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BỘ GIÁO DỤC VÀ ĐÀO TẠO TRƯỜNG ĐẠI HỌC CÔNG NGHỆ ĐỒNG NAI BÀI BÁO ĐĂNG TRONG TẠP CHÍ QUỐC TẾ THUỘC DANH MỤC ISI CHEMOSPHERE ISSN: 0045-6535 (Print); 1879-1298 (Online) Impact Factor: 7.086 (2020) Published online: April 2022 Research Article: Evaluate the role of biochar during the organic waste composting process: A critical review Hong Giang Hoang, Faculty of Health Sciences and Finance - Accounting, Dong Nai Technology University, Bien Hoa, Dong Nai 76100, Vietnam Địa tra cứu tải báo: https://doi.org/10.1016/j.chemosphere.2022.134488 Đồng Nai - Năm 2022 Chemosphere 299 (2022) 134488 Contents lists available at ScienceDirect Chemosphere journal homepage: www.elsevier.com/locate/chemosphere Evaluate the role of biochar during the organic waste composting process: A critical review Minh Ky Nguyen a, b, Chitsan Lin a, b, **, Hong Giang Hoang c, Peter Sanderson d, Bao Trong Dang e, Xuan Thanh Bui f, g, Ngoc Son Hai Nguyen h, Dai-Viet N Vo i, j, Huu Tuan Tran k, l, * a Ph.D Program in Maritime Science and Technology, National Kaohsiung University of Science and Technology, Kaohsiung 81157, Taiwan Department of Marine Environmental Engineering, National Kaohsiung University of Science and Technology, Kaohsiung, 81157, Taiwan Faculty of Health Sciences and Finance - Accounting, Dong Nai Technology University, Bien Hoa, Dong Nai, 76100, Viet Nam d Global Centre for Environmental Remediation (GCER), Faculty of Science, The University of Newcastle, Callaghan, NSW, Australia e HUTECH University, 475A, Dien Bien Phu, Ward 25, Binh Thanh District, Ho Chi Minh City, Viet Nam f Key Laboratory of Advanced Waste Treatment Technology, Vietnam National University Ho Chi Minh (VNU-HCM), Linh Trung Ward, Thu Duc District, Ho Chi Minh City, 700000, Viet Nam g Faculty of Environment and Natural Resources, Ho Chi Minh City University of Technology (HCMUT), Ho Chi Minh City, 700000, Viet Nam h Faculty of Environment, Thai Nguyen University of Agriculture and Forestry (TUAF), Thai Nguyen, 23000, Viet Nam i Center of Excellence for Green Energy and Environmental Nanomaterials (CE@GrEEN), Nguyen Tat Thanh University, Ho Chi Minh City, 700000, Viet Nam j School of Chemical Engineering, Universiti Sains Malaysia, Engineering Campus, 14300, Nibong Tebal, Penang, Malaysia k Laboratory of Ecology and Environmental Management, Science and Technology Advanced Institute, Van Lang University, Ho Chi Minh City, Viet Nam l Faculty of Technology, Van Lang University, Ho Chi Minh City, Viet Nam b c H I G H L I G H T S G R A P H I C A L A B S T R A C T • Using biochar as an additive improved the performance and quality of composting • Biochar affects the dynamic and struc ture of the microbial community during composting • Biochar reduced the availability of heavy metals and odorous gases emissions • Biochar improved the compost maturity by promoting enzymatic activity and germination index A R T I C L E I N F O A B S T R A C T Handling Editor: Derek Muir Composting is very robust and efficient for the biodegradation of organic waste; however secondary pollutants, namely greenhouse gases (GHGs) and odorous emissions, are environmental concerns during this process Bio char addition to compost has attracted the interest of scientists with a lot of publication in recent years because it has addressed this matter and enhanced the quality of compost mixture This review aims to evaluate the role of biochar during organic waste composting and identify the gaps of knowledge in this field Moreover, the research Keywords: Additives Microorganism Nitrogen losses * Corresponding author Laboratory of Ecology and Environmental Management, Science and Technology Advanced Institute, Van Lang University, Ho Chi Minh City, Viet Nam ** Corresponding author Ph.D Program in Maritime Science and Technology, National Kaohsiung University of Science and Technology, Kaohsiung 81157, Taiwan E-mail addresses: ctlin@nkust.edu.tw (C Lin), tranhuutuan@vlu.edu.vn (H.T Tran) https://doi.org/10.1016/j.chemosphere.2022.134488 Received 16 January 2022; Received in revised form 18 March 2022; Accepted 30 March 2022 Available online April 2022 0045-6535/© 2022 Elsevier Ltd All rights reserved M.K Nguyen et al Humification Enzyme activity Greenhouse gases emission Chemosphere 299 (2022) 134488 direction to fill knowledge gaps was proposed and highlighted Results demonstrated the commonly referenced conditions during composting mixed biochar should be reached such as pH (6.5–7.5), moisture (50–60%), initial C/N ratio (20–25:1), biochar doses (1–20% w/w), improved oxygen content availability, enhanced the perfor mance and humification, accelerating organic matter decomposition through faster microbial growth Biochar significantly decreased GHGs and odorous emissions by adding a 5–10% dosage range due to its larger surface area and porosity On the other hand, with high exchange capacity and interaction with organic matters, biochar enhanced the composting performance humification (e.g., formation humic and fulvic acid) Biochar could extend the thermophilic phase of composting, reduce the pH value, NH3 emission, and prevent nitrogen losses through positive effects to nitrifying bacteria The surfaces of the biochar particles are partly attributed to the presence of functional groups such as Si–O–Si, OH, COOH, C– –O, C–O, N for high cation exchange capacity and adsorption Adding biochars could decrease NH3 emissions in the highest range up