Effects of biochar size and type on gaseous emissions during pig manure/wheat straw aerobic composting: Insights into multivariate-microscale characterization and microbial mechanism
Graphical abstract
Introduction
During aerobic composting, microbial activity produces large amounts of greenhouse gases (GHGs) and ammonia (NH3) (Szanto et al., 2007). The latest analysis of observations from the World Meteorological Organization (WMO), Global Atmosphere Watch Programme indicated that globally averaged surface mole fractions of carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O) reached new maxima in 2016, with CO2 at 403.3 ± 0.1 ppm, CH4 at 1853 ± 2 parts per billion (ppb) and N2O at 328.9 ± 0.1 ppb (World Meteorological Organization, 2018). Concurrently, the loss of nitrogen (N) in the form of NH3 can account for 50%–88% of the total N content of livestock manure (Ogunwande et al., 2008). Biochar, produced through the pyrolysis of biomass, is a stable carbon-rich stable substance that is a good adsorbent. Biochar has great potential to reduce GHG and NH3 emissions during composting (Sanchez-Monedero et al., 2018, Wang et al., 2013). This, would not only reduce the environmental impact of GHG and NH3 emissions but also limit C and N losses from fertilizers. Mao et al. (2018) found that adding 5% w/w bamboo biochar aerobically composting pig manure/wood dust decreased the maximum CH4, N2O, and NH3 emissions by 54%, 37%, and 13%, respectively. Awasthi et al. (2017) found that adding 8%–18% w/w wheat straw biochar to compost decreased CH4, N2O, and NH3 emissions while composting by 92.85%–95.34%, 95.14%–97.30%, and 58.03–65.17%. In other studies it has been found that different biochar types and particle sizes have different adsorption and structural characteristics that affect gas emissions during aerobic composting (Nartey and Zhao, 2014, Zhang et al., 2018a). Chen et al. (2017) found that different types of biochar (cornstalk biochar, bamboo biochar, woody biochar, layer manure biochar, and coir biochar) caused variable reductions in CH4 and NH3 emissions, with lowest NH3 and CH4 emissions measured for the cornstalk biochar treatment. He et al. (2017) pointed out that powdered rice straw biochar (RSB) increased CH4 emissions during the initial stages of composting because of its specific particle size. Xiao et al. (2017) concluded that different types of biochar reduced gaseous emissions to varying extents during the aerobic composting. Detailed studies of the effects of different particle sizes and types of biochar on gaseous emissions during aerobic composting will allow the environmental impacts of GHG and NH3 emissions during composting to be better understood and controlled.
The effects of adding biochar to composting material on CO2 and CH4 emissions will mainly be related to effects on the activities of aerobic and anaerobic microbes. The physicochemical properties of biochar will be strongly affected by microbial activity during composting (Sanchez-Monedero et al., 2018, Xiao et al., 2017). Biochar has a high carbon content and contains unstable carbon (Boehm, 1994), so may strongly affect emissions of carbon-containing gases (CO2 and CH4). CH4 is produced during composting by methanogens under strictly anaerobic conditions (Chen et al., 2018), and may also be metabolized by methanotrophs before being emitted (Kalyuzhnaya et al., 2015). The composting material, especially manure, will be indirectly affected by the physicochemical characteristics of added biochar. The added biochar will change the porosity, pH and C/N ratio of the compost and affect the adsorption processes that occur, and these changes will affect microbial diversity during aerobic composting (Zhang et al., 2018a). Different biochar types and sizes will therefore have different effects on the microbial community and therefore affect the metabolic processes involved in the production of GHGs and NH3.
The metabolic processes affecting N during aerobic composting are mainly include the decomposition of organic N by ammonifying bacteria to produce NH3 and NH4+, the production of NO3− by nitrifying bacteria, and the production of N2O by denitrifying bacteria (Kuypers et al., 2018, Zhou et al., 2018). Hydroxyl, carboxyl, and carbonyl groups in biochar can adsorb the metabolites produced during these processes and therefore affect metabolic processes and the gases emissions (Zhang et al., 2018b). Different biochar particle sizes will affect the pore distribution in the compost pile to different degrees and therefore affect the aerobic microenvironments and chemical exposure to functional groups on the biochar surfaces to different degrees. The metabolic processes producing NH3 and N2O will be affected by the adsorption and structural characteristics of the biochar.
This study was designed taking the results of previous studies (He et al., 2018, Liu et al., 2017) into consideration. Two types of biochar (bamboo biochar (BB) and rice straw biochar (RSB)) with two sizes each (powder: ɸ < 1 mm and granular: 10 mm > ɸ > 4 mm)) were added to composting material. Multivariate-microscale characterization methods were used to analyze pore characteristics, surface morphologies, and functional group of the four test mixtures during the composting process. High-throughput methods were used to analyze the microbial metabolic pathways to explore the mechanisms underlying the effects of the biochar on GHG and NH3 emissions. The results provide a theoretical framework for using biochar in aerobic composting processes to reduce GHG and NH3 emissions.
Section snippets
Experimental design
The laboratory-scale composting system (Fig. 1) was a 15-L cylindrical stainless steel reactor (0.40 m high, 0.25 m internal diameter) with a ventilation control system and an insulation case (Ge et al., 2014). Pig manure from a pig farm in Changping, operated by the Chinese Academy of Agricultural Sciences (Beijing, China) was used as the main raw compost material. Wheat straw from the Shangzhuang Experimental Base of the China Agricultural University (Beijing, China), cut into 1–3 cm stalks,
Compost evolution with different biochar additives
The temperature of the four compost piles followed the classic composting pattern, exhibiting mesophilic (days 1–4), thermophilic (days 4–7), cooling (days 7–13), and maturation (days 12–18) phases (Fig. 2). The temperatures of the PRSB, PBB, GRSB, and GBB piles increased rapidly over the first four days reaching the maximum temperatures of 52 °C, 52.3 °C, 53.4 °C and 53.3 °C, respectively. There was no significant difference in the dynamic change of temperature among these four piles
Conclusions
It was concluded that powder bamboo biochar was most suitable than other biochars for controlling GHGs and NH3 emissions. Microscale characterizations and microbial analyses were performed to identify the mechanism involved, but the microscale characterization analyses could not provide information on the compost macrostructures. A new multiscale characterization approach will be required to fully characterize the pores in compost. Competition between microbes will need to be investigated in
Acknowledgments
This work was financially supported by the Research and Development of China (2018YFD08001-02), the China Agriculture Research System (CARS-36) and the Program for Changjiang Scholars and Innovative Research Team in University of Ministry of Education of China (IRT-17R105).
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