger microbial population, leading to a more active biodegradation process that generates more heat, thus accelerating water evaporation. This ultimately results in a lower moisture content at the end of sheep manure composting compared to cattle manure composting.

According to the industry specification “Organic Fertilizer” (NY 525-2012), the moisture content of organic manure produced by composting should not exceed 30% [50]. Detailed testing and analysis of five different proportions of cow and sheep manure compost in this study showed that the moisture levels of all samples were significantly lower than this standard value, well below the upper limit set by the industry. This is sufficient to meet the requirements of the industry specification and also proves the superior performance of the optimized equipment used in this study.

3.2. Oxygen Concentration Changes during Composting

Oxygen concentration plays a vital role in the composting process. A suitable oxygen concentration, usually recommended to be in the range of 15% to 20%, provides a favorable metabolic environment for aerobic microorganisms, which promotes the rapid decomposition of organic matter and maintains compost quality. However, if the oxygen concentration is too low, anaerobic conditions may be triggered in certain areas of the compost. This not only inhibits the action of aerobic microorganisms but may also lead to the production of malodorous gases such as hydrogen sulfide (H₂S) and methane (CH₄). The data from the study, shown in Figure 11, demonstrate the dynamics of oxygen concentration during the composting process of mixed cow and sheep manure. Monitoring and managing oxygen concentration is crucial for safeguarding compost quality, shortening the composting cycle, and preventing toxic emissions.

During the composting process, a gradual decrease in the oxygen concentration in the tank was noted as the fermentation progressed. However, the oxygen concentration increased after each periodic aeration. In particular, during the cool-down phase of composting, as the compost gradually approaches maturity, microbial activity diminishes, and the demand for oxygen subsequently decreases. The semi-closed design used in the fermenter helps to retain the gas inside the tank to a certain extent, resulting in a slight increase in oxygen concentration during the compost maturation stage.

To study this phenomenon in depth, this study conducted a precise quantitative analysis to compare the average rate of oxygen consumption in composts with different cow and sheep manure mix proportions under the same aeration conditions. For example, in the case of the pure cow dung compost, the oxygen concentration rose by 0.5% after aeration, whereas the rebound in oxygen concentration in the pure sheep dung compost was only 0.1%. For the mixed cow and sheep manure compost, the rebound in oxygen concentration ranged from 0.2% to 0.3%. This suggests that the physical structural properties of different compost combinations, such as porosity and moisture retention capacity, significantly affect the ability to propagate oxygen and the metabolic rate of aerobic microorganisms. The results of these analyses clearly indicate that the mix proportions of cow and sheep manure, as well as the aeration strategy, play a key role in regulating and maintaining oxygen levels in the composting process. This finding highlights the importance of choosing the right mixing ratio of cow and sheep manure and optimizing the frequency of aeration to improve the efficiency and quality of composting. It also reveals the value of strategies to promote the ecological functioning of the composting system and its environmental sustainability through the regulation of operational parameters.

3.3. Seed Germination and pH Changes in the Composting Process

In this study, representative crops widely planted in the Inner Mongolia Autonomous Region—corn, mung bean, and wheat—were selected to verify the effect of compost quality on seed germination. The collected compost samples were mixed with local farmland soil in suitable proportions, placed in seedling containers, and subjected to seed germination experiments under controlled conditions. The effect of compost on seed germination potential was assessed by recording the number of germinated plants of each crop and calculating their germination rates. The determination of seed germination rates not only revealed the activity of the seeds themselves but also indicated the facilitating effect of the compost on the soil ecosystem, which was reflected in the germination capacity of the seeds. Figure 12 demonstrates the variation in seed germination rates with different cow and goat manure mix proportions.

In the present study, the effect of organic compost made using different proportions of cow and goat manure on the germination of mung bean, wheat, and maize seeds was examined. Seed germination in the compost was assessed at the end of the external temperature increase phase (day 5). It is noteworthy that the germination of maize seeds in compost treated with artificial aeration and heating ranged between 80% and 90%. The germination of mung bean seeds was also in this range, whereas the germination of wheat seeds was slightly lower, ranging from 75% to 80%. Analysis of the data revealed that maize seeds exhibited the highest germination rate (87%) in organic manure with a ratio of sheep dung to cow dung of 2:1. This result may be related to the high demand of maize for nutrients such as nitrogen during its growth cycle. Sheep manure, due to its rich nutrient content, especially nitrogen, phosphorus, and potassium, is essential for maize, which requires these nutrients. Nutrients are also regulated by the addition of cow dung, whose slow-release properties are thought to reduce potential root burning problems caused by sheep dung and ensure a continuous supply of nutrients.

