Optimization of composting methods for efficient use of cassava waste, using microbial degradation

With the recent revolution in the green economy, agricultural solid waste resource utilization has become an important project. A small-scale laboratory orthogonal experiment was set up to investigate the effects of C/N ratio, initial moisture content and fill ratio (vcassava residue: vgravel) on the maturity of cassava residue compost by adding Bacillus subtilis and Azotobacter chroococcum. The highest temperature in the thermophilic phase of the low C/N ratio treatment is significantly lower than the medium and high C/N ratios. The C/N ratio and moisture content have a significant impact on the results of cassava residue composting, while the filling ratio only has a significant impact on the pH value and phosphorus content. Based on comprehensive analysis, the recommended process parameters for pure cassava residue composting are a C/N ratio of 25, an initial moisture content of 60%, and a filling ratio of 5. Under these conditions, the high-temperature conditions can be reached and maintained quickly, the organic matter has been degraded by 36.1%, the pH value has dropped to 7.36, the E4/E6 ratio is 1.61, the conductivity value has dropped to 2.52 mS/cm, and the final germination index increased to 88%. The thermogravimetry, scanning electron microscope, and energy spectrum analysis also showed that the cassava residue was effectively biodegraded. Cassava residue composting with this process parameter has great reference significance for the actual production and application of agriculture.


Introduction
With the continuous development of the industry worldwide, more and more industrial wastes are generated. Environmental protection and waste recycling have gradually become critical issues of concern globally. Human-generated environmental waste has a different meaning. For economically developed countries, environmental waste usually means greenhouse gas  emissions. The most typical ones are carbon dioxide, methane, and nitrous oxide (Xu et al. 2020). For developing countries, environmental wastes are more of various solid wastes generated in basic human activities and industrial production. In recent years, they have made great progress in cleaner production. Meanwhile, they pay more attention to reducing local environmental problems (Pan et al. 2021b). They also actively use various methods to deal with the generated waste and maximize the use of waste to generate more value.
China is one of the countries with the richest potato resources in the world. Cassava, also known as tree potato, is known as the "king of starches." Cassava residue is one of the large amount of agricultural waste obtained during the production of cassava starch, which is produced in large quantities, has high treatment costs, and causes many environmental problems when dumped in large quantities. Cassava waster contains high concentrations of organic Responsible Editor: Diane Purchase * Wei Gao galaxy@gxu.edu.cn 1 compounds and improper disposal can lead to the eutrophication of rivers. The air can be polluted by the toxic fumes produced, in the process of cassava residues encroaching on agricultural land and deteriorating and rotting. Wastewater can threaten vegetation, soil, water bodies, and humans (Hu et al. 2022). The degradation and reuse of large quantities of cassava residues have become an urgent issue in solid waste management. Sometimes, farm farmers mix cassava residue with some feeds and use it to feed livestock such as pigs and chickens. Meanwhile, cassava residue can also be used to cultivate flat mushrooms. But all these treatments have big limitations. Cassava residue only fills animals' stomachs and does not provide a lot of nutrients, because the digestion and absorption rate of crude fiber is very low in livestock. It is also far less effective than using wood chips or crop straw when used to cultivate flat mushrooms. On the other hand, soils have been degraded in agricultural production and their productivity has declined due to the continuous and excessive misuse of fertilizers (Djuma et al. 2020). The excessive use of fertilizers has caused problems such as soil impoverishment, loss of biodiversity, and increased salinization (Djuma et al. 2020). Therefore, it is important to protect agricultural land and limit the use of fertilizers. Composting effectively treats this cellulosic waste, enriching it with nutrients and rendering it harmless .
Composting is an aerobic, thermophilic, solid-state fermentation process. Microorganisms provide the most important energy for the normal operation of the composting system (Pan et al. 2021a). Inoculation of compost with exogenous microorganisms can effectively accelerate the maturation process of compost (Alfonzo et al. 2021) and promote humus formation . By inoculating the compost with a complex of microbial agents, the biotope diversity is effectively increased, enzyme activity is increased, and microbial community diversity is improved (Li et al. 2018). Microorganisms in the normal growth and reproduction process can decompose organic matter. The more suitable C/N ratio for their growth and reproduction is around 25. Given the high carbon to nitrogen ratio, microorganisms do not have enough nitrogen to grow and reproduce. This may lead to low reproduction rates, slow decomposition of organic matter, low humification coefficient, and poor fermentation effect. If the C/N ratio is too low, the energy source required for microbial growth and reproduction is restricted, the fermentation temperature of the pile is slow to warm up, and excess nitrogen is released in the form of ammonia, resulting in a large loss of organic nitrogen (Pan et al. 2022) and also giving off an unpleasant smell. Therefore, a reasonable adjustment of the C/N ratio of the composting material is one of the most effective ways to accelerate the decomposition of compost. Thomas et al. have successfully composted at lower initial C/N ratios and found that lower initial C/N ratios (10:1) composts reduced the number of pathogens in a short period, although they were slow to warm up (Thomas et al. 2020).