to 98%, the removal efficiency of CH4 emissions has been reported with a wide range greater than 80% Biochar could absorb volatile organic compounds (VOCs) more than 50% in the experiment based on distribution mechanisms and surface adsorption and efficient reduction in metal bioaccessibilities for Pb, Ni, Cu, Zn, As, Cr and Cd By applicating biochar improved the compost maturity by promoting enzymatic activity and germination index (>80%) However, physico-chemical properties of biochar such as particle size, pore size, pore volume should be clarified and its influence on the composting process evaluated in further studies Introduction and change in bulk density affecting microbial colonization/structure and its biomass (Bapat et al., 2022; Jindo et al., 2012a) Biochar mi crospores may be responsible for adsorbing moisture, and biochar par ticles may also contribute some secondary porosity structure, which is essential for improving aerobic conditions during composting (Steiner et al., 2011) The bulk density is influenced significantly by the com posting time and experimental process Reducing bulk density obtained by using biochar in the composts and reaching potential benefits for their application Furthermore, biochar agent has been investigated as a promising and efficient technology to increase the organic matter degradation and adsorbs GHGs (Wang et al., 2018) The previous illus trated that poultry manure mixed with wood biochar showed that the organic matter was approximately degraded by 73–75%, and also GHG emissions were reduced around 42.8% (Chowdhury et al., 2014; Dias et al., 2010) Similarly, coffee husk and sawdust biochar were reported to decrease GHG emissions by around 46% and 55%, respectively (Chen et al., 2010; Jiang et al., 2016) Biochar amendment for composting has also recently been regarded as a cost-effective and environmentally friendly solution to improve composting humification and performance, increasing microbial activ ities and decreasing the available of heavy metal contents and organic contaminants (Agegnehu et al., 2017; Guo et al., 2020; Wu et al., 2018; Xiao et al., 2017) Biochar has favorable physicochemical properties, such as large SSA, high porosity, carbon-residue derived from the thermal conversion related to organic waste and cation exchange ca pacity, which allows it to interact with essential nutrient cycles and promotes microbial development during the composting (Awasthi et al., 2017; Gudimella et al., 2022; Qayyum et al., 2017) The results showed that compost mixed with 8–12% biochar became more humified after composting (35 days), and the compost maturity not only revealed that this could be a much more feasible approach to increased important nutrients such as NO3− , PO43− , Na+, K+, DOC and DON, but also bioavailability of heavy metal (i.e., Zn, Cu, Ni, and Pb) was reduced when compared to control Regarding the effect of biochar on the dy namic of a microbial community, the increased temperature of com posting piles is related to changes in the richness and diversity of compost mixture (Jindo et al., 2012a; Le et al., 2021; Steiner et al., 2011; Thakur et al., 2022; Yen et al., 2020) Biochar impacts the microbial community structure and changes in the phospholipid fatty acid analysis (PLFAs) patterns are related to major composting profiles (i.e., tem perature, C/N ratio, bulk density) as the critical drivers During this stage, the raw materials were mineralized by the biochemical and bio logical processes in which enzyme (i.e., dehydrogenase) reflected the microbial dynamic of the composting process Also, biochar is an effective tool for producing high-quality compost-based on reducing nutrient losses during the composting process and increasing compost maturity (Awasthi et al., 2016; Jiang et al., 2016) On the other hand, Composting is an effective technique to convert organic wastes into the final product called “mature compost” through various metabolisms (Awasthi et al., 2018; Chowdhury et al., 2014; S´ anchez-Monedero et al., 2019) During the composting process, volatile organic compounds (VOCs), greenhouse gases (GHGs: N2O, CO2, CH4), and other odor emission (H2S, NH3) could lead to serious secondary pollution, which may pose adverse environmental and health effects (Chung et al., 2021; Guo et al., 2020; S´ anchez-Monedero et al., 2019) The previous studies illustrated that default N2O and CH4 emissions factors were 0.6 g kg− and 10 g kg− waste during the composting process, respectively (Pipatti et al., 2006) In certain conditions, for instance poor aeration, will lead to anaerobic conditions that favors H2S and NH3 related odorous emis sions (Lin et al., 2021; Tran et al., 2021a) The rapid increase in VOCs and GHGs, leading to the emission of chemical precursors to ozone (O3), has led to the enhancement of ground-level ozone and raised concerns about the impact of its pollution (Wang et al., 2017; Xu, 2020), which may be related to changing ambient environment characteristics (Phan et al., 2020; Pusede et al., 2015; Wu et al., 2017a) The effects of severe secondary and primary pollution caused by VOCs and GHGs from composting process impact natural ecosystems and health issues, which is a concern and needs to be solved Therefore, it is necessary to manage those gaseous emissions, which has become one of the most significant concerns for sustainable development goals Adding biochar as an additive into the compost mixture is a helpful way to decrease GHGs