Variations in physicochemical characteristics
Figure 1a exhibits the changes in pH in different treatments. T3 and T4 showed higher initial pH than that of T1 and T2, owing to the alkaline property of biochar (Table 1). And then, consistent with that reported previously (Yang et al., 2019a), the pH of all the groups dropped notably within the first three days, which may be a result of the production and accumulation of organic acids (Tu et al., 2019; Yang and Zhang, 2022). As the process progressed, the matrix pH fluctuated and remained in the range of 3.2–4.4 due to NH3 emissions and variations of organic acids (Awasthi et al., 2016).
Electrical conductivity (EC) reflects the compost's salinity and degree of organic mineralization. It is a crucial indicator that reveals the impact of compost maturity, potential inhibition, and phytotoxicity on plant growth (Onwosi et al., 2017). As the experiment proceeded, small molecular organic matter and inorganic matter were gradually formed through the degradation of organic matter and thus leading to a gradual increase in the EC of T2, T3, and T4. Finally, the EC was 0.3, 0.24, 0.18, and 0.14 mS·cm− 1 for T1 to T4 (Fig. 1b), respectively, all of which were within the standard safety range of 4.0 mS·cm− 1 (Yang et al., 2021). The biochar-amended group achieved a lower EC value, perhaps because of the addition of biochar is conducive to the formation of humus. Moreover, it is worth noting that the biochar could act as a buffer and stabilize the EC values, suggesting the cation exchange and adsorption capacity of biochar. Furthermore, in line with Wang et al. (2022a) and Zhang et al. (2013), T1 exhibited the lowest pH and the highest EC. The explanation may be that most mineral salts dissolve more readily at lower pH (Yang and Zhang, 2022).
Effects of microbial agents on gas content
Changes in the content of CO2 and O2
By degrading organic matter, degrading bacteria obtain the material and energy needed for their growth, often accompanied by CO2 production (Chen et al., 2019). Figure 2a illustrates the change in CO2 concentration throughout the experiment process, which could indirectly reflect the microbial activities (Awasthi et al., 2016). The emission of CO2 was the result of the rapid decomposition and mineralization of total organic carbon (TOC). Since the initial environment was suitable for the growth of microorganisms, the CO2 concentration increased quickly. Contrary to Jiang et al. (2021), who reported a higher concentration of CO2 concentration in the experimental group. The CO2 concentration in the T2, T3, and T4 recorded a peak value (1, 1.31, and 1.04%, respectively) on days 2–3, and the peak value (1.37%) in T1 was on day 4 (Fig. 2b), the peak value was decreased by 27.01%, 4.38%, and 24.09% for T2, T3, and T4 treatments in comparison with T1. The fact that biochar served as a bulking agent and facilitated the diffusion of O2 may account for the decrease in this study.
During IRBR process, microorganisms consumed oxygen to biodegrade organic substances (Xu et al., 2021a). As shown in Fig. 2b, all treatments witnessed a decrease in oxygen concentration followed by an increase within the six days. Microorganisms consumed oxygen to biodegrade organic substances. The decline in oxygen content was due to the intensive biodegradation of easily organic matter. Subsequently, with the depletion of easily biodegradable organic matter, microbial biological activity decreased and oxygen content increased. Possibly due to poor ventilation or strong microbial metabolism, T1 had the lowest average O2 concentration, while low oxygen content would promote the release of NH3 and H2S.
Changes in the emission of NH3 and H2S
The dynamics of NH3 concentration are shown in Fig. 2c. Specifically, the NH3 concentration increased dramatically at the beginning and gradually decreased, in agreement with the results of Manu et al. (2021) and Xu et al. (2021b). The accumulative emissions of NH3 in all treatments were 1244.04, 625.90, 512.68, and 682.50mg/m3, respectively. The ORB and biochar could significantly reduce NH3 emissions. The results could be ascribed to the changes in microbial community and biochar’s absorption capacity of NH4+-N and NH3 because of its large surface functional groups (Cai et al., 2022; Godlewska et al., 2017). Additionally, biochar could create favorable conditions for nitrifying bacteria to transform NH4+ to NO3− and thus reduce NH3 emissions (Hoang et al., 2022; Yang et al., 2020; Yin et al., 2021). Furthermore, it’s worth noting that ODB could not play a synergistic effect with ORB and biochar in which the NH3 reduction rate decreased from 58.79% (T3) to 45.14% (T4).