Moisture influences, to a large extent, the porosity of the compost pile and the rate of gas exchange as well as the oxygen supply capacity and microbial metabolism, thus affecting the conversion of organic matter and expressing these effects through changes in pile temperature. Numerous studies have shown that too high or too low moisture content are not conducive to composting . Increased pile compaction, reduced free space (Jain et al. 2018), and permeability result in insufficient oxygen supply in a pile, difficulties in warming the compost, lower organic matter degradation rates, and longer composting cycles when the moisture content (White et al. 2011) is too high. The metabolism of composting microorganisms is hampered by low humidity levels and excessive moisture, leading to a decrease in microbial activity and difficulties in compost maturation. In general, compost quality can be expressed by the stability and maturity of the compost ). Stability represents the growth activity of microorganisms. This change can be observed by measuring the respiration index by looking at chemical element transitions in the organic matter during composting. The organic matter degradation amount of phytotoxicity represents maturity, and the seed germination index can well reflect it (Said et al. 2007). Adding a stable and mature compost product to the soil can positively affect soil improvement ). It allows the soil to have better nutrients, allowing for more plant growth (Proietti et al. 2016). On the contrary, immature compost will fix the nutrients in the soil (Hannet et al. 2021), compete with the plants growing in the soil for nutrients and oxygen, and inhibit plant growth. Immature compost products can often release toxic substances that damage plant roots.
During the composting process, we introduced Bacillus subtilis and Azotobacter chroococcum. Bacillus subtilis is able to secrete cellulase, which has a strong ability to decompose lignocellulose. Bacillus subtilis will reproduce heat-resistant and resistant bacilli so that there is no need to worry about killing them even during the thermophilic period of composting. At the same time, Bacillus subtilis itself is a biological fertilizer. At the end of composting, Bacillus subtilis enters the soil along with the mature fertilizer, which not only promotes the growth and development of plant seeds, seedlings, and roots (Cabello et al. 2019) and enhances the disease resistance of plants, but also creates a probiotic environment in the soil (Ganchev 2021). Azotobacter chroococcum is also a bacterial fertilizer that has a strong nitrogen fixation effect and can fix atmospheric N 2 into NH 3 . Adding Azotobacter chroococcum to the pile can increase the nitrogen content in the final pile and improve the nutrient content of the fertilizer. Bacillus subtilis and Azotobacter chroococcum, these two microorganisms were added in a ratio of 1:1, and the effective viable count of each bacterial suspension was about 2 × 10 8 CFU/ ml. The addition amount is 0.5% of the dry mass of the pile, because at this rate, it can speed up the composting process, promote compost maturation, reduce carbon loss, and improve the quality of compost (Azzawi and Kamal 2022), although some researchers have explored how the C/N ratio, moisture content, and pile porosity affect compost quality. However, most of them focus on a certain factor to explore. There is not a good representation of the cross-influence of different factors affecting composting results and the strength of their influence. To better investigate the effect of cassava residue composting and to be as close as possible to its practical application in agricultural production, we only used cassava residue as a substrate material for composting without the addition of swelling agents such as straw or wood chips and to scale up the range of experimental parameters appropriately. Because of the high moisture content of the cassava residue itself and the high denseness of the pile and other characteristics, if there is no expansion agent filling, the pile is slow to warm up and the fermentation effect is poor; in order not to affect the composition of the material elements of the cassava residue pile, we added gravel to the pile to change the denseness of the cassava residue pile. Gravel is not easily decomposed by microorganisms within the pile and does not participate in chemical reactions within the pile. The microporous structure of gravel does not impede the flow of water within the pile and is superior to the normal foam structure. Therefore, we put gravel in the fertilizer.
The main factors affecting the effectiveness and maturity of pure cassava residue composting were investigated using orthogonal tests: the C/N ratio of the compost (15, 25, and 35), the water content (50%, 60%, and 70%), and the ratio of the filling ratio (v cassava residue : v gravel ) (6, 5, and 4). Using orthogonal tests, the best process parameters can be found quickly. The composting process of cassava residue, its morphology, and the change of element content was demonstrated by thermogravimetric (TG), scanning electron microscope (SEM-EDS), and other detection methods.
