Effects of different ratios of pig manure to fungus residue on physicochemical parameters during composting

ABSTRACT

This study examined physicochemical parameters to assess their effectiveness as stability and maturity indicators during the process of composting pig manure and fungus residue at different ratios. The results showed that composting mixtures with all ratios of pig manure to fungus residue maintained a temperature exceeding 50 °C for more than 10 days during composting and met the requirement for pathogen destruction. The treatment containing mainly pig manure showed higher nitrogen loss and a shorter thermophilic phase and maturity time than the treatment containing mainly fungus residue. The germination index (GI) values indicated that compost maturity was achieved in the final compost with initial ratios of pig manure to fungus residue of 9:1–7:3 (GIs of 101.4%, 91.2%, and 81.3%); the ratio of 6:4 did not reach compost maturity (GI of 63.8%) and had an inhibitory effect on seed germination. The results of this study suggest that a ratio of pig manure to fungus residue of approximately 8:2 can be considered suitable for the efficient and quality composting of pig manure and fungus residue.

Implications: Co-composting of pig manure and edible fungi residue with appropriate proportion can effectively reduce the risk of environmental pollution caused by agricultural wastes, as well as achieve a safer and high-quality organic fertilizer, which can be used to improve physical and chemical properties of the soil, increase crop yields, and promote agricultural sustainable development. Therefore, technique of co-composting of pig manure and edible fungi residue has a wide prospect of application in practical production all over the world.

Introduction

With the rapid development of animal husbandry and the planting industry, increasingly more agricultural waste is produced in China. In 2008, the amount of livestock manure produced by large-scale farms and edible fungus residue produced by cultivation reached 776 (Yang et al., 2010) and 4.57 (Wei et al., 2010) million tons, respectively, among which pig manure accounted for 49.6% of the total livestock manure. Many wastes are rich in organic nutrients, such as organic matter, amino acids, and protein, and inorganic nutrients, such as nitrogen, phosphorus, potassium, and microelement, and are thus pools of organic and inorganic resources with great potential. However, the utilization ratio of agricultural wastes in China is only 34% (Huang et al., 2006b) because of the lagging technology and industrialization, and most agricultural wastes are inefficiently used, exposed to the public, or even directly discharged into the aquatic environment. This causes environmental problems, such as odor pollution (Pagans et al., 2006), methane emissions (Yan et al., 2015), water eutrophication (Selvam et al., 2012), the growth of mold and pests, and the spread of diseases (Wei et al., 2010). “The Bulletin on the First National Census on Pollution Sources” (Shi et al., 2011), which was published in 2010, confirmed that pollution by the livestock industry had become one of the most important sources of agricultural non–point source pollution in China. Therefore, there is an urgent need for a suitable environmentally and economically feasible technology to manage agricultural wastes. Currently, the most fundamental, economical, and effective method of processing livestock manure, fungus residue, and other solid agriculture wastes is aerobic composting. Composting is an aerobic process that involves the decomposition of organic matter under controlled temperature, moisture, oxygen, and nutrient conditions, which can destroy pathogens, parasites, and weed seeds (Millner et al., 2014), remove antibiotics (Selvam et al., 2012), and finally result in a safe and high-quality organic product that can be used as a nutrient source for plant growth or as a conditioner to improve the physical properties of soil (Huang et al., 2006).

Sun et al. (2008) showed that pig manure and edible fungus residue with inoculation bacillus for composting could be successfully transformed into high-quality organic fertilizer. Li et al. (2008) observed that the ammonia volatilization of compost with pig and mushroom residue was significantly lower than that of compost with pig and straw or rape straw. Zhang and Sun (2014) demonstrated that green waste composting with an addition of 35% spent mushroom greatly increased the nutrient content and shortened the composting time. However, Guo et al. (2012) studied the effect of different ratios of raw material on nitrogen transformation during the compost process of spent mushroom dreg mixed with cow manure, and the results showed that the loss of nitrogen was significantly increased with increasing spent mushroom dreg. Other studies have indicated that products of compost with edible fungus residue could increase the yield of paddy rice (Hu et al., 2011) and the potential for employing spent mushroom compost in the ex situ bioremediation of petroleum hydrocarbon polluted soil (Lau et al., 2003; Okerentugba et al., 2015). However, due to the high content of cellulose, hemicellulose, and lignin in fungus residue, large-scale composting requires appropriate amounts of fungus residue to ensure effectiveness and high efficiency. This paper describes a composting experiment with different ratios of pig manure to fungus residue, investigates the dynamic change of temperature, nutrients, carbon content, pH, and germination index (GI) during composting, and finally obtains optimal ratios of pig manure to fungus residue. This research is intended to provide a technical basis for the nonhazardous disposal of agricultural wastes on a large scale, the production of high-quality organic fertilizer, and the utilization of agricultural wastes as resources.