For mung bean seed germination, the highest germination rate (90%) was achieved under the organic manure treatment with a 1:1 ratio of sheep manure to cow dung. Since mung bean does not require large amounts of nitrogen and other nutrients for growth, the more balanced nutrient release from cow dung and the presence of beneficial microorganisms provided a steady and moderate supply of nutrients to mung bean during germination. This suggests that for vegetable crops such as mung bean, the selection of organic fertilizers with lower nitrogen content may improve seed germination. For wheat, germination was generally lower than that of maize and mung bean, although the highest germination rate (80%) was achieved under a sheep dung to cow dung ratio of 2:1. The lower germination rate of wheat may be related to its different requirements for starting nutrient content in the soil. Wheat seeds have relatively low initial nutrient requirements at germination, so organic fertilizers with high nitrogen content may have a slight inhibitory effect on their germination. Additionally, bio-abiotic factors such as differences in the soil microenvironment, microbial activity, and soil temperature may affect germination.

By analyzing the seed germination of three crops receiving organic fertilizers with different percentages of cow and sheep manure, it was concluded that the mixing ratio of organic fertilizers is important to meet the needs of a particular crop. In practice, the precise application of appropriate cow and sheep manure ratios not only enhances seed germination but also helps to ensure the nutrient requirements of the crops in the subsequent growth stages, providing a scientific basis for achieving sustainable agricultural production and soil health management.

The acid–base balance, or pH, of the composting process is critical to the quality of the finished compost. In Figure 13, the dynamic trend of pH value during composting of cow dung mixed with sheep dung in different proportions is shown.

During the composting process, a gradual shift in the pH of the compost from initially weakly alkaline to weakly acidic was observed from the mid to late stages of composting, showing a pattern of increasing and then decreasing pH values. In the middle stage of composting, thermal decomposition activity was accompanied by the release of large quantities of ammonia, leading to a temporary rise in pH. The decomposition of proteins and other nitrogenous substances due to high temperatures releases large quantities of ammonia, which in its partially free state causes an increase in the pH of the compost mixture. As the temperature of the compost decreases and the microbial ecology changes, decomposition activity becomes slower and more anoxic. During this maturation stage, microorganisms begin to convert the ammonia produced by decomposition into nitrite and nitrate, depleting the alkaline component of the compost, which leads to a gradual decrease in pH. In the later decomposition stage of composting, more complex organic matter is broken down into more stable forms such as humic and fulvic acids. The complexes formed between these stable organic matters and minerals help to further stabilize the pH. The buffering capacity of the humic acid complexes plays a key role in pH regulation, eventually stabilizing the compost pH at a slightly acidic level suitable for plant growth. According to the industry standard document “Organic Fertilizer” (NY 525-2012), the acidity (pH) of organic fertilizers that have been composted and rotted should be between 5.5 and 8.5, and the organic fertilizer involved in the present test satisfies this condition.

3.4. Changes in Conductivity EC Values during Composting Process

Electrical conductivity (EC) reflects the concentration of dissolved salts in the compost, serving as an indirect indicator of nutrient content and potential salt stress. Changes in the ionic composition of the solution primarily alter the conductivity value. According to the relevant literature, there is a positive correlation between soluble salt content and conductivity, meaning an increase in conductivity often corresponds to a rise in soluble salt content, and vice versa [51].

Figure 14 demonstrates the variation in electrical conductivity (EC) of composts with different mixing ratios of cow and sheep manure. The highest EC value, 6.3 mS/cm, was observed for whole sheep manure, while the lowest value, 4.5 mS/cm, was recorded for whole cow manure. This suggests differences in salt release and organic matter decomposition between the two types of manure during the fermentation process. In the early stages of composting, EC values are usually low because microbial activity and organic matter decomposition have not yet become pronounced. As composting progresses, microorganisms begin decomposing organic matter, producing small molecules like organic acids, ammonium ions, bicarbonate ions, and nitrate ions, which increase the EC values by reflecting the rising dissolved salt content.