The results of this study may provide a certain reference for the following research: (i) the ratio of C/N ratio, moisture content, and filling ratio (v cassava residue : v gravel ) of composting to the heap temperature, organic matter, how nitrogen, phosphorus, potassium, pH, E4/E6, EC, and GI change and (ii) the importance of these three parameters at different stages affecting each outcome.

Raw materials
The cassava residue was taken from the Hua Rui Starch Factory in the Wuming District of Nanning City, Guangxi. The newly obtained cassava residue contained high moisture content and was treated appropriately to adjust its moisture content. The inoculated Bacillus subtilis and Azotobacter chroococcum were purchased from the Guangdong Microbial Strain Collection Management Centre. To change the density of the pile and adjust its porosity, we added gravel to the pile, which was purchased from Guangxi Suge Horticulture Co. The physicochemical properties of the cassava residue, the raw material for the compost, were determined before the start of the experiment, 85% organic matter (OM), 0.2% total nitrogen (TN), 0.1% phosphorus (TP), 8.8 (pH), and 5.7 mS/cm electrical conductivity (EC).

Composting units
The composting was performed using a homemade two-layer composting device, the inner layer made of cylindrical plastic drums with a diameter of 25 cm, a height of 33 cm, and a volume of 10 L. The outer layer was wrapped in large drums with a diameter of 28.1 cm, a height of 38.5 cm, and a volume of 25 L. The bottoms of both barrels are filled with foam to use the air to provide better insulation and reduce heat loss.

Composting programs
This experiment was started on October 1, 2021, and compost was carried out for 28 days. After initial exploration of the one-factor method, a three-factor, a three-level orthogonal test was set up following the standard L9 (3 3 ) orthogonal matrix of the orthogonal test design method. The total weight of each pile was designed to be 4 kg for each of the 9 sets of experiments. After calculation, different weights of urea, water, and gravel were added to the pile to adjust the C/N ratio, initial water content, and porosity of the pile. The parameters of the three factors for the nine treatment groups are listed in Table 1.

Determination of basic properties of pile samples
During the composting process, temperatures were recorded at 9 a.m. and 3 p.m. each day, with thermometers inserted at the top, middle, and bottom of the pile and averaged at multiple points while the ambient temperature was measured and recorded. Samples were obtained from three different heights of each reactor, each with a total weight of about 200 g, and mixed thoroughly. The fresh samples obtained were air-dried. After grinding and passing through a 1 mm sieve, the powder obtained was used to measure the organic matter and nutrients of the compost product. A fresh sample of 5.0 g of compost was weighed into a conical flask, 50 mL of deionized water was added, mixed, and placed in a constant temperature water bath shaker (SHA-B, Changzhou Jinnan Instruments) for 24 h and then centrifuged. The supernatant after centrifugation was filtered through a filter paper with a pore size of 0.45 μm to obtain the leachate of fertilizer. The pH of the leachate was measured using a pH meter (PHS-2F, Remag Shanghai). The organic matter content was determined using potassium dichromate and concentrated sulphuric acid, based on the oxidation of potassium dichromate; the nitrogen content was determined by Kjeldahl; the phosphorus content was determined by molybdenum yellow chromogenic photometry; the potassium content was determined by boiling the sample in sulphuric acid and hydrogen peroxide, diluting it and determining it by flame photometry ). The UV-visible spectrum of the leachate was measured using an ultraviolet spectrophotometer (UV-2200, Riley, North Branch, China) to obtain the absorbance ratio at 465 nm and 665 nm, i.e., the value of E4/E6 (Xu et al. 2019). In Petri dishes (10 cm in diameter), 10 seeds were distributed on filter paper and moistened with 10 mL of compost water extract. Three replicate Petri dishes of each sample were incubated in the dark at 25 °C for 48 h. The number of germinated seeds and root length were determined using distilled water as a control sample. SGI was calculated according to the following equation (Yang et al. 2013) In the above equation, S: Seed germination rate of compost leachate. C: Compost extract seed root length. G: Germination rate of seeds in distilled water. D: Distilled water seed root length.
The thermogravimetric curves of the samples were measured using a thermogravimetric analyzer, DTG-60 (H) (Shimadzu, Japan). A field emission scanning electron microscope (SEM) SIGMA 300 (Zeiss, Germany) was used to scan the surface of the compost samples and energy spectra of the elements were obtained. The multi-factor ANOVA function in SPSS analysis software was used to statistically examine the effect of three factors and three levels on the composting results. The significance of the effect of three factors and three levels on the results was determined using the significant difference test at a 95% confidence level (P < 0.05).The data obtained from the experiments were expressed as the mean of the values measured in parallel samples, and the data were processed and analyzed using Origin and plotted graphically.