Figure 2d depicts the H2S content during the IRBR process. The H2S concentration showed a similar trend in several studies (Gu et al., 2018; Yuan et al., 2019). All experimental groups rose rapidly in the early stage and then declined to a low level. Since the decomposition of organic substance generated H2S via sulfate-reducing bacteria (SRB), a kind of anaerobe (Zhu et al., 2021), the porous structure of biochar was conducive to improving ventilatory capacity and delaying the peak H2S of T3 and T4 (Cai et al., 2022). The amount of H2S produced by the T2 and T3 with ORB-amended was considerably lower than that of the control group, and the accumulative H2S emissions were reduced by 51.62% and 40.57%, respectively. Meanwhile, adding ODB could weaken ORB and biochar’s efficiency in eliminating H2S.
As discussed above, although the odor emissions in treatment T4 were higher than T2 and T3 due to the addition of ODB, which accelerated the decomposition of organic matter, thus increasing odor emissions, the odor release was still lower than that of the T1.
Effect of biochar and microbial agents on nitrogen transformation
A similar variation of ammonium nitrogen was observed for T1 and T2 (Fig. 3a). With the degradation of organic matter, the NH4+-N contents of T1 and T2 increased in the early stage and peaked at 425.11 mg·kg− 1 and 471.18 mg·kg− 1 on day 4, respectively. After day 4, the NH4+-N content of T1 and T2 began to decline due to the conversion of NH4+-N to NO3−-N by nitrifying bacteria and the transformation of NH4+-N to NH3. Finally, the NH4+-N content of T1 and T2 were 342.8 mg·kg− 1 and 309.79 mg·kg− 1, respectively. This trend which showed an initial increase followed by a decrease, was similar to previous research (Wang et al., 2013; Zhang et al., 2016). The content of NH4+-N in T3 and T4 was consistently lower than that of T1 during the experiment, providing less precursor for NH3 formation, thus resulting in less NH3 emissions (Fig. 2c).
Figure 3b showed the NO3−-N variations during the process. Within the first three days, the NO3−-N content fluctuated in all treatments, and then an increase was observed from day 3 to day 4, which might be ascribed to the decrease of temperature, thus benefiting the growth of nitrifying bacteria and resulting in the formation and accumulation of NO3−-N (Guo et al., 2020; Hoang et al., 2022; Wang et al., 2022b). At the end of the experiment, the NO3−-N content in four experimental groups was 149.65, 157.97, 79.81, and 134.48 mg·kg− 1, respectively. The NO3−-N content of treatment T2 was higher than CK. Similar conclusions were reached by Zhao et al. (2020) during sewage sludge composting. Moreover, the lower contents of NO3−-N in T3 and T4 could result from the lower contents of NH4+-N, which acted as the precursor of NO3−-N (Hoang et al., 2022). These findings suggested that biochar and microbial agents could mitigate odor emissions by altering nitrogen transformation.
Effect of additives on organic matter
The organic composition of the products was also monitored before and after the experiment. As shown in Fig. 4a, the protein content of T1 to T4 decreased from 18.24%, 18.74%, 16.6% and 17.87–13.65%, 16.11%, 12.77% and 7.51% respectively, indicating that protein is the main component in the biodegradation process. Due to the characteristics of large surface area and high porosity, biochar could regulate the environmental conditions of microbial habitats and accelerate organic bio-degradation by promoting metabolism-related functional microbial activity (Mandal et al., 2018; Oldfield et al., 2018). It’s obviously that T3 and T4 with the addition of biochar achieved higher degradation rate. Furthermore, due to the addition of organic-degrading bacteria, T4 achieved the highest protein degradation rate.
Low solubility and high viscosity of WCO make it easy to create an oil film on the water surface, which inhibits oxygen diffusion and makes it difficult for microorganisms to thrive and reproduce (Ke et al., 2021). The lipid degradation rate in T4 was the highest and was 18.55 ~ 28.87% higher than other treatments (Fig. 4b), which benefited the growth of microorganisms and the reduction of OW (Zhou et al., 2021). Furthermore, the increased lipid content was observed in the final product of T1 and T2, which may be related to the fact that lipid degrade much more slowly than proteins and carbohydrates.
Figure 4c and Fig. 4d show the proportions of the organic compounds (protein, lipid, and starch) before and after the experiment. In the end, the organic matter of T2, T3, and T4 decreased by 15.76%, 16.86%, and 20.09%, respectively. In comparison, that of T1 decreased by only 6.28%, proving that adding microbial agents and biochar could promote the decomposition of organic substances.
Changes in the bacterial community structure
Alpha Diversity
All samples obtained a coverage index of > 99% (Table 2), indicating that the sequencing results reflected the bacterial community. As the experiment progressed, the four indexes increased significantly in four treatments, suggesting a gradual increase in the richness and diversity of bacterial communities due to enough readily biodegradable organic substance, which benefited the growth of bacteria and improved the microorganisms’ succession. It resembled the conclusion of Mao et al. (2018). Notably, the richness of T1 was the highest, demonstrating that microbial agents might inhibit the growth of indigenous bacteria, which may relate to odor emissions.