During composting, the decomposition of organic matter causes EC values to fluctuate, first rising, then falling, then rising and falling again. During the fermentation stage, vigorous microbial activity and accelerated decomposition of organic matter typically cause the EC value to rise. Nutrients such as nitrogen, phosphorus, and potassium in cow and sheep manure are gradually utilized by microorganisms and converted into forms more readily absorbed by crops, contributing to the increase in EC value. Additionally, the nitrification process produces large amounts of nitrate, further raising the EC value. In the later stages of fermentation, the rate of organic matter decomposition slows, microbial activity weakens, and compost temperature decreases, leading to stabilization or a slight decrease in EC value. Ultimately, the EC value reflects the compost’s maturity and salt content, which is influenced not only by the type of feedstock but also by the management and regulation of the composting process.

3.5. Changes in Nitrogen, Phosphorus and Potassium Content of Composting Processes

During the composting process, three important nutrients—nitrogen (N), phosphorus §, and potassium (K)—undergo significant dynamic changes that are critical for evaluating the quality, maturity, and nutrient content of compost products. These rapid morphological changes are essential for accurately assessing the maturity and nutrient value of compost, helping to determine its role in soil health and its contribution to crop growth. Figure 15 illustrates the evolution of nitrogen, phosphorus, and potassium contents in composts made with different proportions of cow and sheep manure.

Figure 15a demonstrates the variation in nitrogen content in composts with different mixing ratios, showing an initial increase followed by a decrease. Whole sheep manure compost had the highest nitrogen content of 2.35% at the end of fermentation, while whole cow manure compost had a relatively low nitrogen content of 1.36%. This variation reflects the microbial conversion of organic nitrogen to inorganic nitrogen (including ammonia NH₃ and ammoniacal nitrogen NH₄⁺) at the beginning of fermentation. As fermentation progressed, the rate of microbial decomposition accelerated, leading to an increase in ammoniacal nitrogen content. During the thermal decomposition stage, increased temperatures promoted microbial activity and intensified the volatile loss of nitrogen, causing a decrease in total nitrogen content. In the later stages of composting, microbial activity slowed, and ammoniacal nitrogen was further converted to nitrate nitrogen (NO₃⁻) through nitrification. Therefore, by the end of the composting process, the total nitrogen content may decrease slightly, but its bioavailability is enhanced due to the increase in inorganic nitrogen.

According to Figure 15b, the phosphorus content in all proportions of compost showed a gradual increase. Whole sheep manure compost had the highest phosphorus content of 1.62% at the end of fermentation, while pure cow dung had the lowest phosphorus content of 1.02%. This trend is attributed to the decomposition of organic phosphorus by microorganisms in the early stages of composting, converting it into inorganic phosphorus that can be directly absorbed by plants. Throughout the fermentation process, organic phosphorus continued to convert to the inorganic state, resulting in an increase in fast-acting phosphorus content. At the maturity stage, the inorganic phosphorus content tends to stabilize, and since phosphorus is not volatile, its content usually remains constant or increases slightly. Changes in potassium content, as shown in Figure 15c, also displayed a gradual increase. The highest potassium content of 2.52% was found in whole sheep manure compost at the end of fermentation, while whole cattle manure had a lower potassium content of 1.24%. Potassium is relatively stable during the composting process, does not easily undergo chemical changes, mainly exists in ionic form, and is readily absorbed by plant roots. The reduction of water content or water evaporation can lead to the relative concentration of potassium content, so the potassium content increased throughout the compost fermentation process.

A comprehensive analysis, according to the national standard GB 7959-2016 “Organic Fertilizer”, indicates that the nitrogen, phosphorus, and potassium content of the mixed organic fertilizer from cow and sheep manure in this test exceeds the standard requirements of 1.2%, 0.6%, and 1.2%, respectively [50]. The samples from each heap meet this standard and satisfy the relevant requirements for organic fertilizer.