Materials and methods

Raw materials

Pig manure was collected from ZheJiang TianPeng Livestock Industry Limited Company (Jiangshan, China), edible fungus residue was provided by JiangShan GenGen Edible Fungus Company (Jiangshan, China), and rice bran was purchased from the PuCheng Rice Processing Plant (Jiangxi, China). The basic properties of these experimental materials are shown in Table 1.

Table 1. Basic characteristics of the compost materials.

Material	Moisture content (%)	TOC (%)	Total N (g·kg^−1)	Total P (g·kg^−1)	Total K (g·kg^−1)	C/N ratio	pH
Pig manure	68.5	35.1	28.3	27.6	12.3	12.4	7.77
Fungus residue	50.3	51.4	21.8	6.5	6.2	23.6	7.05
Rice bran	4.70	50.0	4.2	1.4	5.3	119.0	7.23

Experimental design

The total amount of each experimental compost sample was approximately 2000 kg. Except for the 250 kg of rice bran that was added as a carbon resource to adjust the C/N ratio, the four treatments were designed with different ratios of fresh pig manure to fungus residue (w/w): treatment A 9:1, treatment B 8:2, treatment C 7:3, and treatment D 6:4. The initial water content of each treatment was adjusted within the range from 50% to 55%. The composting period was from July 15 to August 30, 2014. After being homogeneously mixed in a blender, the raw material was piled in long groove (constructed of concrete) with approximate dimensions of 100 m (length) × 2 m (width) × 1 m (height); 4 m above the groove, a roof was constructed to prevent rainfall from moistening the compost. The distance interval of each treatment was 2 m. During composting, the mixture was shifted 2 m forward every 2 days with turning by a mixing machine such that the distance moved corresponded to the days composted.

Testing items and methods

During composting, the compost temperature and ambient temperature were both monitored at 8:00–9:00 a.m. each morning by a thermometer inserted into the compost at a depth of 40 cm, and the reading was directed recorded once it was stabilized. Five points were measured for each treatment, and the average value was taken. Samples were collected immediately after turning the piles every 2 days from the 1st day to the 47th day, and each composite sample (approximately 2 kg) consisted of five separate samples taken from around the compost pile. To analyze the variance of different treatments, initial and final samples were taken from piles in triplicate; only one of the other samples was taken. After homogeneously mixing, all samples were divided to two parts: one part was directly used to measure the moisture and pH, and the other part was air dried, ground, and then sent to the laboratory of ZheJiang TianPeng Livestock Industry Limited Company or the Agricultural Product Quality Testing Center of JiangShan for analysis of the physical and chemical properties.

Sample analysis

The pH of distilled water extracts (fresh weight:volume [FW:V] = 1:10) was measured using a digital pH meter (PHS-3CT; Kangyi Instrument Co., Ltd., Shanghai, China). The moisture content of the materials was determined by fresh samples at 105 °C for 8 hr. The total carbon matter (TOC) was measured in dried and milled samples with potassium dichromate titration. The total nitrogen (TN) was determined by the Kjeldahl method (KDN-08A; Shanghai, China). The total phosphorus (TP) was determined using a spectrophotometer (722; Shanghai, China) at 440 nm via vanadium-ammonium molybdate colorimetry, and the total potassium (TK) was determined with a flare photometer (FP640; Shanghai, China). These determination methods were performed according to the method of Bao (2000), and all nutrient contents in this study are given on a dry weight basis.