Stack temperature and ambient temperature
Temperature is an important reference to measure the maturation process of composting (Han et al. 2012). During the composting process, temperature changes can indicate to some extent the decomposition of organic matter by micro-organisms (Zhao et al. 2022), and high temperatures accelerate the decomposition of the pile. However, too high a temperature may kill the microorganisms in a pile (Sudharsan Varma and Kalamdhad 2014). The composting process can be divided into three such phases: the warming phase, the thermophilic phase, and the curing phase. The flow chart of the composting study is shown in Figure 1. Figure 2 shows the changes in pile temperature and ambient temperature. As the cassava residue pile continued to biodegrade, the temperatures in all nine treatment groups continued to rise during the first 2 days, rising above 50 °C by day 3, reaching thermophilic conditions and remaining largely above that level for at least 7 days. This indicates that active biodegradation of the organic matter within the pile has occurred at this stage. The low C/N ratio, limited by the energy source and the growth and reproduction of microorganisms to some extent, explains the slower warming of treatment groups 1, 2, and 3 compared to the other treatment groups and their significantly lower maximum temperatures during the thermophilic phase.
In the first stage of the process, the warming phase, microorganisms metabolize organic nitrogen, converting it into ammonium nitrogen. The microorganisms multiply rapidly and break down the easily degradable material in a pile, continuously rise the temperature. In the second, or thermophilic stage, the microbial activity releases a large amount of heat, causing the temperature to rise and maintaining a hightemperature environment. This allows thermophilic bacteria within the pile to dominate, and these microorganisms can rapidly break down some of the simpler, low-molecular-weight compounds. At the same time, the continuous high-temperature environment during this stage kills many  1  15  50  6  2  15  60  4  3  15  70  5  4  25  50  4  5  25  60  5  6  25  70  6  7  35  50  5  8  35  60  6  9  35  70  4 pathogenic micro-organisms and plant seeds, satisfying the hygiene requirements and creating the conditions for the subsequent practical use of the fertilizer. In all treatments, the final temperature change levelled off. The temperature dropped below 40 °C and continued to converge to the ambient temperature, which meant that the composting was ending (Martins et al. 2021). Analysis showed that there was a significant difference (P < 0.05) in the effect of the C/N ratio (P = 0.041) on temperature during the thermophilic phase. A suitable C/N ratio enables the microorganisms in the pile to grow and multiply faster, which enables the pile to enter the thermophilic period faster and maintain a higher temperature for a longer period during the thermophilic period, accelerating the decomposition rate of the pile. Water content (P = 0.722) and fill ratio (P = 0.531) were not significantly different (P > 0.05) for temperature effects. Therefore, at a C/N ratio of 25, the pile can be heated up quickly and kept at a high temperature for a longer period of time.

Organic matter and nutrient content during composting
The changes in organic matter (OM) content during the composting process are shown in Fig. 2. Microorganisms mineralize the organic carbon in organic matter as a source of starting energy . The OM content tends to decrease during the composting process, and its OM content varies significantly. As shown in Fig. 3, OM content gradually decreases from the composting process. At the end of composting, the amount of organic matter degraded decreased by about 30%, with the highest rates of decrease in treatment groups 3, 5, and 8. This is because the right amount of water content and nutrients provide a suitable living environment for microorganisms in the pile, which promotes the interaction between the compost and microorganisms, enabling the microorganisms to multiply and the organic matter in the cassava residue to be consumed more and converted into more stable humus. Lignocellulose would be resistant to oxidation by potassium dichromate, resulting in incomplete oxidation of the organic carbon in some samples . A multi-factor ANOVA revealed that the differential effect of the C/N ratio (P = 0.006) on the final amount of organic matter degraded at the end of the compost pile was extremely significant (P < 0.01). The differential effect of water content (P = 0.015) on the final result was significant, whereas the effect of fill ratio (P = 0.090) on the results was not significantly different (P > 0.05) and not statistically significant but has some reference value. Therefore, the C/N ratio has the greatest effect on the results of organic matter degradation. An appropriate C/N ratio accelerates the decomposition of organic matter and increases the final amount of organic matter decomposed in the pile. The second-largest effect is water content, and the fill ratio has some effect but no significant difference.  