3.6. Changes in Carbon-to-Nitrogen Ratio during Composting

The carbon to nitrogen (C/N) ratio in the composting process significantly affects microbial activity, composting rate, and the quality of the final product. An ideal C/N ratio balances the carbon and nitrogen requirements of microorganisms, promoting their growth and accelerating the degradation of organic matter, ultimately producing a high-quality and mature compost product. A high C/N ratio may indicate an insufficient nitrogen source, limiting microbial activity, slowing down the compost maturation process, and potentially causing nitrogen loss. Conversely, a low C/N ratio often indicates an excess of nitrogen, which can increase ammonia volatilization and organic nitrogen loss, potentially impacting the environment and reducing the available nitrogen in the end product. Therefore, adjusting the C/N ratio to meet the nutrient requirements of microorganisms is crucial for achieving efficient and sustainable composting.

As shown in Figure 16, the carbon to nitrogen (C/N) ratios of different cow and sheep manure proportions in the composting process showed a gradual decreasing trend. The initial decrease was rapid, while the rate of decrease slowed down in the later stages, eventually stabilizing in the range of about 15 to 16. Specifically, the initial C/N ratios of whole sheep manure were relatively low, those of whole cow manure were relatively high, and those of mixed composts fell in between. These differences were mainly due to the composition of the initial materials. Cow dung had a relatively high initial C/N ratio because of its high content of cellulose and other difficult-to-decompose organic matter. As the composting process progressed, microorganisms broke down this complex organic matter, releasing carbon as carbon dioxide, while the nitrogen content remained relatively stable, resulting in a gradual decrease in the C/N ratio. In contrast, the organic matter in sheep manure was more readily decomposed by microorganisms, resulting in a relatively low starting C/N ratio. During the composting process, the readily decomposable components in sheep manure were rapidly converted, leading to faster carbon release and greater nitrogen stability, ultimately resulting in a lower C/N ratio for sheep manure compared to cattle manure.

When the C/N ratio drops to about 15, the compost is usually considered sufficiently mature, providing good fertilizer efficiency and high stability for plants, making it suitable for agricultural use. Therefore, monitoring and adjusting the C/N ratio is a critical step in process adjustment and product quality control to ensure that the final output of organic fertilizer meets the criteria for efficient agricultural use.

3.7. Changes in Germination Index during Composting

The seed germination index (GI) is an important indicator of the presence of plant growth-inhibiting substances in compost, which can cause phytotoxicity. Additionally, the GI is widely recognized as a valid and reliable measure for assessing compost maturity, as it combines seed germination and seedling growth. The GI is calculated as follows: GI (%) = (average germination rate of seeds in the treatment group × average root length of seedlings in the treatment group)/(average germination rate of seeds in the control group × average root length of seedlings in the control group) × 100%. A high GI value usually indicates low toxicity in the compost sample and suitability for plant growth, thereby indicating high compost maturity. The variation curves of seed germination percentages for compost with different proportions of cow and sheep manure mixes are shown in Figure 17.

The seed germination index (GI) showed a slow increasing trend in the early stage of the composting process, indicating the presence of plant growth inhibitors in the compost. These inhibitors were gradually reduced or eliminated as the composting process progressed. In the middle stage, the rate of increase of GI significantly accelerated, reflecting a notable increase in compost maturity and a rapid decrease in phytotoxicity. In the final stage of compost maturation, the rate of increase of GI slowed down again, indicating that the compost was approaching an optimal maturity level. The compost sample with a 2:1 ratio of sheep manure to cow dung showed the highest GI of 103%, while the sample with a 1:2 ratio had the lowest GI of 95%. According to industry standards, when the GI value exceeds 80%, the composted material is considered sufficiently decomposed and essentially free of toxic substances for plant growth.

In summary, all tested compost ratio configurations in this study met this standard, indicating low phytotoxicity and suitability for use as organic fertilizer.

4. Comprehensive Evaluation and Analysis of Aerobic Composting Effect

In the previous subsection, an experimental study of composting a mixture of different proportions of cow and sheep manure was conducted, recording data on changes in various indicators during the composting process. Despite this, accurately capturing the changes in the degree of decomposition of the compost was not possible. Traditionally, single indicators such as the C/N ratio or germination index (GI) are used to judge compost decomposition. However, these single indicators can only reflect decomposition from one perspective and cannot fully capture the complexity of the organic matter decomposition process. Therefore, recent research has focused on using multiple indicators to comprehensively assess compost maturity from different perspectives.