The germination index (GI) was used to evaluate the negative effects of the compost on plant growth. Aqueous compost extracts were used to evaluate the inhibitory effect on GI. In this study, 20 grains of Chinese cabbage seeds were placed on filter paper (No. 1) in a Petri dish with a diameter of 10 cm, in which approximately 10 mL of each sample filtrate (prepared earlier) was added; distilled water (10 mL) was used as the control sample. Triplicate samples were analyzed for each treatment. The Petri dishes were placed in the light incubator for 48 hr at a temperature of 26 °C. The number of germinated seeds was counted, and the lengths of roots were measured. The responses were calculated by a GI that was determined according to the formula described previously (Nolan et al., 2011):

Data processing

The data were analyzed by standard analysis of variance (ANOVA) procedures using DPS software (Zhejiang University, Hangzhou, China), and the significant differences were determined based on the P < 0.05 level in the least significant difference test.

Results and discussion

Change in temperature, pH, and moisture during composting

Temperature evolution is an indicator of microbial activity during composting and may thus be considered as a good indicator of the end of the bio-oxidative phase. Figure 1a shows that the temperature of all treatments increased rapidly at the beginning of composting, reached 50 °C after 3 days and 60 °C after 9 days, and then maintained a thermophilic phase (>50 °C) for 24–31 days. This behavior can be explained by the high proportion of pig manure in the piles, which induced a high initial concentration of active heterotrophic biomass (Oudart et al., 2015). The influence of cellulose hinders the decomposition of the fungus residue (Wei et al., 2010), which prolongs the duration of the thermophilic phase with an increasing proportion of fungus residue. All treatments reached the hygienic standard for thermophilic composting (GB 7959-2012; Ministry of Health, PRC, 2013). After 5–8 days of cooling, each treatment reached stabilized maturity, the temperature of which tended to be close to ambient temperature.

Figure 1. Changes in temperature (a), pH (b), and moisture content (c) of different treatments.

The change in pH is an important parameter in the composting process. A suitable pH level can enable microbes to function effectively, retain effective nitrogen, and reduce the loss of ammonium ions (NH₄⁺), whereas the efficiency and quality of composting will be damaged if the value of pH is too high or too low (Jimenez and Carcia, 1991). The suitable pH range is generally considered to be 6.5–7.5, which is most suitable for the growth of microbes, especially bacteria and actinomycetes (Nakasaki et al., 1993). In this study, the pH value profiles of the four treatments were similar (Figure 1b), presenting a tendency to rapidly increase at the initial stage, coinciding with the highest temperatures in the compost and thus leading to the production of a large quantity of ammonia (NH₃) by the mineralization of the organic nitrogen due to the higher microbiological activity at the higher temperature (Tiquia, 2005; Eklind and Kirchmann, 2000; Pagans et al., 2006). The subsequent decrease in pH was caused by the emission of ammonia, resulting in N loss, eventually reducing the alkalinity of the composting pile (Gao et al., 2010) and releasing a large quantity of H⁺ produced by nitrification (Eklind and Kirchmann, 2000). At the end of composting, the pH levels of the four treatments were 7.71, 7.56, 7.23, and 7.35, representing increases of 0.30, 0.43, 0.01, and 0.34, respectively, compared with the initial pH (Table 2). Among them, the pH levels of treatments A and B were significantly higher than those of treatments C and D. This may be caused by the higher proportion of pig manure in treatments A and B, which was related to the fact that the pH of the raw pig manure was higher than that of the fungus residue (Table 1). The pH values of all end compost products not only reached the standard of a neutral or weak alkaline environment that was most suitable for microbial activity but also met the criteria for the pH of commercial organic fertilizer (NY 525-2012; Ministry of Agriculture, PRC, 2012).

Table 2. Characteristics of the initial and final compost materials.

Treatments		TN (g·kg^−1)	TP (g·kg^−1)	TK (g·kg^−1)	TOC (%)	C/N ratio	pH (1:5)	Moisture (%)	GI (%)
A	Initial	21.7a	19.8bc	10.3bcd	41.70bc	19.22ab	7.41bc	50.77a	46.9d
	Final	18.8a	27.0a	13.5a	34.87e	18.55ab	7.71a	15.38b	101.4a
B	Initial	20.8a	19.6bc	9.5cd	42.65abc	20.50ab	7.13cd	51.50a	36.3d
	Final	19.8a	23.1b	12.8ab	36.48de	18.42ab	7.56ab	15.57b	91.2ab
C	Initial	19.8a	18.5c	9.2d	44.29ab	22.37ab	7.22cd	54.56a	9.2e
	Final	21.4a	22.7b	12.3abc	38.29cde	17.89b	7.23cd	16.78b	81.3b
D	Initial	20.1a	14.8d	9.7cd	46.09a	22.93a	7.01d	54.03a	5.3e
	Final	22.0a	16.9cd	12.4abc	40.57bcd	18.44ab	7.35bc	17.85b	63.8c

Notes: LSD (least significant difference) test for the significant difference (P < 0.05) among the means of treatment. Values followed by the same letters in each column are not significantly different at the 0.05 level (least significant difference).