and VOCs emissions during the composting (Chowdhury et al., 2014; Dias et al., 2010; He et al., 2017; Wang et al., 2018) The porous structure of biochar could enhance aeration rate and reduce bulk density of the compost pile, also provide main shelters for microorganisms, thus reducing the anaerobic zone generated, and as a result, minimize GHGs and odors emissions (Jindo et al., 2012a; Steiner et al., 2011) Owing to physical properties (e.g., high nano-porosity and large SSA), biochar can enhance aeration capacity and water retention The Brunauer-Emmett-Teller (BET) method revealed the specific surface area (SSA) of nonactivated and rice straw–derived biochars varied from 0.2 to 35.4 m2 g− which have been demonstrated by its helpful appli cations (Hwang et al., 2018; Lap et al., 2021; Nguyen et al., 2018; Tran et al., 2022) Additionally, the BET SSA of nonactivated biochars lightly changed consistent with the increase related to pyrolysis temperature Biochar formed from various feedstocks has a SSA of more than 520 m2 g− 1, and the SSA of biochar derived from diverse feedstocks is as follows: shell, straw, wood, manure, and sludge (Yuan et al., 2016) The benefits of the application of compost combined with the presence of biochar have reduced the bulk density inside the composting piles The exist a strong correlation between bulk density and the microbial community (bacterial and fungi), indicated depending on the initial organic wastes, M.K Nguyen et al Chemosphere 299 (2022) 134488 composting sewage sludge and animal waste with biochar has been shown to reduce mobility and bioavailability of toxically heavy metals (Antonangelo et al., 2021) Biochar⸻as an amendment to reduce the bioavailability of heavy metals and enhance the composting effec tiveness during composting by adding 1, 3, and 7% biochar into a mixture (Liu et al., 2017b) During co-composted biochar investigated that these heavy metals are usually formed strongly complex with the organic substances in composting materials The biochar pyrolysed at temperature conditions between 450 and 500 ◦ C demonstrated excellent ability in immobilization of heavy metals (particularly Zn, Cu) during biochar blended composting (Li et al., 2015) Biochar was observed to reduce the bioavailability and toxicity of Cd and Zn to E fetida earth worms in the vermicomposting experiments as well as enhancing the number of juveniles, cocoons in the compost mixture In recent years, numerous studies have investigated the biochar ef fect on the composting process (Guo et al., 2020; Sanchez-Monedero et al., 2018) Sanchez-Monedero et al (2018) indicated that as an ad ditive, biochar enhanced the composting performance and humification process, resulting in enhanced quality and maturity of the compost mixture Similarly in Guo et al (2020) reported that biochar is a valu able technique to enhance microbial activities, reducing GHGs and odors emissions, and the availability of heavy metals However, the interaction between microbial community and biochar during the composting process has not been addressed yet Also, a comprehensive review of the role of biochar on compost quality is still ambiguous Therefore, the aims of the study are to evaluate the role and effect of biochar combined with composting as an amendment on VOCs, GHGs emissions, enhancement of compost quality/maturity and their rela tionship with the microorganism activities during the organic waste composting process The key benefits that can be reached by the biochar mixed composting will also be investigated, especially related to the organic matter degradation, microbial community, humification, reduction of nitrogen losses, VOCs, and GHGs emissions Finally, the future perspectives will be pointed out in this review composting process (Fig 1) Also, biochar addition could enhance the efficiency and decrease available nutrients due to their characteristic such as larger porosity, functional groups, water holding capacity (WHC), cation exchange capacity (CEC) (Guo et al., 2020; Schmidt et al., 2014) The critical effects of added biochar on composting physico chemical profiles and their performance are shown in Table 2.1 Moisture Biochar was added into the compost mixture as a bulking agent, that could decrease the initial moisture content by absorbing excess moisture of the mixture (Zhang et al., 2016) Also, biochar properties reduced bulk density and enhanced the aeration of compost, leading to a decrease the initial moisture content (Chowdhury et al., 2014; Wang et al., 2013a) On the other hand, biochar addition has been illustrated to increase the water retention capacity of the mixture as an absorbent, implying it prolonged the optimal moisture content (50–60%) for composting, and enhanced the performance and humification of the ´pez-Cano et al., 2016; Prost et al., 2013) The composting process (Lo positive effect of biochar addition on the humification related to organic matters during the composting process and has been illustrated in pre vious research (Dias et al., 2010; Jindo et al., 2012b; Zhang et al., 2014a) For instance, the combined addition of compost (35%) mixed biochar (20%) contributed efficiently to moisture content during the composting (Zhang et al., 2014a) Also, Awasthi et al (2017) showed that 8–12% biochar was blended into biosolids co-composting to improve humification within 35 days of the experiment 2.2 Oxygen content Oxygen content plays a crucial role during the composting process, their