To assess compost decomposition more comprehensively, an analytical model based on fuzzy mathematical evaluation is developed in this chapter. By integrating multiple indicators, the model provides a comprehensive reflection of the degree of compost decomposition from multiple levels, offering a more accurate and holistic evaluation than previous single-indicator approaches. Using the fuzzy mathematical model, a more detailed evaluation of compost maturity can be derived, providing a stronger scientific basis for the preparation and use of compost.

4.1. Changes in Carbon-to-Nitrogen Ratio during Composting

In aerobic composting of cattle and sheep manure, assessing the degree of decomposition is crucial. Based on the results of the above experiments, four indicators were selected to comprehensively evaluate the degree of decomposition of aerobic compost: apparent score, water content, C/N ratio, and germination index (GI). These indicators collectively reflect the physical properties, chemical quality, and biological activity of composted cow dung and sheep manure, providing a highly reliable measure of compost decomposition.

In previous studies, the duration of the high-temperature phase (temperature ≥ 50 °C) in the composting process is also considered a key indicator. This phase is crucial for destroying seeds and killing harmful organisms such as insect eggs, pathogenic bacteria, parasites, and spores. However, this study uses small-scale ventilated heated composting equipment, which operates similarly to artificial heating and differs somewhat from natural composting. Although the principles are similar, the duration of high temperature was not used as an indicator in this evaluation to ensure the rigor and controllability of the test. The indicators were defined as grades of 4 (fully decomposed, basically decomposed, slightly decomposed, and undecomposed), and the specific grading of the evaluation indicators is shown in

Table 1. Criteria for Defining Evaluation Indicators.

Indicator Level	${\mathit{u}}_1$ Moisture Content (%)	Retrieve a Value	${\mathit{u}}_2$ C/N	Retrieve a Value	${\mathit{u}}_3$ Apparent Score	Retrieve a Value	${\mathit{u}}_4$ GI (%)	Retrieve a Value
Fully decomposed (I)	≤15	15	≤16	16	≥8	9	≥80	90
Basically decomposed (II)	15~30	30	16~19	19	6~8	6	60~80	60
Slightly decomposed (III)	30~45	45	19~22	22	3~6	3	30~60	30
Undecomposed (IV)	>45	50	>22	25	<3	2	<30	20

.

4.2. Establishment of a Fuzzy Evaluation System for Putrefaction

4.2.1. Establishment of a Fuzzy Evaluation System

Four factors that can influence the advantages and disadvantages of aerobic composting were selected to form a comprehensive evaluation parameter set U_1,2,3,4 = {Moisture content (%), C/N, Apparent score, GI (%)}. Matrix of the composting effectiveness rating of the unit $V_{1,2,3,4}$ = {Fully decomposed, Basically decomposed, Decomposed, Undecomposed}.

$$V=\left[\begin{array}{cccc}{k}_{11}& k_{12}& k_{13}& k_{14}\\ k_{21}& k_{22}& k_{23}& k_{24}\\ k_{31}& k_{32}& k_{33}& k_{34}\\ k_{41}& k_{42}& k_{43}& k_{44}\end{array}\right]$$

(1)

Let i (take 1, 2, 3, 4) one-factor evaluations correspond to $R_{\mathrm{i}}=({\gamma }_{\mathrm{i}1}, {\gamma }_{\mathrm{i}2}{, \gamma }_{\mathrm{i}3}{, \gamma }_{\mathrm{i}4})$, which is a fuzzy subset of its evaluation set V, then ${\gamma }_{\mathrm{ij}}$ (j takes 1, 2, 3, 4) is the degree of affiliation of the evaluation of the i-th factor to the j-th evaluation level, then the combined evaluation level matrix of the 4 factors is

$$R=\left[\begin{array}{c}{R}_1\\ R_2\\ R_3\\ R_4\end{array}\right]=\left[\begin{array}{cccc}{\gamma }_{11}& {\gamma }_{12}& {\gamma }_{13}& {\gamma }_{14}\\ {\gamma }_{21}& {\gamma }_{22}& {\gamma }_{23}& {\gamma }_{24}\\ {\gamma }_{31}& {\gamma }_{32}& {\gamma }_{33}& {\gamma }_{34}\\ {\gamma }_{41}& {\gamma }_{42}& {\gamma }_{43}& {\gamma }_{44}\end{array}\right]$$

