Effects of biochar size and type on gaseous emissions during pig manure/wheat straw aerobic composting: Insights into multivariate-microscale characterization and microbial mechanism

Highlights

• Granular-biochar improved pore volume and was benefit to methanotrophs activities.

• Higher aromatic compounds on surface of bamboo biochar and NO₃⁻ formed π–π EDA.

• Rice straw biochar is rich in reactive functional groups to adsorb NH₄⁺.

• Powder bamboo biochar was most suitable for controlling GHG and ammonia emissions.

Abstract

Greenhouse gas and ammonia emissions during composting with different biochar types and particle sizes were investigated. Compared with powder-biochar, granular-biochar improved pore connectivity and was benefit to methanotrophs activities, like Methylococcaceae, reducing CH₄ emissions. At the same particle size, bamboo biochar (BB) had a higher pore volume and more aerobic microenvironment within the compost than rice straw biochar (RSB), reducing GHG emissions. Bamboo biochar had high aromatic compound and NO₃⁻ concentrations and therefore surface π–π electron donor/acceptor interactions, causing low N₂O emissions and inhibiting denitrifying bacteria (e.g., Bacteroidales). More CO and CO bonds in rice straw biochar than bamboo biochar caused lower NH₃ emissions using rice straw than bamboo biochar. Powdered biochar had more exposed reactive functional groups and decreased NH₃ production better than granular biochar. Powdered bamboo biochar controls gaseous emissions better than other biochars during aerobic pig manure/wheat straw composting.

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.