presence in the biochar contributes to the adsorption process, which is also the key factor for aerobic microbial to degrade the organic substrate of the mixture (Tran et al., 2021b) Biochar addition into the mixture was observed enhancing the effectiveness of oxygen supply during the composting process due to higher porosity and their large SSA (Mujtaba et al., 2021; Tran et al., 2022) For instance, the presence of biochar has increased the range from 21% to 37% in oxygen (O2) uptake rates on the first day of the sludge composting process (Zhang Effect of biochar on the physio-chemical properties during composting process As an additive agent, biochar helps to improve the performance of Fig Effects of biochar addition on physicochemical properties during composting M.K Nguyen et al Chemosphere 299 (2022) 134488 Table Main effects of added biochar on compost physico-chemical properties and their performance Composting Biochar Amended rate Scale Periods Effects on composting performance and their quality References Poultry manure Wood 50% (w/w) Wood A high polymerization degree of humic compounds leads to reduce TN losses Increased TN around 45% Bamboo 2.5, 5, 10% (w/w) 0.03% (w/w) 210 days 20 days (Dias et al., 2010) Cow dung + hydrilla + sawdust Pig manure + sawdust Conical piles: 1.5 m high The turned-pile system A rotary drum composter: 550 L A tractor-pull windrow turner 74 days (Wang et al., 2014) Poultry litter + sugarcane straw Chicken litter + sawdust Sewage sludge + wheat straw Poultry litter + sugarcane straw Municipal solid waste Poultry litter 10% (w/w) 220 L compost bin 60 days Hardwood shaving Wheat straw 5, 10% (w/w) Spherical plastic bins (153 L) 130-L PVC composter reactors 220 L plastic compost bins Real conditions Full-scale 133 days 56 days A shorter time for thermophilic phase and a higher temperature during thermophilic phase Decreasing N2O and CH4 emissions Increased TN by 40% The increase in CEC was 6.5 times Green waste 2, 4, 8, 12% (w/w) 10% (w/w) 60 days Wood chips 1.5, 3, 5% (w/ w) Dewatered sewage sludge + wheat straw Poultry manure + barley straw Wheat straw biomas 2, 4, 6, 8, 12, 18% (w/w) 130-L reactor in-vessel 56 days Holm oak 3% (w/w) 19 weeks Poultry manure + wheat straw Woodchips 5, 10% (w/w) Pilot-scale Trapezoidal piles (1.5 m high) Turned pile (windrow) system Reactors: 165 L Corn wastes Corn wastes 1, 2% (w/w) Hen manure + wheat straw Bamboo 5, 10, 20% (w/ w) Swine manure + maize straw N/A 5, 10% (w/w) Green waste Yellow pine N/A Farm yard manure + vermicompost Distilled grain waste Rice husk due Food waste digestate + sawdust + mature Fresh chicken manure 72 days 42 days Plastic pots (35 cm height and 25 cm diameter) Small laboratory reactor Cylindrical (inner diameter: 0.25 m, total height: 0.40 m) Medium-scale PVC reactors (100 L) 150 days 28 days 52 day N/A N/A Plots Lab-scale Plot size (3.6 m × 2.6 m) Coconut shells 5, 10, 15, 20% (w/w) A computer-controlled 28 L reactor 65 days Tobacco stalk 2.5, 5, 10% (w/w) 3, 5, 10% (w/ w) 20 L composters 42 days 100-L plastic, cylindrical vessels Pilot-scale 50 days Rice husk N/A (Jain et al., 2018) (Agyarko-Mintah et al., 2017a) (Khan et al., 2016) Decreasing and degrading volatile fatty acids (VFAs) and odor emission index Decreased TN losses by 51% Improved N retention A positive impact on the compost quality and reduction of nitrogen losses Indicated higher moisture level and lower density Increased water-soluble nutrients (i.e., NO3–, DOC, DON, PO43–, K+ and Na+) Reduced bioavailability of heavy metals Biochar accelerated organic matter (OM) degradation Reduce the composting time by around 20% (Awasthi et al., 2018) (Agyarko-Mintah et al., 2017b) (Malinowski et al., 2019) Addition of biochar caused increasing temperature and shortened the thermophilic phase Biochar increased CO2 emission Improves the physicochemical properties of soil Exhibited high CEC and soil organic carbon (SOC) Reduced CO2, CH4, N2O and NH3 emissions (Czekała et al., 2016) Decomposition of dissolved organic carbon (DOC) Biochar promoted the composting humification and increased the P-bioavailability Biochar and compost divergently impacted functional groups of soil Improving soil hydro-physical properties, crop yield Adding 10% biochar reduced nitrogen loss up to 25.69% and accelerated OM degradation, thereby shortening the composting cycle 10% biochar distribute to reduce 58% of NH3 emission and 50% of nitrogen loss Significant reduction in gaseous emissions (GHGs, NH3 and CO2), microbial pathogens (Awasthi et al., 2017) (S´ anchez-García et al., 2015) (Liu et al., 2021a) (Liu et al., 2017a) (Cui et al., 2022) (Hale et al., 2021) (Sharma et al., 2021) (Wang et al., 2021) (Manu et al., 2021) (Chung et al., 2021) Remarks: CEC: Cation exchange capacity, TN: Total nitrogen, OM: Organic matter, DOC: Dissolved organic carbon, DON: Dissolved organic nitrogen, PVC: Polyvinyl chloride, N/A: Not available et al., 2014a) Similarly, Steiner et al (2011) indicated that a com posting pile with biochar doses (20% v/v) improved oxygen content availability, accelerating organic matter decomposition through faster microbial growth Biochar with large surface area provided a home for microorganisms, that significantly enhanced the microbial structure (richness and diversity) of mixture (Laird et al., 2010) Furthermore, biochar’s porous properties can help enhance the physical structures of compost by increasing pile porosity, which prevents anaerobic fermen tation by