In addition to providing a carrier for soluble nutrients required for microbial metabolism and physiological activity, moisture can also dissolve organic matter, participate in the metabolism of microbes, remove heat through evaporation, and adjust the temperature of composting (Sharma and Canditilli, 1997). Thus, water content is one of the important parameters of composting and the key factor in process control. Figure 1c shows that the water content of all treatments decreased rapidly during the first 28 days, corresponding with the thermophilic phase, and then stabilized in the remaining days. Because the experiments were conducted in summer and in an open environment condition (composting mixture was not placed in a closed reactor such as in laboratory tests), the water loss from the composting mixture occurred quite rapidly (Table 2)—the water content decreased from 50.8–54.6% in the initial mixture to 15.4–17.8% in the final compost product. However, the temperature also remained above 50 °C for a longer time, whereas the moisture content decreased to below 40% after day 9 (Figure 1a and c). This phenomenon was inconsistent with the results of most earlier studies. Normally, the composting process features a moisture content between 40% and 70% (Haug, 1993; Rynk, 2000). If the moisture content of compost were below 40%, the biological activity would be low due to insufficient moisture and lead to a low temperature. The phenomenon in this study may be closely related to the scale of the test (the mass of raw materials was approximately 2000 kg) and strategy of turning the pile (one time every 2 days): the small amount of heat produced by the low microbial activity was difficult to radiate out due to the high amount of compost material and inadequate aeration, and finally, the temperature increased with the accumulation of heat in the piles. Rynk (2000) also noted that the critical moisture content range for supporting spontaneous combustion was approximately 20–45%; below 20%, there was not a sufficient amount of moisture to sustain the biological activity that initiates the increase in temperature. In other words, the microbial activity was still maintained in moisture greater than 20%.

Change in total nitrogen, total phosphorus, and total potassium nutrition

During the entire composting process, the organic matter decomposed constantly under the influence of microbes and evaporated in the form of CO₂, H₂O, and NH₃, causing the absolute amount of nitrogen and carbon and the total mass of dry matter to gradually decrease. Phosphorus and potassium could not be lost by volatilization; thus, the absolute amount of total phosphorus and total potassium would not change considerably. There were distinguishing differences with respect to changes in the relative amount of nitrogen, phosphorus, and potassium due to the different proportions of fungus residue and different rates of microbiological degradation.

The TN, TP, and TK content evolution during the composting process is shown in Figure 2. The total nitrogen content of all treatments first rapidly decreased and then gradually increased because of the large amount of NH₃ having been volatilized into the environment within the first few days (Kader et al., 2007; Oudart et al., 2015), resulting in a rapid reduction of the total nitrogen content (Figure 2a). Afterward, with the reduction of the ammoniacal nitrogen and the continuing loss of carbon matter, the reduced total mass of dry matter was significantly larger than that of total nitrogen; nitrogen was condensed gradually, and total nitrogen content increased slowly. Table 2 presents the following tendency: the higher ratio of pig manure results in a greater loss of nitrogen. This is related to the high content of nitrogen, particularly easily degradable nitrogen in pig manure, whereas the fungus residue was a secondary degradation material. Most of the easily degradable nitrogen was consumed by edible fungus, and the remainder (such as cellulose, hemicellulose, and lignin) was extremely resistant to chemical and enzymatic degradation (Nolan et al., 2011; Michel et al., 2004). Sánchez-Monedero et al. (2001) found that nitrogen losses during composting depended on the materials used and the pH values of the initial mixtures; mixtures with a higher lignocellulose content and lower pH would incur lower nitrogen losses. At the end of composting process, the TN content reached 18.8, 19.8, 21.4, and 22.0 g·kg⁻¹ in treatments A, B, C, and D, respectively. Compared with the initial values of compost, the TN content final values for treatments A and B declined by 13.4% and 4.8%, respectively, whereas those of treatments C and D increased by 8.1% and 9.5%. This result indicated that the fungus residue functioned to conserve nitrogen, consistent with the finding reported by Li et al. (2008).

