1.3 Nitrogen retention and deodorization effects of biochar combined with MDWR
1.3.1 Effect of temperature and pH changes
As shown in Fig. 1, the CB treatment temperature increased the fastest, reaching the high-temperature stage (50°C) in 2 days. The CC and CMB treatments entered the high-temperature stage on the third day, while the CM treatment reached the high-temperature stage on the fourth day. This result indicates that MDWR addition affected the fermentation efficiency at the initial stage, but biochar addition weakened this effect and shortened the delayed heating period due to MDWR addition. The highest temperatures under the four treatments, i.e., CC, CB, CM, and CMB, were 62.8, 64.8, 60.4, and 62.6°C, respectively. Regarding safety, each treatment completely met the sanitary requirements of China’s faecal harmlessness standards (GB 7959 − 2012).
The pH is a key indicator affecting the microbial activity in compost [24]. The initial compost pH was 8.6 (CC). As shown in Fig. 1, at the initial composting stage, due to the degradation of small molecular organic substances to generate organic acids and carbon dioxide and MDWR addition under the CM treatment, the pH under the CC and CM treatments decreased over time. The CB and CMB treatments contained biochar, which provided a suitable buffering effect. With regular aeration in the aerobic composting process, carbon dioxide was discharged from the pile. With increasing compost temperature, some protein was degraded to produce NH3, and the pH of the pile gradually increased. The trends were relatively consistent among the various treatments. Since SO32− in MDWR was gradually converted into SO42−, thereby increasing the acidity during composting, the pH under the CM treatment was low and decreased to 7.99 by the end of the composting process, which could increase ammonia volatilization inhibition. Biochar addition under the CMB treatment resulted in the smallest change in pH among all treatments, at only 0.31 units. This could not only alleviate the problem of high product acidity likely caused by MDWR acidification but could also adsorb the generated ammonia in the composting process. By the 14th day, the pH under each treatment reached its maximum value. From day 21 (the late composting period), the pH under all treatments except the CB treatment decreased synchronously, and at the end of composting, the pH values under the four treatments were 8.37, 8.44, 7.99 and 8.16. The acidification effect under the two treatments involving MDWR was obvious, and the difference was significant compared to the acidity under the CC treatment according to the multiple comparison test method (P < 0.05).
1.3.2 Effect of changes in NH3-N, NO3-N and TN
Ammonia volatilization is the main form of nitrogen loss during composting (Delaune et al. 2004). As shown in Fig. 2, the overall trend in NH3-N under each treatment first increased, then decreased, and then stabilized. On the 4th day, the NH3-N under the CM treatment began to decline and then stabilized, indicating that MDWR addition resulted in effective nitrogen nutrient retention in the system. Under the CMB treatment, NH3-N stably declined from the 7th day, mainly attributable to MDWR addition. Some NH3-N formed MAP, but the remainder, accounting for a large proportion, was adsorbed onto biochar. However, the ammonia nitrogen content under the CC and CD treatments without MDWR addition began to rapidly decline from 14–15 days. This phenomenon occurred for two reasons: one reason was that nitrification began to increase; the other reason was that the proportion of nitrogenous organic substances that could be degraded by microorganisms decreased. At the end of composting, the NH3-N concentrations under the CC, CB, CM, and CMB treatments were 4.05, 4.21, 6.82, and 6.71 g/kg, respectively. After correlation analysis, the difference between the CM and CMB treatments and CC and CB treatments, i.e., the treatments with and without MDWR addition, respectively, was significant (p < 0.05), indicating that MDWR addition significantly influenced NH3-N content enhancement in the compost pile.
As shown in Fig. 2, except under the CMB treatment, the NO3-Na under the other three treatments hardly increased over the first 7 days. During the high-temperature period, the increase in NO3-N was limited. However, starting from the 14th day, as the pile temperature gradually decreased, nitrification began to increase, and the NO3-N content rapidly increased. The NO3-N under the CM and CMB treatments indicated that MDWR addition did not affect the activity of nitrifying bacteria. In contrast, when the reactor pH was lowered and the NH3-N increased, the NO3-N under the CM and CMB treatments was significantly higher than that under the CC and CB treatments. At the end of composting, the NO3-N concentrations under the four treatments, i.e., CC, CB, CM, and CMB, were 21.22, 23.31, 25.42, and 26.02 g/kg, respectively.