(2)

According to the different influencing factors correspond to different evaluation levels, therefore, different weights are defined for them according to the degree of their influence, and the weight allocation matrix is

$$A=\left[\begin{array}{cccc}{a}_1& a_2& a_3& a_4\end{array}\right]$$

(3)

Establishment of fuzzy evaluation model based on fuzzy transformation theory:

$$B=A\circ R=\left\{\begin{array}{cccc}{b}_1& b_2& b_3& b_4\end{array}\right\}$$

(4)

$$B_{\mathrm{G}}{=b}_{\mathrm{j}}=\frac{b_{\mathrm{i}}}{{\displaystyle \sum b_{\mathrm{j}}}}(\mathrm{j}=1,2,3,4)$$

(5)

where $\circ$ is the sign of the fuzzy matrix calculation and B is the comprehensive evaluation result set.

4.2.2. Establishment of the Evaluation Factor Affiliation Function

In order to accurately assess the various types of impact indicators in the device composting test, this study classifies these indicators into several well-defined grades. This hierarchical approach reveals the degree of compliance of each impact factor with the established criteria, i.e., its affiliation. The affiliation describes the intensity of the impact factor’s effect and, accordingly, is assigned a specific range of values, which correspond to large, small and intermediate grades, respectively. Therefore, an affiliation function needs to be established and defined.

$$\mathrm{Grade} \mathrm{I}: {\gamma }_{i1}=\{\begin{array}{cc}1& x_i\ge \mathrm{I}\\ \frac{x_i-\mathrm{II}}{\mathrm{I}-\mathrm{II}}& \mathrm{II}<x_i<\mathrm{I}\\ 0& x_i\le \mathrm{II}\end{array}$$

(6)

$$\mathrm{Grade} \mathrm{II}: {\gamma }_{i2}=\{\begin{array}{cc}0& x_i\ge \mathrm{I} \mathrm{or} x\le \mathrm{III}\\ \frac{\mathrm{I}-x_i}{\mathrm{I}-\mathrm{II}}& \mathrm{II}<x_i<\mathrm{I}\\ \frac{x_i-\mathrm{III}}{\mathrm{II}-\mathrm{III}}& \mathrm{III}<x_i<\mathrm{II}\\ 1& x_i=\mathrm{II}\end{array}$$

(7)

$$\mathrm{Grade} \mathrm{III}: {\gamma }_{i3}=\{\begin{array}{cc}0& x_i\ge \mathrm{II} \mathrm{or} x_i\le \mathrm{IV}\\ \frac{\mathrm{II}-x_i}{\mathrm{II}-\mathrm{III}}& \mathrm{III}<x_i<\mathrm{II}\\ \frac{x_i-\mathrm{IV}}{\mathrm{III}-\mathrm{IV}}& \mathrm{IV}<x_i<\mathrm{III}\\ 1& x_i=\mathrm{III}\end{array}$$

(8)

$$\mathrm{Grade} \mathrm{IV}: {\gamma }_{i4}=\{\begin{array}{cc}0& x_i\ge \mathrm{III}\\ \frac{\mathrm{III}-x_i}{\mathrm{III}-\mathrm{IV}}& \mathrm{III}<x_i<\mathrm{IV}\\ 1& x_i\le \mathrm{IV}\end{array}$$

(9)

Based on Equations (6)–(9), we can derive the affiliation function of water content (%).

$${\gamma }_{11}=\{\begin{array}{cc}1& 0\le x_i\le 15\\ \frac{30-x_i}{30-15}& 15<x_i<30\\ 0& x_i\ge 30\end{array}$$