Due to the inherent characteristics of cassava residue, the nitrogen, phosphorus, and potassium contents are low. Figure 4 shows the changes in N, P, and K content during the composting process. More urea was added to the three treatment groups (7, 8, and 9) before the composting started, so the N content of the three was a little higher at the beginning. As the composting process progresses, the nitrogen content changes and stabilizes at the end due to the multiple effects of urea decomposition, ammonia volatilization (Ravindran et al. 2021), and the growth and reproduction of Azotobacter chroococcum. At the end of composting, the C/N ratio (P = 0.011) and water content (P = 0.026) had a significant effect on the final elemental nitrogen content (P < 0.05). On the other hand, the fill ratio (P = 0.086) had no significant effect on the elemental nitrogen content (P > 0.05) and was not statistically significant, but is of some reference value. To adjust the C/N ratio we added urea. However, it differed from the materials used by others to adjust the C/N ratio (pig manure, Fig. 3 Change of organic matter content during composting Fig. 4 The content of nitrogen, phosphorus, and potassium in each sample during composting straw, etc.) in that the urea decomposes or evaporates quickly, as the material itself has a low N content. Hence, it causes a consistent low elemental N content in a pile. The measured TN content curve is rather heterogeneous, considering the addition of measurement errors. Urea pellets were not completely dissolved or dissolved but under the action of water transport, the distribution of elemental N throughout the pile was not very uniform, resulting in higher N content in groups 2 and 3 at the beginning. The N content of group 2 was kept at a higher level during the composting process, probably because the gravel added to it was the highest level, which led to a high porosity in the pile, and the Azotobacter chroococcum was able to have more contact with the air, and at the same time, in the low N pile environment, it prompted the Azotobacter chroococcum to fix more N 2 from the atmosphere into NH 3 . Figure 4 shows that phosphorus tends to rise steadily during the composting process. In the presence of saprophytic microorganisms, organic compounds containing phosphorus form phosphoric acid, becoming a nutrient that plants can take up. According to the analysis, the leachate produced in the initial stage takes away some phosphorus, which may lead to a decrease in its content. Then, there is degradation of organic matter and volatilization of gases, leading to an increase in the relative concentration of phosphorus (Chan et al. 2016). Treatment groups 5, 3, 8, and 7 showed the greatest increases, all reaching 0.52%, 0.41%, 0.40%, and 0.36%, respectively. At the end of composting, the multifactorial ANOVA showed that the C/N ratio, water content, and fill ratio all produced highly significant differential relationships on phosphorus content at the end of composting (P = 0.002, P = 0.000, and P = 0.000). It was possible to obtain that the C/N ratio, water content, and fill ratio produced extremely significant differential relationships in phosphorus content. Therefore, adjusting the appropriate impact parameters (i.e. C/N ratio = 25, moisture content = 60%, and packing ratio of 5) can effectively contribute to the increase in phosphorus content during the composting process. Figure 3 shows that the potassium content tends to rise steadily as the composting progress. The rise was greater during the warming and thermophilic stages, and the increase was relatively slower during the curing stage. Analysis of the data revealed that at the end of composting, water content (P = 0.004) produced an extremely significant differential relationship with the final potassium content (P < 0.01). The C/N ratio (P = 0.016) also had a significant effect on the final potassium content (P < 0.05). The fill ratio (P = 0.119) did not produce a significant differential relationship with the potassium content. Therefore, the C/N ratio had the greatest effect on the potassium content, followed by the water content, and the fill ratio did not significantly affect the potassium content. pH, E4/E6, and seed germination index of composting products during composting.
The leachate of the compost product during the composting process can provide us with a great deal of useful information to help us determine the state of fermentation of the pile.