The TN in organic fertilizer is an important index to measure its quality (Ouatmane et al. 2000). At the early composting stage, due to organic matter decomposition, the loss of carbon and dry matter occurred faster than that of nitrogen, resulting in a continuous increase in the TN of each treated sample (Cerda et al. 2018). As shown in Fig. 2, the TN in each treated sample first increased and then decreased. Due to MDWR addition, the fermentation process under the CM and CMB treatments slowly progressed, while that under the CC and CB treatments rapidly progressed. The first increase in the TN was reached on the 3rd day of composting. After the start of the continuous high-temperature period on the 3rd day, much ammonia was volatilized, so the TN in each treated sample decreased to varying degrees. Because other nitrogen conservation measures were not implemented, the TN under the CC treatment rapidly decreased, reaching 3.51% and 12.07% on the 7th and 14th days, respectively. However, the TN under the CB, CM, and CMB treatments decreased by 2.29%, 5.76%, and 2.89%, respectively, on the 14th day. These three treatments yielded ideal nitrogen retention effects during the high-temperature period of the composting process. At the middle and late composting stages (14–28 days), the nitrification process in the pile was relatively intense, ammonia volatilization was reduced, and TN reduction decreased, whereas that under the CM and CMB treatments increased. During ageing (28–35 days), the TN under the four treatments slightly increased due to the influence of comprehensive factors, such as a decrease in the reactor temperature, improved biochar adsorption, increased MAP formation and enhanced nitrification. At this point, organic matter decomposition continued, increasing the ratio of TN to the dry matter mass. By the end of composting on the 35th day, the TN loss ratios under the CC, CB, CM, and CMB treatments reached 39.9%, 36.9%, 28.9%, and 17.2%, respectively. After multiple comparative analysis, the differences between the treatments were significant (p < 0.05). In contrast to the CB and CM treatments, the CMB treatment exhibited the ideal synergistic behaviour between biochar and MDWR.
1.3.3 Effect of changes in ammonia, hydrogen sulfide and VOC emissions
In this study, we monitored the ammonia emission dynamics of each treated sample from the 3rd day because the daily emissions of each sample were very low beyond the 12th day. Therefore, the dynamic tracking time in this experiment ranged from 3–14 days. As shown in Fig. 3-a, from days 3–8, the highest ammonia emissions were observed under the CC treatment (22.99 g/kg); the second-largest amount was observed under the CB treatment (15.93 g/kg). The sample under the CB treatment produced the highest cumulative emissions from days 9–11 (6.7 g/kg); the sample under the CM treatment exhibited the highest cumulative emissions from days 12–14 (1.88 g/kg). Since the pH of the sample under the CM treatment with MDWR always remained low, ammonia volatilization could be controlled. In addition, magnesium salt could form MAP with ammonia and phosphate so that the ammonia concentration in the reactor was relatively low, which was also a factor of ammonia volatilization reduction. Ammonia volatilization always remained low under the CMB treatment involving the addition of both MDWR and biochar, reflecting the synergy between these two additives and indicating that this measure could optimally suppress ammonia volatilization in compost. During the 12-day monitoring period, the cumulative ammonia volatilization amounts under the four treatments, i.e., CC, CB, CM, and CMB, were 27.13, 23.56, 13.32, and 10.82 g/kg, respectively. The CMB treatment, which exhibited the best effect, reduced the cumulative ammonia volatilization amount by 60.12%. After correlation analysis, the differences between the four treatments were significant (P < 0.05).
Hydrogen sulfide, another typical odorous inorganic molecule involved in the composting process, has attracted much attention due to its low odour threshold and notable harmfulness. As shown in Fig. 3-b, due to MDWR addition under the CM and CMB treatments, the hydrogen sulfide emissions were significantly higher than those under the CC and CB treatments, and the cumulative emissions at 12 days reached 121.9 and 112.2 g/kg, respectively. This result indicated that biochar addition could reduce hydrogen sulfide emissions to a certain extent. From this perspective, when MDWR is used as a nitrogen-retaining agent during composting, it is necessary to implement and study related supporting technologies to reduce hydrogen sulfide emissions and prevent secondary pollution attributed to this technology.
VOCs not only cause environmental pollution but also cause respiratory, endocrine, circulatory and nervous system damage in humans and animals and may seriously endanger the health of industrial workers and residents. Therefore, composting companies must consider VOC emissions. Table 1 indicates that the different treatments exhibited notable differences in the corresponding control effects for the various types of gases emitted. Among the 16 VOCs above the detection limit, 8 were not controlled by one or several measures designed in this experiment, indicating that VOC control during composting entails a relatively complicated process. From an overall VOC control perspective, the CB, CM, and CMB treatments yielded notable effects, with VOC emissions reduced by 27.17%, 41.67%, and 44.63%, respectively, compared to those under the CC treatment. Among the 8 controlled odorous gases in China, dimethyl disulfide and styrene were measured via GC–MS. Under the CB, CM, and CMB treatments, dimethyl disulfide emissions were reduced by 30.42%, 54.34% and 55.27%, respectively, indicating optimal emission reduction effects of biochar and MDWR addition. However, styrene emissions remained unaffected. In contrast, the CM and CMB treatments achieved notable improvement. The deodorization measures in this experiment did not reduce styrene and other VOC emissions, and further research is needed.