promoting oxygen supply (Xiao et al., 2017) et al., 2009; Tran et al., 2021a) The previous studies illustrated that critical condition for composting is affected by optimal pH during the compost mixture (6.5–7.5) (Godlewska et al., 2017; Hoang et al., 2022; Tran et al., 2021a; Yunus et al., 2020; Zainudin et al., 2020) Also, the mobility of ions (e.g., heavy metals) is usually determined by pH The solubility of their toxic metals is reduced in compost with a higher pH, lowering its toxicity when used as fertilizer Because some soluble alkaline components in biochar leak away, biochar can elevate compost pH shortly after being added (Li et al., 2015; Tran et al., 2021b) The pH profile during biochar mixed composting varies depending on biochar characteristics, composting methods/techniques, their compositions (e g., C/N ratio, nutrients, etc.) and composting materials such as food waste, animal waste, biosolids, yard waste, etc (Godlewska et al., 2017) Furthermore, He et al (2017) indicated that negative charge surfaces on biochar could absorb the generated ammonia/ammonium, resulting in a pH reduction 2.3 pH pH is a crucial factor that indicates the microbial activities and population community during the composting process (Tran et al., 2020) Increased pH at a later stage (i.e., in composting development) was thought to impair biochar’s potential for phenolic component retention, thus causing biochar materials to degrade even more (Bernal M.K Nguyen et al Chemosphere 299 (2022) 134488 2.4 Temperature profile of biochar mixed compost varies based on biochar properties, composting techniques, and their compositions (e.g., C/N ratio, nutri ents, etc.), and the ideal pH during the composting mixture should be reached 6.5–7.5 For aerobic organic waste composting, a suitable initial C/N ratio should be about 20–25:1, which is essential for C sequestration Temperature is known as a key vital indicator of the composting process and reflects the activities of microorganisms and also the organic matter degradation in the compost piles (Huang et al., 2019) The added biochar to the compost has been observed to activate the process, evi denced by a temperature increase and extending the thermophilic phase (Chen et al., 2010; Steiner et al., 2010) The presence of biochar in the composting process leads to temperature rising quickly, and so does the duration of the thermophilic phase The temperature during composting process increased faster in case of biochar addition than compared to the control without biochar (Wei et al., 2014) Biochar added at the start of the composting process, leads to increased water holding capacity (WHC), thus ensuring the desired moisture level in the range from 50% to 60% w/w (Prost et al., 2013) The biochar addition resulted in obtaining higher temperature, ascribed fewer heat losses, and increased microbial activity (Li et al., 2015) The higher temperature from the composting may accelerate the abiotic oxidation process of the biochar surface, resulting in more hydrophilic functional groups (e.g., carbonyl groups, hydroxyl) that are available for microbial breakdown (Cheng et al., 2006) The inclusion of biochar enhances aeration and hence the number of microorganisms, speeding up the transformations and increasing the heat produced (Godlewska et al., 2017) Effect of biochar on the dynamic of microbial community during the composting process 3.1 Effect of biochar on the dynamic of microbial community The biochar itself possesses a highly porous structure that contains valuable substances such as inorganic nutrients, labile aliphatic com pounds, and minerals (Quilliam et al., 2013; Xiao et al., 2017) Conse quently, the biochar amended compost essentially contributes to an increase in natural ventilation, temperature, moisture content, a favor able niche, and nutrition for native microorganisms (Atkinson et al., 2010) In this regard, added biochar could positively improve the per formance by changing the microbial community in the compost pile (Agegnehu et al., 2017) However, how much biochar dosage to ensure maximum activity and diversity of the microbial community is still being evaluated Table presents some studies on the microbial community changes driven by different raw materials and biochar dosage Most studies suggested that biochar is hugely compatible with various feedstock (Bello et al., 2020; Du et al., 2019b; Li et al., 2021) The biochar dosages are commonly used from 1% to 20% (w/w) Besides, adding a high dosage of biochar (10%) could reduce NH3, hydrogen sulfide (H2S), and total VOCs, while N-cycling microorganisms such as genus Pusillimonas and Pseudomonas became more active (Li et al., 2022; Liu et al., 2017a) Moreover, the enzyme’s activity of the bacterial community was strengthened by the biochar addition (10% and 20%) into sewage sludge and sawdust mixtures (Li et al., 2022) It indicated that the high dose of biochar might accelerate the metabolic activity of the adapted strain However, the observed richness (Chao1) and diversity (Shannon-Wi ener) varied with biochar dosage Some studies have suggested that increasing biochar dosage to 20% recorded lower alpha diversity indices than control compost (Zainudin et al., 2020) Possibly, a high dose of biochar might reduce the biodiversity index It is well known that increasing the decomposition rate can help facilitate treatment times, but high microbial diversity could promote the complete breakdown of a broad type of persistent