pH is closely related to the type of microorganisms that grow during the biodegradation of organic matter and the biochemical reactions. The lower the pH, the better the nitrogen retention in the compost. Too low pH can inhibit microbial growth activity (Wang et al. 2017). Generally accepted that a pH of 5-8 is optimal for compost products (Zhang and Sun 2016). Too high or too low pH can hurt microbial activity and therefore the composting process. Most bacteria prefer to live in environments with a pH of 6.8 to 7.4. Fungi can grow normally in environments with a pH of 5.0 to 8.0. Figure 5(b) shows that the pH was alkaline at the beginning of the composting period. As the composting progress, the pH of the nine treatment groups gradually decreases and reaches a peak during the high-temperature period. During the second half of the composting process, the pH stabilized. During the first 8 days of composting, the pH of all 9 treatment groups decreases rapidly, which can be attributed to the organic acids, such as formic and acetic acid, produced by the organic reactions accompanying the degradation process. Under high-temperature conditions, organic nitrogen is biodegraded to inorganic nitrogen such as NH 4 + and volatilized as NH 3 . After day 8, the organic nitrogen is mineralized by microbial activity and the pile temperature drops. Nitrification gradually intensifies with time. Due to the rapid conversion of ammonium nitrogen (NH 4 + -N) to (nitrate) NO x -N, the NO x -N content of the treatments increases. Inorganic acids and organic acids are continuously produced, and the presence of H + causes the pH to decrease and stabilize gradually. The results of the gradual decrease in pH are very close to those of the composting carried out by Feng and Zhang (2021). Most notably, at the end of composting, the pH of trea-5 and trea-6 decreased by 1.46 and 1.49, respectively, and the pH of these two treatment groups was closer to neutral. Using a multi-factor ANOVA, an extremely significant difference (P < 0.01) was obtained in the effect of the C/N ratio on pH at the end of composting. The effect of water content, and fill ratio on pH was significantly different (P < 0.05), and the effect of C/N ratio (P = 0.004) on pH was the greatest compared to water content (P = 0.026) and fill ratio (P = 0.031). Therefore, we conclude that a C/N ratio of 25, a water content of 70%, and a filling ratio of 6 resulted in the best pH of the samples at the end of degradation, which was closer to neutral.
The ratio of the absorbance of compost leachate at wavelengths of 465 nm and 665 nm is known as E4/E6 (Yin et al. 2019) and is considered an important parameter for the rapid evaluation of compost maturity. This ratio changes continuously as the composting process progresses. During the composting process, the value of E4/E6 decreases as the molecular weight of humic acid and the degree of aromatization continue to increase. The number of humic acid molecules does not cause this value to change. In recent years, it has been increasingly recognized that the microbial decomposition of organic matter is the main source of humic acid (Cai et al. 2020). Therefore, using the E4/E6 ratio to describe the degradation effect on the samples is considered appropriate. Before composting, the organic matter content of the manure is high, and the molecular weight of humus and aromatization in the manure is low. Consequently, the E4/E6 values are high. As the composting progressed, the organic matter decomposed considerably, and high molecular weight humic acids were formed (Cai et al. 2021). The ratios of all experimental groups decreased continuously, and, in comparison with the other groups, the samples from treatment group 5 had the fastest rate of decrease in their ratios and the smallest ratios during the corresponding high-temperature period. In contrast, treatment group 5 showed good temperature indicators during the high-temperature period. The organic matter degradation effect and the accumulation of nutrients are also more considerable. At the end of composting, the E4/E6 values decrease significantly, indicating degradation and mineralization of degradable organic matter. The phenolic compounds underwent oxidation and were bound to methoxy or aliphatic side chains. This indicates an increased degree of OM humification. Among them, the E4/E6 ratio decreased to 1.61 and 1.65 for trea-5 (60% water content) and trea-6 (70% water content) with a C/N ratio of 25. The E4/E6 ratio decreased to 1.71 for trea-8 with a C/N ratio of 35 and 60% water content. The ratio for all treatment groups was less than 2, indicating that the required state of decay was achieved in all treatment groups under this test indicator. The better the pile temperature, pH, and organic matter degradation and nutrient conditions, the higher the degree of humification here, proving the better the degradation and humification effect. The ANOVA showed that the C/N ratio (P = 0.024) and water content (P = 0.037) had a significantly different relationship (P < 0.05) on the degree of decomposition of the compost at the end of the compost pile. In contrast, the fill ratio (P = 0.662) had no significant effect on the final E4/E6 results and was not statistically significant. Therefore, it was concluded that the best degree of decay was achieved at a C/N ratio of 25 and an initial moisture content of 60%.
Electrical conductivity (EC) is directly related to the concentration of salt-based ions (Huang et al. 2004) and can be used to determine the salt content of the final compost product, indirectly reflecting the potential toxicity of the product for affecting plant growth. This is because high salt concentrations can affect the germination rate of seeds, causing plants to wilt, inactivate, and produce "burned seedlings." The presence of a low EC value indicates that the compost is mature and ready to use. At the beginning of Fig. 5 a The compost product of (i) pH, (ii) E4/E6, (iii) EC experimental analytical instrument diagram, and (iv) schematic diagram of the seed germination index. Compost products of b pH, c E4/E6, d EC, e seed germination index, and f picture of mature compost composting, the EC increases due to the release of inorganic salts (e.g., ammonium, nitrate, potassium) as the microorganisms degrade the organic matter . In contrast, we did not find an increase in EC at the beginning of composting. Considering that it is possible that the high ambient temperature at the beginning of this composting promoted the decomposition of urea a lot and the volatilization of ammonia, etc. (Ravindran et al. 2022), this is why the EC value decreased compared to the beginning of the composting. Later, as the composting process progresses, mineral salts precipitate, ammonia volatilization increases, and stable humus begins to form. The results of Makan's study show that composting with EC levels not exceeding the 3 mS/cm limit does not hurt plant growth (Makan et al. 2014).