Table 2
Bacterial diversity and richness at day 1 and day 7 of four treatments
Items | T1 | T2 | T3 | T4 |
Time (d) | 1 | 7 | 1 | 7 | 1 | 7 | 1 | 7 |
Chao1 | 464 | 517 | 426 | 467 | 460 | 488 | 412 | 435 |
Shannon | 3.51 | 4.33 | 3.74 | 4.28 | 2.96 | 3.92 | 2.97 | 4.62 |
Simpson | 0.828 | 0.861 | 0.828 | 0.843 | 0.750 | 0.843 | 0.753 | 0.896 |
ACE | 448 | 512 | 424 | 469 | 451 | 473 | 409 | 436 |
Succession of microbial community
Microorganisms are the main participants in the conversion of materials during the IRBR process. Nitrogen metabolism and sulfur metabolism driven by microorganisms played important roles in the release of ammonia and hydrogen sulfide, respectively. Hence, it is necessary to investigate the composition and succession of microbial community. The evolution of the bacteria community through the IRBR process is depicted in Fig. 5a. Firmicutes, Proteobacteria, and Actinobacteria dominated throughout the experiment (Fig. 5a). Similar results were founded by Mao et al. (2018). The higher abundance of Firmicutes in T1 explained most of the higher NH3 emissions as it was reported to facilitate the decomposition of organic substances and show a positive relationship with NH3 emissions (Wang et al., 2022a; Xu et al., 2021a). Studies have shown that Proteobacteria have a nitrogen fixation function, which is closely associated with nitrogen preservation during composting (Xi et al., 2016). In this study, the treatment T2, T3 and T4 increased the abundance of Proteobacteria due to the addition of Kocuria rosea and Acetobacter pasteurianus, which could explain the reduction of NH3 emissions. Furthermore, the nitrogen and carbon cycles are all impacted by Actinomycetes and Bacteroides, which participate in the degradation of organic substances (Storey et al., 2015; Yang et al., 2019b).
The heatmap analysis of bacterial communities in the genus level was depicted in Fig. 5b. In the beginning, the phylum Firmicutes mainly comprised Lactobacillus and Streptococcus in T1 and T2. With the addition of ORB and ODB, the genera Acetobacter and Kocuria belonging to Proteobacteria and the genera Bacillus belonging to Firmicutes also became dominant genera. As a common anaerobic bacterium, Lactobacillus could produce large amounts of lactic acid from fermentable carbohydrates and contribute significantly to the emission of H2S. Streptococcus is a facultative anaerobe commonly found in nature and belongs to the indigenous microorganisms in organic waste. With the progress of the experiment, its abundance gradually decreased, indicating that the experiment could destroy pathogenic microorganisms and make the samples meet the requirements of health standards. In addition to these dominant genera, the genus Bacillus was reported to be a typical thermophilic bacterium that could produce thick-walled endospores to endure high temperatures and take part in the degradation of organic substances (Ma et al., 2019). The degradation of organic matter is inevitably accompanied by the production of odor, so the reason for the high degradation rate and odor emission in T4 can be explained by the high abundance of Bacillus. From what has been discussed above, ORB and ODB could propagate in the system and alter the bacteria dynamics to achieve the purpose of odor mitigation and waste reduction.
Factors that affected changes in the Odor emissions and bacterial community
Microorganisms are affected by various environmental factors (EC, NH3, H2S, etc). RDA was performed to show the correlation between environmental factors and microbial communities (Fig. 6). The samples from T2, T3, and T4 were significantly separated from T1, demonstrating that exogenous additives altered the microbial communities. Notable variations in the microbial communities in different groups were also observed on day 7, revealing the additives’ considerable influences on the microbial community's succession.
NH3 was positively correlated with Firmicutes and negatively correlated with other dominant phyla such as Proteobacteria and Bacteroidetes, which further explained the high NH3 emissions in treatment T1 (Fig. 2c). Furthermore, there were significant positive correlations between H2S and dominant genus such as Proteobacteria and Actinobacteria. Actinobacteria was detected in the early stage, which were positively participated in the NH3 emission. Additionally, NH4+-N is transformed by microorganisms through degradation and ammonification of organic nitrogen (Hoang et al., 2022). NH4+-N could reflect the degree of decomposition of organic matter. Figure 6 also indicated that NH4+-N were closely related to Firmicutes and Chloroflexi. Thus, these dominant phyla had important roles in odor emissions and organic degradation.