pollutants (Bird et al., 2011; Novak et al., 2016) Thus, this creates a trade-off to balance the crucial benefits between metabolic rate and microbial community diversity driven by biochar dosage Excessive biochar (>10%) might cause severe heat dissipation and water loss, adversely affecting on the composting process (Liu et al., 2017a) In addition, the relative abundance of heavy metals resistant bacteria (HMRB) in composting process was decreased with elevated biochar dosages (0–10%) Heavy metals and biochar content signifi cantly reduced Firmicutes (52.88–14.32%), Actinobacteria (35.20–4.99%), while increased phylum of Bacteroidetes (0.05–15.07%) and Proteobacteria (0.01–20.28%) (Zainudin et al., 2020) A moderate addition of biochar (6%) was considered to have the most abundant HMRB among all treatments (poultry manure, wheat straw, and added chicken manure biochar (0–10%) (Li et al., 2021) Furthermore, the added 7.5% biochar enhances the removal of recalcitrant keratinized waste during pig manure composting (Duan et al., 2020) Taken together, these results point to the use of an appropriate amount of biochar that can help balance the diversity and metabolic activity of the community 2.5 C/N ratio Biochar is used popularly to amend the elemental composition dur ing the composting, and the C/N ratio is one of the most primary factors that have influenced this process (Lin et al., 2021; Nguyen et al., 2020) Because of the refractory carbon produced from the biochar addition, most studies indicated that adding biochar enhanced the C/N ratio (Chowdhury et al., 2014; Jindo et al., 2012a; Zhang et al., 2014b) A suitable initial C/N ratio should be reached around 25:1 for aerobic organic wastes composting (Wu et al., 2017c) The carbon to nitrogen ratios (C/N) of various feedstock-derived biochars and composts varies, which has a direct effect on the rate of organic matter decomposition (Godlewska et al., 2017) The rates of labile carbon mineralization remained high due to these biochar features, and the biochar in the compost provided more excellent durability when utilized as the soil amendments, which has crucial implications for C sequestration (Dianey et al., 2021; Doan et al., 2022; Godlewska et al., 2017; Kamaruzaman et al., 2022; Steiner et al., 2010) A high mineralization intensity leads to decompose/oxidize to easily available forms and conservation of N levels that are favorable during the N-rich composting process, e.g., manures and organic wastes Adding raw materials that contain a high C/N ratio (e.g., biochar) could improve immobilizing N compounds, and the nitrogen retained might be plant available for their growth Owing to the presence of not only acidic functional groups but also in the condi tion of low pH, biochar has been reported as an absorber of water-soluble NH4+ or NH3, thus reducing N losses during composting process (Kastner et al., 2009; Steiner et al., 2010) Biochar may impact the C/N ratio, which is an essential factor in compost microbiology (Wang et al., 2015) Biochar produces a favorable micro-environment for nitrifying bacteria, which convert ammonia (NH4+) to nitrate (NO3− ), resulting in higher nitrogen content in biochar-treated compost (Zhang et al., 2014a) In short, biochar was added to the compost mixture as a bulking agent that might reduce the initial moisture content of the mixture by absorbing excess moisture The presence of oxygen in biochar contrib utes to the adsorption process, which is a significant component for aerobic microorganisms to decompose the organic substrate of the mixture during the composting process The optimal conditions of ox ygen content (15–20%) and moisture content range of 50–60% could boost enzyme production and accelerate microbial activity When bio char is mixed into the composting piles, it increases the water-holding capacity (WHC), ensuring the optimum moisture content The pH 3.2 Effect of biochar on the structure of microbial community Biochar creates a distinct microbial population as a result of its intervention in the composting process Proteobacteria, Bacteroidetes, No Materials Composting ratios Biochar (%) Max Temp (◦ C) Max pH Max Shannon Max Chao1 Phylum Major Genus (Early-Middle periods) Major Genus (Middle -Later periods) Refs Poultry manure + rice straw 2:1 20 58.9 9.9 5.2 887 Firmicutes, Proteobacteria, Bacteroidetes 2:1 20 71.5 11 4.9 653 Firmicutes, Proteobacteria, Bacteroidetes Pusillimonas, Pseudomonas, Pseudofulvimona, Petrimonas, Sinibacillu Halomonas, Pusillimonas, Pseudofulvimona, Nitriliruptor, Truepera (Zainudin et al., 2020) Poultry manure + rice straw + biochar Sinibacillus, Ammonibacillus, Pseudofulvimonas, Pusillimonas, Petrimonas Ammonibacillus, Sinibacillus, Halomonas, Pusillimonas, Pseudofulvimonas Chicken manure + peanut straw (3–5 cm) 2.5:1 60.2 8.94 4.1 312 Firmicutes, Bacteroidetes, Proteobacteria, Halanaerobiaeota, Actinobacteria Gallicola, Proteiniphilum, Bacillus, Ammoniibacillus (Li et al., 2022) Chicken manure + peanut straw + biochar 2.5:1 10 64.6 8.71 3.9 318 Firmicutes, Bacteroidetes, Proteobacteria, Halanaerobiaeota, Actinobacteria Gallicola, Proteiniphilum, Bacillus, Ammoniibacillus Pseudomonas, Pusillimonas, Ignatzschineria, Thiopseudomonas, Flavobacterium Pseudomonas, Pusillimonas, Ignatzschineria, Thiopseudomonas, Flavobacterium Sewage sludge + corn cob (7 mm) Sewage sludge + corn cob + biochar 5:3 (v/v) 54 8.08 4.6 – 5:3 (v/v) 8.2 55 8.07 4.0 Alicycliphilus, Ochrobactrum, Proteiniphilum