The EC values for treatment groups 5 and 9 were below 3 mS/cm, which is in general agreement with the results tested by Li et al. (2021), indicating that the compost end-product does not pose a toxicity risk to plant growth. The low EC values for treatment group 5 indicate that the treatment facilitates the compost decomposition. The ANOVA showed that the C/N ratio (P = 0.685), water content (P = 0.655), and fill ratio (P = 0.586) did not significantly differ from each other in the final conductivity values. Here, the conductivity failed to distinguish well between the effects of each factor.
The seed germination index (SGI) is a visual indicator of the toxicity of the seed leachate. Therefore, by immersing seeds in the leachate of compost products and incubating them away from the light, the germination rate and root length of the plant seeds can be observed and measured to obtain an accurate picture of the effect of the leachate on the germination of the seeds and thus an accurate test of the decomposition of the compost. As a rule, mature compost products have a low concentration of salt-based ions and a low content of pathogenic micro-organisms, so the corresponding phytotoxicity is lower. Therefore, the higher the degree of compost product decomposition, the less toxic the compost product and the higher the seed germination index. As shown in Fig. 5(e), the SGI values for all treatments increased gradually with the composting process. In the pre-composting stage, the germination index of all treatment groups was relatively low, and the fertilizer was more phytotoxic. It is presumed that the main reason for this is that the pre-composting products have not undergone the hightemperature treatment of the thermophilic stage, so they contain more pathogenic microorganisms and are very toxic to the seeds. We found that in cultivating seeds, 48 h later, a large number of white molds grew in the pre-compost product leachate seed pan, and some seeds became moldy. As the composting continued, the SGI of each treatment showed an increasing trend. This trend indicates that the toxic substances in the compost products gradually decreased with the increase in composting time. It is generally accepted that if the SGI is > 50%, the compost is considered largely decomposed, and when the SGI reaches 80-85%, the compost is considered fully decomposed and not toxic to plants (Gabhane et al. 2012). Treatment groups 7, 8, and 9 all had SGI values of 80% or more, indicating that the compost was largely mature at the end of composting. In contrast, the products produced by trea-1, trea-2, and trea-3 are not mature and contain some amount of phytotoxic compounds. At the end of the composting stage, a multi-factor ANOVA study determined that the C/N ratio (P = 0.002) produced a highly significant difference in the seed germination index (P < 0.01), the effect of water content on the seed germination index was also significant (P = 0.050), and the fill ratio (P = 0.950) did not produce a statistically significant difference in the germination index. We found that the seed germination index was also highest when the temperature of the thermophilic period was higher and the decay of the compost product was higher. This proves that its toxicity is minimal and that the final quality of the compost is the best. Luiz Antônio de Mendonça Costa also refer to that the faster the decay rate, the higher the stability of the final compost (Costa et al. 2021). Thus, the least toxic compost product and the highest seed germination index were considered when the C/N ratio was 25 with a moisture content of 60%.
As shown in Fig. 5(f), at the end of the composting process, the cassava residue fertilizer showed a dark brown, loose granular structure with a soft and loose feel. No mold or other stray colonies were observed to form, and the foul odor at the beginning disappeared. Furthermore, no significant leaching or clumping of the pile material was observed during the composting process in any treatments, demonstrating that the composting was effective.

Thermal decomposition of composting products at different composting stages
To better observe the effect and extent of degradation of substances during the composting process, typical samples were analyzed by thermogravimetric (TG) analysis, and first-order linear differentiation of TG was performed to obtain DTG curves. The overall trend in the composition and content of the material and the change in the maturity of the samples remained the same for all treatment groups. A representative sample of treatment group 5 was selected for analysis. Figure 6(a), (b), and (c) show the TG and DTG curves for treatment group 5 on days 0, 12, and 28 of the composting process. It can be observed in the graphs that the curves have a small heat absorption peak at around 50-150 °C, which is mainly caused by the loss of residual water after the samples have been dried (Marhuenda-Egea et al. 2007).