(10)

$${\gamma }_{12}=\{\begin{array}{cc}0& x_i\le 15 \mathrm{or} x_i\ge 45\\ \frac{x_i-15}{30-15}& 15<x_i<30\\ \frac{45-x_i}{45-30}& 30<x_i<45\\ 1& x_i=30\end{array}$$

(11)

$${\gamma }_{13}=\{\begin{array}{cc}0& x_i\ge 50 \mathrm{or} x_i\le 30\\ \frac{x_i-30}{45-30}& 30<x_i<45\\ \frac{50-x_i}{50-45}& 45<x_i<50\\ 1& x_i=45\end{array}$$

(12)

$${\gamma }_{14}=\{\begin{array}{cc}0& x_i\le 45\\ \frac{x_i-45}{50-45}& 45<x_i<50\\ 1& x_i\ge 50\end{array}$$

(13)

The same method was used for the affiliation functions of C/N, apparent score, GI and so on.

4.2.3. Determine the Weights of Each Factor

The use of a weighting methodology to estimate the level of impact of different evaluation indicators is an effective approach. By assigning a specific weight to each evaluation indicator, the method enables a quantitative measure of the importance of the indicator, a process that provides a structured framework for a thorough understanding of the role that each indicator plays in the overall evaluation system. Through this weighted average approach, it not only ensures flexibility and detailed consideration of the evaluation system but also promotes in-depth analysis of the connotations of diverse indicators. The weighted affiliation values of the impact factors confer a differentiated comparison with each other in the evaluation process, enabling a more systematic and objective definition and understanding of the correlations and differences between the evaluation levels.

For the i-th influence factor, the affiliation function presents the standardized values $k_{i1},k_{i2},k_{i3},k_{i4}$ for the different criteria levels, which are then combined and averaged to calculate a composite affiliation value for the factor.

$${\mathsf{\omega }}_i=\frac{x_i}{\overline{k_i}}=\frac{x_i}{{\displaystyle \sum _{j=1}^4{k}_{ij}(i=1,2,3,4)}/4}$$

(14)

According to Equation (14) the weight ${\mathsf{\omega }}_i$ of the i-th factor can be obtained, normalized to give:

$$a_i=\frac{{\mathsf{\omega }}_i}{{\displaystyle \sum _{i=1}^n{\mathsf{\omega }}_i}}$$

(15)

4.3. Changes in the Degree of Decomposition of Mixed Compost in Different Proportions

The evaluation results of the method are visualized in graphical form for the five different proportions of compost tests carried out, as shown in Figure 18. The data obtained from the above experiments were measured three times per day, so the horizontal coordinates of the graphs shown below appropriately reflect the time points that coincide with the experiments. This ensures that the scale of the horizontal coordinates in the graph is consistent with the times recorded in the previous experiment.

As shown in the figure, in the compost fermentation test, the fermented product obtained by mixing and treating sheep and cow manure in a ratio of 2:1 had the shortest cycle time to reach complete putrefaction, taking only 10 days and 16 h. In contrast, the fermentation cycle for whole cow dung was significantly longer, totaling 12 days and 8 h. This difference in duration may be attributed to the respective compositional properties of cow and sheep manure affecting fermentation efficiency. Comparatively, whole sheep manure, sheep manure mixed 1:1 with cow manure, and sheep manure mixed 1:2 with cow manure followed each other in their respective complete putrefaction cycles.

The fermentation process can be subdivided into four stages: unrotted, slightly rotted, basically rotted, and fully rotted. The duration of these stages for the five different ratios of fermentation products in this experiment showed slight variation. The unrotted and slightly rotted stages lasted about 3 to 4 days each, while reaching the basically rotted stage took about 3 days before transitioning to the fully rotted stage.

It is particularly noteworthy that sheep and cow manure mixed in 2:1 and 1:1 ratios showed faster progress in the early stages of fermentation. This phenomenon is likely significantly correlated with the optimization of microbial activity and nutrient balancing under these ratios. Microbial activity is a core driver of compost fermentation, and the nutrient sources required by microorganisms play a crucial role in their growth and metabolism. Sheep manure is usually high in nitrogen, whereas cow manure is rich in fiber, and a reasonable ratio of the two may create a more suitable environment for microbial proliferation and organic matter decomposition.