Figure 2. Changes in total nitrogen content (a), total phosphorus content (b), and total potassium content (c) of different treatments.

Because phosphorus and potassium cannot be lost by volatilization, changes in phosphorus and potassium contents directly reflect the evaporation rates of NH₃ and CO₂. With the degradation of organic matter in compost, the phosphorus and potassium contents both exhibited an increasing tendency as a result of the net loss of dry mass, which tends to concentrate phosphorus and potassium in compost piles (Figure 2b and c). By the end of composting, the phosphorus content increased by 36.4%, 17.9%, 22.7%, and 14.2% in treatments A, B, C, and D, respectively (Table 2), and the potassium content increased by 30.5%, 34.7%, 33.0%, and 28.1% in treatments A, B, C, and D, respectively. The phosphorus and potassium contents of the treatments were in the order A > B > C > D, mainly because the phosphorus and potassium contents in the pig manure were higher than those in the fungus residue.

Change in TOC and C/N ratio in the composting process

During the degradation of organic matter, 30–40% of carbohydrates were used as the constituents of bacteria, and 60–70% of carbohydrates were decomposed by microbes and evaporated in the form of CO₂ (Hellmann et al., 1997). Thus, the amount of organic carbon presented a declining trend; this study achieved a similar result (Figure 3a). After the composting began, the TOC content exhibited a significant decrease throughout the composting process for all treatments (i.e., by 16.3%, 14.5%, 13.6%, and 11.9% for treatments A–D, respectively; Table 2). However, the degradation rate of this experiment was lower than that found in most previous studies (Huang et al., 2004; Nolan et al., 2011), which may be due to the low microbiological activity given the deficiency of water in the compost pile after 8–12 days as a result of the rapid evaporation of water. Figure 3a also shows that a greater amount of edible fungus residue in the raw material leads to a smaller reduction in TOC content due to the same cause of nitrogen losses. This trend indicated that the degradation of organic carbon was significantly influenced by the composition of the raw materials.

Figure 3. Changes in total organic carbon content (a) and C/N (b) of different treatments.

The C/N ratio is usually used to evaluate the maturity degree of composting. The maturity degree of composting is typically considered to be less than 20 (Huang et al., 2004, 2006). Figure 3b shows the evolution of the C/N ratio for the different ratios of pig manure to fungus residue. The C/N ratio of the initial mixtures (19.2–22.9) was slightly lower than the optimal C/N ratio of the initial mixtures (25–35) (Haug, 1993). As shown in Figure 3b, the C/N ratio initially increased in all treatments and then decreased in each treatment during composting, mainly because the C/N ratio was comparatively low in the compost. Moreover, in the early stage of high temperatures, along with the massive reproduction of microbes, a large proportion of organic nitrogen in the nitrogen-rich compost components degraded into ammoniacal nitrogen to be emitted, and thus the nitrogen content decreased rapidly; the degraded amount of organic matter in compost was significantly less than the amount of nitrogen loss, resulting in an increase in the C/N ratio. The peak C/N ratios obtained for treatments A, B, C, and D were 29.3 on day 13, 26.4 on day 9, 27.0 on day 11, and 27.0 on day 9, respectively. Additionally, Figure 3B indicates that the rate of increase in the C/N ratio for treatment A was higher than that of other treatments, possibly due to the greater nitrogen losses in treatment A. Subsequently, the strong activity of substantial amounts of microbes enabled increases in the degradation rate of organic matter, the carbon content decreased more rapidly compared with the loss of nitrogen, and the C/N ratio again began to decrease gradually. At the end of composting (Table 2), the C/N ratios of the treatments were 18.6, 18.4, 17.9, and 18.4, respectively; they were all less than 20, which met the requirement for maturity (Huang et al., 2004, 2006).