Bacillus, Ochrobactrum, Rhodanobacter Sewage sludge + corn cob + biochar 5:3 (v/v) 15.2 55 8.13 3.9 – Proteobacteria, Bacteroidetes, Firmicutes, Actinobacteria Bacillus, Ochrobactrum, Proteiniphilum Sphingobacterium, Glutamicibacter, Ochrobactrum, Rhodanobacter, Enterobacter Glutamicibacter, Microbacterium, Ochrobactrum, Enterobacter, Rhodanobacter (Liu et al., 2021c) – Proteobacteria, Bacteroidetes, Firmicutes, Actinobacteria Proteobacteria, Bacteroidetes, Firmicutes, Actinobacteria Cow manure + sugarcane straw (1 cm) 5:1 (v/v) 60 – – – Firmicutes, Actinobacteria, Proteobacteria, Chloroflexi, Bacteroidetes Thermopolyspora, Thermomonospora, Ureibacillus Cow manure + sugarcane straw (1 cm) + biochar 5:1 (v/v) 68.5 – – – Firmicutes, Actinobacteriota, Proteobacteria, Chloroflexi, Bacteroidetes Corynebacterium, Romboutsia, Pseudoxanthomonas, Thermomonospora, Clostridium Corynebacterium, Romboutsia, Pseudoxanthomonas, Thermomonospora, Clostridium Sewage sludge + straw (1 cm) 4:1 59.9 8.4 4.7 582 Firmicutes, Proteobacteria, Chloroflexi, Actinobacteria, Bacteroidetes Vulgatibacter, Anaerolineaceae, Thermobifida Sewage sludge + straw (1 cm) + biochar 4:1 3.85 61.6 8.2 4.6 556 4:1 61.4 8.2 5.0 619 Proteobacteria, Chloroflexi, Firmicutes, Actinobacteria, Bacteroidetes Pseudomonas, Chloroflexi, Pedobacter, Planomicrobium, Microtrichales Pseudomonas, Pedobacter, Planomicrobium Bacillus, Ochrobactrum, Rhodanobacter (Yan et al., 2021) Thermopolyspora, Thermomonospora, Ureibacillus (Xue et al., 2021) Psychrobacillus, Paenisporosarcina, Ureibacillus (continued on next page) Chemosphere 299 (2022) 134488 M.K Nguyen et al Table Different raw materials and biochar dosages drive microbial communities to change over time No Materials Composting ratios Biochar (%) Max Temp (◦ C) Max pH Max Shannon Max Chao1 Sewage sludge + straw (1 cm) + aerobic bacteria 7 Major Genus (Early-Middle periods) Major Genus (Middle -Later periods) Proteobacteria, Chloroflexi, Firmicutes, Actinobacteria, Bacteroidetes, Pseudomonas, Chloroflexi, Pedobacter, Planomicrobium, Microtrichales Pseudomonas, Microtrichales, Planomicrobium, Chloroflexi Pseudomonas, Microtrichales, Chloroflexi, Pedobacter, Planomicrobium Microtrichales, Chloroflexi, Pedobacter, Planomicrobium Vulgatibacter, Thermobifida, Ureibacillus, Chelativorans Sewage sludge + straw (1 cm) + aerobic microorganism agent + biochar 4:1 3.82 62.4 8.25 5.0 576 Proteobacteria Chloroflexi, Firmicutes, Actinobacteria, Bacteroidetes Sewage sludge + straw (1 cm) + facultative anaerobic agent 4:1 61.1 4.9 583 Proteobacteria, Chloroflexi, Firmicutes, Actinobacteria, Bacteroidetes Sewage sludge + straw (1 cm) + facultative anaerobic agent + biochar (3–5 mm) 4:2 3.82 62.2 8.2 5.0 594 Proteobacteria and Chloroflexi, Firmicutes, Actinobacteria, Bacteroidetes Pig manure + wheat straw 2:1 64 7.2 5.5 820 2:1 68 8.1 4.8 840 Pig manure + wheat straw + biochar (10%) 2:1 10 68 8.3 5.5 900 Bacilli, Clostridia, Tenericutes, Actinobacteria Bacilli, Clostridia, Genus of Tenericutes and Actinobacteria Bacilli, Clostridia, Tenericutes, Actinobacteria Firmicutes, Bacteroidetes, Proteobacteria, Chloroflexi Pig manure + wheat straw + bean dregs (15%) Pig manure + wheat straw + bean dregs (15%) + biochar (10%) 2:1 10 69 8.15 6.0 820 Actinobacteria, Firmicutes, Bacteroidetes, Chloroflexi, Proteobacteria Actinobacteria, Firmicutes, Bacteroidetes, Chloroflexi, Proteobacteria Actinobacteria, Firmicutes, Bacteroidetes, Chloroflexi, Proteobacteria Actinobacteria, Firmicutes, Bacteroidetes, Chloroflexi, Proteobacteria Bacilli, Clostridia, Tenericutes, Actinobacteria Firmicutes, Bacteroidetes, Proteobacteria, Chloroflexi Tomato stalk + chicken manure + biochar 6:5 56 7.2 2.58 – Proteobacteria, Bacteroidetes, Firmicutes, Actinobacteria Tomato stalk + chicken manure + peat bog 6:5 50 7.3 2.03 – Proteobacteria, Firmicutes, Actinobacteria Acinetobacter, Chitinophaga sp., Flavobacterium, Actinobacterium Rhizobiales sp., Acinetobacter sp., Chitinophaga sp Pusillimonas sp., Tomato stalk + chicken manure + zeolite 6:5 50 1.46 – Proteobacteria Flavobacterium, Actinobacterium, Chitinophaga sp., Pseudomonas sp Pusillimonas sp., Microbacterium sp., Geobiacillus sp., Pseudomonas sp., Rhizobiales sp Rhizobiales sp., Chitinophaga sp., Acinetobacter sp., Actinobacterium Cattle manure + maize straw 5:1 75 8.1 4.57 871 Firmicutes, Actinobacteria, Proteobacteria, Chloroflexi, Bacteroidetes Thermopolyspora, Thermobifida, Anaerolineaceae Cattle manure + maize straw + biochar 5:1 10 75 8.2 4.65 913 Actinobacteria Firmicutes, Proteobacteria, Chloroflexi, Bacteroidetes Corynebacterium, Bacillus, Atopostipes, Marinilabiaceae, Turicibacter Corynebacterium, Bacillus, Atopostipes, Marinilabiaceae, Turicibacter Refs Thermopolyspora, Anaerolineae, Chloroflexi, Limnochordaceae, Ureibacillus Bacillus, Chelativorans, Thermobifida, Pseudoxanthomonas Pseudoxanthomonas, Anaerolineaceae, Thermobifida, Anaerolineae, Chloroflexi (Yang et al., 2020) Firmicutes, Bacteroidetes, Proteobacteria, Chloroflexi Firmicutes, Bacteroidetes, Proteobacteria Chloroflexi (Wei et al., 2014) Pseudomonas sp., Chitinophaga sp., Rhizobiales sp., Pusillimonas sp., Actinomadura, Longispora, Streptomyces (Bello et al., 2020) Chemosphere 299 (2022) 134488 Phylum M.K Nguyen et al Table (continued ) M.K Nguyen et al Chemosphere 299 (2022) 134488 Firmicutes, and Actinobacteria were the most predominant phyla in bio char amended compost due to their excellent compatibility with biochar and feedstock (Duan