5. Discussion and Conclusions

Using the optimized small-scale aeration and heating composting equipment for mixed fermentation of different proportions of cow and sheep manure, the organic fertilizer produced met relevant standards for pH value, humidity, nitrogen, phosphorus, potassium content, and germination index. The equipment’s core control system regulates the maximum temperature and minimum oxygen concentration in the compost, ensuring efficient and timely feedback and regulation performance. From the comparative analysis of the germination rates of mung bean, corn, and wheat seeds, different ratios of organic fertilizers showed significant differences in promoting seed germination. Notably, the organic fertilizer made by mixing sheep dung and cow dung in a 1:1 ratio was particularly effective in improving the germination rate of mung bean seeds. When mixed in a 2:1 ratio, the resulting organic fertilizer showed the best results for maize and wheat seed germination. These results reveal the variability in the response of different crops to mixed organic fertilizers and demonstrate that mixed fermented organic fertilizers are superior to single fermented fertilizers in overall benefits. Sheep manure provides critical nutrients for crop growth due to its rich nutrient content, which is especially important for high nutrient-demanding crops such as maize. However, excessive nutrient concentrations may trigger root burn. In contrast, cow dung performs better in terms of nutrient regulation and slow-release properties, reducing the risk of root burn and ensuring continuous nutrient absorption by crops. In the mixed fermentation process, the right proportions of sheep and cow manure fully utilize their complementary and synergistic effects. In terms of nitrogen, phosphorus, and potassium content, sheep manure contains higher concentrations of these elements compared to cow manure. Among the fermented composts with different mixing ratios, those containing higher percentages of sheep manure usually showed higher levels of nitrogen, phosphorus, and potassium. Additionally, in the determination of the seed germination index, organic manure mixed with sheep manure and cow dung at a 2:1 ratio exhibited the best germination index, further demonstrating the advantages of optimized mixing ratios in promoting crop growth.

This study not only demonstrated the positive effects of different animal manure mixing ratios on soil nutrition and crop germination but also provided practical guidance for future agricultural production, forming an important scientific basis for combining resource recycling and enhancing sustainable agricultural development. The results of the compost fermentation test demonstrated that the mixture of sheep and cow manure in a 2:1 ratio had the shortest cycle time to reach complete putrefaction, achieving this state in just 10 days and 16 h. This rapid decomposition can be attributed to the optimal balance of nutrients and microbial activity in the mixture. In comparison, whole cow dung required a significantly longer fermentation cycle of 12 days and 8 h, highlighting the influence of manure composition on fermentation efficiency. The fermentation cycles of whole sheep manure, sheep manure mixed 1:1 with cow manure, and sheep manure mixed 1:2 with cow manure followed sequentially, indicating a consistent trend in the influence of mixing ratios on the decomposition process.

According to the research of a large number of scholars, it was found that their fuzzy evaluation models were effectively verified, but they were not applicable to the evaluation of the degree of decomposition of organic fertilizers. So by combining the evaluation levels and evaluation indexes of many scholars, the fuzzy evaluation model that was most suitable for this experiment was finally developed. The subdivision of the fermentation process into four stages—unrotted, slightly rotted, basically rotted, and fully rotted—revealed slight variations in the duration of these stages across different manure ratios. The initial stages of unrotted and slightly rotted lasted about 3 to 4 days each, followed by approximately 3 days to reach the basically rotted stage before transitioning to the fully rotted stage. These findings suggest that the early stages of fermentation are relatively consistent, regardless of the manure ratio, but the transition to complete putrefaction is influenced by the specific composition of the manure mix.

Notably, the mixtures of sheep and cow manure in 2:1 and 1:1 ratios exhibited faster progress in the early stages of fermentation. This accelerated progress is likely due to the enhanced microbial activity and balanced nutrient availability in these ratios. Sheep manure, rich in nitrogen, and cow manure, abundant in fiber, together create an environment conducive to microbial proliferation and effective organic matter decomposition. The results underscore the importance of optimizing manure ratios to enhance the efficiency of the composting process, thereby reducing cycle times and improving the quality of the composted product. These findings have significant implications for the practical application of composting, suggesting that strategic mixing of different types of manure can lead to more efficient and effective composting outcomes.