Change in the seed germination index during composting

Determining the phytotoxicity of compost by biological methods is an effective means for testing the maturity degree of compost (Zucconi, 1981a). The GI is an indicator used to evaluate the harmlessness and stabilization degree of compost by testing whether the compost produces an inhibitory effect on the germination of plants, which can not only detect the phytotoxicity of the compost sample but also predict the change in the phytotoxicity of the compost sample (Zucconi, 1981b). Figure 4 shows the changes in the GI during the different treatments. At the beginning of the test, differences in the GI among the four treatments were observed; the GIs for treatment A (46.9%) and treatment B (36.3%) were significantly higher those that for treatment C (9.2%) and treatment D (5.3%) (Table 2); thus, a higher ratio of fungus residue results in a stronger phytotoxicity of the extract to the seeds. The GI values of treatments A and B underwent a brief decline and then increased rapidly, which may be closely associated with the high amount of easily decomposable nitrogen compounds in treatments A and B and the correspondingly large accumulation of ammonium salts in the air-dried samples, which is considered to have an inhibitory effect on seed germination (Gao et al., 2010). Thereafter, the GI for all treatments increased steadily. The GI value reached more than 80% in treatments A, B, and C on days 23, 33, and 47 (last day of the test), respectively. At the end of the composting, the GIs in treatments A, B, C, and D reached 101.4%, 91.2%, 81.3%, and 63.8%, respectively. Zucconi (1981b) suggested that a GI content of more than 80% was an indication of a phytotoxic-free medium. According to this guideline, treatments A, B, and C achieved compost maturity, and treatment D did not reach compost maturity and had an inhibitory effect on seed germination. The study results also indicated that a larger proportion of fungus residue in compost caused the time required for the compost to achieve maturity to increase.

Figure 4. Seed germination index of different treatments.

Conclusions

The composting of agricultural wastes, such as animal manure, straws, and fungus residue, has been demonstrated to be an effective method for the production of end products that are stabilized and sanitized, ensuring the maximum benefit for agriculture. However, the compost should be of high quality to guarantee its marketability.

In this study, the physicochemical and phytotoxicity changes during the composting of pig manure and fungus residue have been investigated at raw material ratios of 9:1, 8:2, 7:3, and 6:4 (fresh weight of pig manure to fungus residue). The composting of all treatments maintained a temperature exceeding 50 °C for more than 10 days during composting and met the requirement for pathogen destruction. The higher proportion of fungus residue in composting mixtures decreased due to the losses of nitrogen, whereas the degradation of organic matter was slower and prolonged the composting period. The GI value exceeded 80% in raw material ratios of 9:1, 8:2, and 7:3 on days 23, 33, and 47, respectively, but the GI in raw material ratios of 6:4 never reached 80% during composting. Therefore, the suitable ratio of pig manure to fungus residue to shorten the composting period and reduce the nitrogen losses for large-scale composting is approximately 8:2. Moreover, because the C/N ratio of both pig manure and fungus residue is slightly lower, the amount of rice bran or other additives with a high C/N ratio should be increased to further increase the composting efficiency.

Funding

This research was funded by the Science and Technology Department of Jiangshan City (2014C16) and the Science and Technology Department of Zhejiang Province (2015C32123), People’s Republic of China.

Acknowledgments

The authors express their sincere thanks to technicians of ZheJiang TianPeng Livestock Industry Limited Company for their valuable help in the composting experiment.

Additional information

Funding

This research was funded by the Science and Technology Department of Jiangshan City (2014C16) and the Science and Technology Department of Zhejiang Province (2015C32123), People’s Republic of China.

Notes on contributors

Jiangming Zhou

Jiangming Zhou is an agricultural technician (associate professor) at the Agricultural Technique Popularization Centre of Jiangshan City, Jiangshan, Zhejiang, People’s Republic of China.

Litong Wang

Litong Wang is an assistant manager at ZheJiang TianPeng Livestock Industry Limited Company, Zhejiang, People’s Republic of China.

Houming Wang

Houming Wang is an agricultural technician at the Agricultural Technique Popularization Centre of Xiakou Town, Jiangshan, Zhejiang, People’s Republic of China.

Long Jiang

Jiang Long and Xinyou Jiang are agricultural technicians at the Agricultural Technique Popularization Centre of Jiangshan City, Jiangshan, Zhejiang, People’s Republic of China.

Xinyou Jiang

Jiang Long and Xinyou Jiang are agricultural technicians at the Agricultural Technique Popularization Centre of Jiangshan City, Jiangshan, Zhejiang, People’s Republic of China.