Hemicellulose undergoes degradation in the temperature range of 170-210 °C, while other easily degradable substancendergo decarboxylation. The region of 210-300 °C corresponds to the low-temperature degradation of biodegradable materials and includes the thermal degradation of carbonaceous biomass, the degradation of aliphatic compounds (Raclavska et al. 2021), and the slow degradation of some easily degradable aromatic structures. The 300 and 400-450 °C correspond to the organic polymers present in the sample or produced during the curing phase (Diaz et al. 2021). In the high-temperature range, from 450 to 600 °C, are reactions of organic components with different thermal stability that are more difficult to degrade. Above 600 °C are substances that are not biodegraded during the composting process and the peak in DTG is attributed to the volatilization of carbon from the compost (Dhyani et al. 2018). As the compost process proceeds, the remaining solids become more mineralized and it is possible to find an increasing amount of carbon remaining, proving that the molecular weight and aromatization of the humus are increasing during the composting process and that the degree of manure maturity is increasing.

Micromorphological and elemental analysis of composting materials
The microstructure of compost samples from treatment group 5 was observed by SEM. This study used only cassava residue fermented compost, with lignocellulose primarily consisting of cellulose, hemicellulose, and lignin (Ajmal et al. 2021).
As shown in (a) and (d) in Fig. 7, at the beginning of composting, the feedstock had a dense microscopic morphological structure, with cassava residue in strips with square pore-like structures intact on the surface and a slight breakdown of the external wall tissue structure, with some of the internal tissue exposed. Therefore, it can be assumed that the lignocellulose of the inoculated compost is degraded to some extent at this stage (Xu et al. 2016). Figure 7(b) and (e) show that on day 12, the compost product is structurally loose, with the thin-walled tissue on the outside largely destroyed and the thick-walled tissue on the inside heavily broken down, facilitating the entry and degradation of microorganisms and their hydrolytic enzymes, which is consistent with the results expressed by Xu and Yang (2010). The surface of the densely porous material broke up into smaller parts, creating more voids. However, it can also be seen that the cassava residue maintains a largely intact support structure. Figure 7(c) and (f) show that after 28 days of composting, it was observed that the solid material continued to decrease in size, and the internal thin and thick-walled organization was severely disrupted. The square pore-like structure that was intact at the beginning was almost missing, the overall structure was disrupted, fractures occurred, and the void ratio on the surface became larger. The difference in the microscopic morphology of the compost before and after composting demonstrates that microorganisms are effectively degrading and maturing the compost material. Figure 8(a), (b), and (c) show the EDS spectra of the compost products of treatment group 5 on days 0, 12, and 28 of the composting process, respectively. The EDS spectra of the compost samples show the presence of five naturally occurring elements, as evidenced by the small amounts of nitrogen, phosphorus, and potassium present and evenly distributed in the samples from the energy spectra.
Since the distribution of elements in compost samples is not perfectly homogeneous, it is impossible to accurately represent the changes in the content of the elements in the compost product. However, it is possible to semi-quantify, to a certain extent, the changes in the substance content before and after composting. As the composting process continues, the elemental content of C decreases, with the weight percentage dropping from an initial 55.26 to 51.82%, corresponding to the decomposition of organic matter during the composting process. There was a significant increase in nitrogen, phosphorus, and potassium peaks, with the percentage elemental content increasing from 5.34%, 0.24%, and 0.43% to 5.98%, 0.5%, and 0.65%, respectively. This demonstrates an increase in the corresponding elemental content, consistent with our previously measured trends in organic matter, nitrogen, phosphorus, and potassium. The compost product also had the highest carbon content and adequate oxygen content, which would have resulted in an anaerobic environment if the oxygen content was insufficient, which aided the aerobic composting process of cassava residue.

Conclusion
The effects of the C/N ratio, initial moisture content, and filling ratio on the maturity of cassava residue compost were investigated with the addition of Bacillus subtilis and Azotobacter chroococcum as raw materials. A C/N ratio of 25 was effective in promoting the maturation of the compost and improving the maturity of the compost product. Too low a Fig. 8 EDS spectra of composting products on the day a 0, b 12, and c 28 of the composting process C/N ratio will inhibit the growth of microorganisms, slowing down the maturation of the compost and increasing its toxicity. The initial moisture content also influences the outcome of compost decomposition, with the decomposition rate significantly reduced at too high a moisture content. However, when the moisture content is too low, it does not meet the needs of microbial growth, and organic matter is difficult to degrade. The filling ratio has a certain influence on the compost quality, but the effect is not significant. Under these conditions, the organic matter degraded by 36.1%, the E4/E6 ratio was 1.61, the conductivity value decreased to 2.52 mS/ cm, the final germination index increased to 88%, and the thermogravimetric, SEM, and energy spectroscopy analyses also showed that the cassava residues were effectively biodegraded. The analysis also indicated that the cassava residue underwent effective biodegradation.