Microbial driving mechanism of biochar and bean dregs on NH3 and N2O emissions during composting
Graphical abstract
Introduction
With the rapid development of the large-scale and intensive poultry breeding industry in China, the production of pig manure (PM) has been increasing year by year, and bring huge pressure on the environment. At the same time, the development of the tofu-producing industry generates a large amount of bean dregs (BD), and it will also cause environmental pollution and waste of resources without used properly. The organic fertilizer produced by the above two types of agricultural waste can both realize the resource utilization of wastes and avoid environmental pollution. Aerobic composting is the main way to achieve the harmlessness, reduction and resource utilization of agricultural solid organic waste (Gajalakshmi and Abbasi, 2008). However, the composting process will cause different degrees of nitrogen loss (Gu et al., 2011). Generally, the amount of nitrogen loss is 16%-76% of the initial total nitrogen (Barrington et al., 2002). The pathways of nitrogen loss mainly included NH3, N2O and N2 emissions, in which the nitrogen lost in the form of NH3 volatilization can reach 47% to 77% of the total nitrogen loss (Cáceres et al., 2018). Nitrogen loss can reduce the quality of the end product and cause secondary pollution in the environment. Meanwhile, as a nitrogen-rich material, BD could accelerate the production of NH3 owing the large amount of NH4+ contained itself. Besides, it could cause the sharp N2O emission due the anaerobic environment generated in the compost. Based on this, biochar (BC) could change the anaerobic environment caused by BD during the composting (Yang et al., 2019). Therefore, the characteristics of nitrogen and microbial communities during the composting with BC and BD amendments is very important for improving the quality of compost.
Nitrification is an important process in the nitrogen cycle. Nitrifying microorganisms instruct the nitrification process, and its distribution becomes the key factor to regulate the nitrification in compost. The nitrification can reduce the NH3 emission by converting NH4+ to NO3− (Cáceres et al., 2018, Maeda et al., 2011). Ammonia-oxidizing microorganisms including bacteria (AOB) and archaea (AOA) can convert NH4+ to NO2− by oxidation, which is a rate-limiting step in the nitrification (Yin et al., 2016, Kowalchuk et al., 1999). The amoA gene is considered as the symbol for AOA and AOB in many researches. Zeng et al. (2011) found the AOA and AOB were actively participated in the nitrification during the agricultural waste composting. Yamamoto et al. (2012) researched the abundance of AOB was significantly higher than that of the AOA in the poultry manure composting. However, Yamada et al. (2013) indicated the AOA was not detected in the cattle manure composting. The contributions of each nitrifying microorganisms need to be further researched.
In addition, denitrification is an another important process concerning nitrogen cycle during the composting (Li et al., 2016, Maeda et al., 2017, Zhang et al., 2015). The denitrification was confirmed as a main pathway for generating N2O (Chen et al., 2014). N2O is an important greenhouse gas with a strong global warming potential. Therefore, regulating the denitrification genes was of great significance to reduce the N2O emission in the compost. The denitrifying genes mainly composed of nitrate reductase, nitrite reductase, NO reductase and N2O reductase, which are regulated by the genes encoding napA or narG, nirS or nirK, nor and nos Z, respectively (Zumft, 1997). The nirS, nirK and nosZ genes were selected for identifying the dynamics of denitrifying microorganisms during the composting. For example, Chen et al. (2014) indicated the temperature could influence the structure of nirS-type and nirK-type denitrifying genes. Zhang et al. (2015) found the abundance of nosZ-type gene was significantly higher than that of the nirS and nirK. Besides, the nirK was the main functional gene in the compost. Furthermore, the effect of additives on the denitrification during the composting has been studied. Li et al. (2016) found biochar could reduce the N2O emission, which was closely related to the abundance of nirK-type gene. However, the effect of BD and BC on the dynamics of denitrification genes as well as the relationship between different denitrification genes and greenhouse gases were still not clear.
Based on the above review, the dynamics of nitrifiers and denitrifiers as well as their contributions to NH3 and N2O emission during the composting with BC and BD amendments was still limited reported. Therefore, the microbial driving mechanism of BC and BD on NH3 and N2O emissions during composting was investigated in this study. The purpose of the research was 1) to identify the dynamics in the abundance and structure of nitrification gene (AOB amoA) and denitrification genes (nirS, nirK and nosZ) during the composting with BD and BC amendments; 2) to understand the contribution of this genes to the production of NH3 and N2O during the composting.
Section snippets
Experimental materials
Fresh PM was collected from a pig farm nearby the university in Yangling, Shaanxi province, China. The pig manure was air-dried with a moisture less than 30% before mixing with other materials. Wheat straw (WS) was collected from a field at university and it was chopped into pieces with a length less than 1 cm. Besides, BD and BC were gathered from a tofu factor and Yangling biochar private company, respectively.
Experiment setup
The 2-fold of PM and 1-fold WS were mixed in the 100-L reactor and the compost
Changes of physicochemical properties
The selected physicochemical properties are shown in Table 1. The temperature was increased from day of 0 to 7, and the temperature of all treatments were higher than 55 ℃, indicating the existence of thermophilic period. Then the cooling period of composting was presented from day 7 to day 28 and maturity stage was within the day of 28–56. The pH was increased from 6.70 to 7.67 to 7.98–8.18 in the day of 28, which was related to the production of volatile ammonia and ammonium (Chen et al., 2010
Conclusion
The results showed the BD plus BC amendments could inhibit the NH3 and N2O emissions by limiting the nitrification and denitrification during the composting. The abundance of nirS was more closely associated with N2O and nosZ was more functional denitrifiers to the production of N2O. Among all nitrogen functional genes, Nitrosospira belonged to AOB amoA genes was mainly observed in the early stage of composting, and it was significantly positive related to NH3 and N2O emission. Marinobacter for
CRediT authorship contribution statement
Yajun Yang: Conceptualization, Data curation, Formal analysis, Writing - original draft, Methodology. Mukesh Kumar Awasthi: Formal analysis, Methodology, Writing - review & editing. Lulu Wu: Project administration, Resources, Validation, Software. Ying Yan: Supervision, Validation, Visualization, Methodology, Visualization. Jialong Lv: Funding acquisition, Investigation, Writing - review & editing.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This study was supported by the Key special projects of the Ministry of science and technology for the 13th Five Year Plan of China (Project No. 2017YFD0200205) and Integration and demonstration of agricultural non-point source pollution control technology of China (No. 2016slkj-15).
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