Table 1
Volatile organic compound emissions under the different treatments during the high-temperature composting period (V/V %)
Items
|
Component
|
CC
|
CB
|
CM
|
CMB
|
1
|
Dimethyl sulfide
|
6.13
|
5.63
|
8.78
|
7.25
|
2
|
Dimethyl disulfide
|
56.77
|
39.5
|
25.92
|
25.39
|
3
|
Dimethyl trisulfide
|
7.12
|
5.93
|
4.51
|
4.15
|
4
|
Dimethyl tetrasulfide
|
0.18
|
0.23
|
0.12
|
0.11
|
5
|
S-methyl 3-methylbutanethioate
|
0.37
|
0.18
|
0.07
|
0
|
6
|
2-Methylpyrazine
|
0.31
|
0.19
|
0.21
|
0.18
|
7
|
2-Heptanone
|
0.41
|
0.3
|
0.61
|
0.64
|
8
|
Styrene
|
0.41
|
0.41
|
0.72
|
0.81
|
9
|
Cyclohexanone
|
1.12
|
1.11
|
1.27
|
1.43
|
10
|
1,3-Dithiane
|
0.35
|
0.25
|
0.22
|
0.29
|
11
|
2-Ethylhexanol
|
1.3
|
1.08
|
1.4
|
1.33
|
12
|
2-Nonanone
|
0.22
|
0.22
|
0.39
|
0.37
|
13
|
N-dodecane
|
0.12
|
0.11
|
0.13
|
0.15
|
14
|
2,6-Dimethylphenol
|
0.01
|
0.01
|
0.02
|
0.02
|
15
|
N-capronitrile
|
0.21
|
0.12
|
0
|
0
|
16
|
2,6-Lutidine
|
1.04
|
0.13
|
0
|
0
|
Total
|
76.07
|
55.40
|
44.37
|
42.12
|
1.4 Research on the deodorization mechanism of biochar combined with MDWR
1.4.1 SEM–EDS analysis of the crystals formed in the compost
As shown in Fig. 4, SEM was used to observe the crystals formed in the compost under 500× magnification, and we could observe that there existed obvious distributions of flake-, X- and Y-shaped rhomboid crystals. These structures are typical of MAP. The relative content of elements was obtained via SEM-energy dispersive spectroscopy (EDS) (Table SM4). The relative content ratio between nitrogen, magnesium, and phosphorus was 1:1.24:0.77, indicating that the crystals were not pure MAP (NH4MgPO4). The reason for this ratio may be the poor solubility of MDWR, which was mixed into MAP during crystallization, introducing impurities; additionally, it was confirmed that nitrogen under the MDWR and biochar treatments was partially converted into MAP as a slow-release nitrogen fertilizer to achieve nitrogen retention.
1.4.2 X-ray diffraction pattern of the crystals formed in the compost
The XRD pattern shown in Fig. 5 reveals that the diffraction angle was very close to the standard angle of MAP crystals, which proves that the crystals comprised MAP, but the purity was lower than that of standard MAP crystals. Some nitrogen in the composting process was converted into MAP slow-release nitrogen fertilizer, which further proves that magnesium residue and biochar could retain nitrogen as composting additives in chicken manure compost.
1.5 Effects of the composting products on the vegetable yield and quality
1.5.1 Effects of the different treatments on the vegetable yield
The pot experiment results demonstrated that the change in yield under each lettuce treatment was basically the same (Table 2). When the two compost product treatments, CB and CMB, which contained biochar, were applied, the lettuce yield was higher than that under the other treatments, but there was no significant difference in the yield between the two treatments (P < 0.05). Among them, the treatment with the highest yield was the CB treatment. The yield under this treatment reached 783.5 g/3 plants. Compared to the control treatment, CC, the yield increased by 7.3%. There was no significant difference in the lettuce yield between the CMB and CM treatments involving compost treated with MDWR (P < 0.05).
Table 2
Effect of the different treatments on the lettuce yield
Treatments
|
Yield (g/3 plants)
|
CC
|
730.0 ± 20.61c
|
CB
|
783.5 ± 25.93ab
|
CM
|
747.0 ± 38.21bc
|
CMB
|
779.9 ± 48.23ab
|
1.5.2 Effects of the different treatments on the vegetable quality
According to the pot experiment results, the vitamin C content in the plants under the CB and CMB treatments did not significantly differ from that under the CC control, and the treatment yielding the lowest content was the CMB treatment (145.8 mg/kg). The vitamin C content under the CM treatment was significantly higher than that under the CC, CB and CMB treatments, and the content was 212.4 mg/kg. Regarding the soluble sugar content, the content under the CM and CMB treatments was significantly higher than that under the other treatments (P < 0.05), and the contents were 25.7 and 24.4 g/kg, respectively.
The treatment providing the highest nitrate content was the CC treatment (943.0 mg/kg), followed by the CM treatment, with a nitrate content of 894.3 mg/kg. The nitrate content under the CB and CMB treatment, both involving biochar, was significantly lower than that under the remaining treatments (P < 0.05) (Fig. 6-b). The treatment generating the lowest content was the CB treatment, with a content of 726.1 mg/kg.