et al., 2019a; Li et al., 2022; Yang et al., 2020) Biochar is likely to promote the proliferation of certain individual bac terial phyla rather than entire communities due to biochar affecting physicochemical properties during composting (Dang et al., 2021) Proteobacteria was related to the nitrogen and carbon cycle, while Acti nobacteria played an essential part in degrading lignin and refractory cellulose Actinobacteria can utilize biochar as a source of C and miner alize it to CO2; these biochars may have guided the metabolism of Actinobacteria and enriched it through a process known as Copiotrophs (Bello et al., 2020) The relative abundance of this bacterium can be altered by C/N value, biochar dosage, composting phase, and temperature (Czepiel et al., 1996) Biochar simultaneously increases the temperature and prolongs the thermophilic phase compared to non-biochar compost piles, thereby promoting the metabolism of adapted strains as well as changing the bacterial community over time (Zainudin et al., 2020) The Actinobacteria and Firmicutes play an important role in decomposing complex organic materials during the thermophilic phase and are thermos-tolerant (Bello et al., 2020) With increased temperature, the relative abundance of genus belonging to Bacteroidetes and Proteobac teria decreased and later increased as the compost temperature declined (Bello et al., 2020) Genera of Proteobacteria and Bacteroidetes are known to be resistant to antibiotics, and this fact may explain why increased peak temperature by biochar amendment compost would be beneficial to enhance the elimination of antibiotic resistance genes (ARG) (Fu et al., 2021; Li et al., 2017; Siedt et al., 2021) Effect of biochar on VOCs and GHGs emission during composting process During composting, VOCs (e.g., hydrocarbons, alcohols, aldehydes, esters, etc.), GHGs (N2O, CO2, CH4), and odorous gases (H2S, NH3) are emitted into the ambient air, which may pose an environmental risk and health concerns Biochar amendment is considered as an efficient solu tion to adsorb VOCs, GHGs, odorous gases through various previous studies (Awasthi et al., 2016; Chowdhury et al., 2014; Dias et al., 2010; He et al., 2017; Janczak et al., 2017; Jiang et al., 2016; Wang et al., 2018) Table and Fig illustrate the effects of biochar on the VOCs and GHGs emissions during composting 4.1 Volatile organic compounds (VOCs) The biochar addition significantly reduced the emissions of oxygenand nitrogen-containing VOCs during the composting process ´nchez-Monedero et al., 2019; Tran et al., 2018) During the ther (Sa mophilic phase belongs to composting process, VOC were classified based on its abundance into three main groups, including nitrogenous, oxygenated and other compounds By using 90% poultry manure plus 10% straw (within 3% biochar addition), improving aerated conditions reduced up to 50% these concentrations during the thermophilic phase in composting process The greatest efficiency was illustrated in the OVOCs compounds, with most ketones, phenols and volatile fatty acids (VFAs) concentrations reduced significantly in a pile mixing biochar The addition of biochar promoted the aeration rate in the composting matrix due to its higher porosity, leading to increasing gas exchange and preventing the anaerobic zones formation, which could be a source of VOCs (Sanchez-Monedero et al., 2018) Pore structure and surface acid functional groups in biochar could trap toxic emissions, thus preventing toxic volatilization and reducing their pollution Biochar’s strong sorp tion capacity may represent a mechanism for VOCs elimination in the ´nchez-Monedero composting piles, that is aided by their large SSA (Sa et al., 2019) For example, wooden biochars showed high removal ef ficiencies for acetone, toluene and cyclohexane in the value from 50 to 100 mg VOC g− (Zhang et al., 2017) Janczak et al (2017) investigated that 10% biochar can decrease VOCs emissions during biosolid com posting Biochar could absorb TVOCs (about 17.55%) in the experiment based on distribution mechanisms and surface adsorption (Li et al., 2021) Furthermore, the most efficient VOCs reduction was investigated in OVOCs compounds (e.g., phenols, ketones, and organic acids) and dramatically reduced the amounts of volatile nitrogen compounds, which are produced by microbial modification of N-compounds These results indicate the importance of biochar application not only impacts the composting progress but also their sorption capacity as key drivers for VOCs reduction In addition, the environmental conditions are characterized such as temperature, moisture, etc during composting that may affect the sorption of VOCs on biochar surface (Hwang et al., 2018; ´nchez-Monedero et al., 2019; Tran et al., 2018; Zhang et al., 2017) Sa The impact of biochar could modify these conditions inside the com posting matrix, and affecting on vital parameters including aeration, temperature, moisture, microbial activity that is related to the for mation/degradation of VOCs compounds during composting (Mauli ni-Duran et al., 2014) More clearly, composting biochar increases pH, aeration, enhances water holding capacity, and thus improving oxygen content and redox conditions (Wu et al., 2017b) Interestingly, the composting piles mixed biochar enhanced favorable environmental conditions for microbial growth, leading to the reduction of VOCs This work may explain that biochar is a promising alternative sorbent and favorable effective in reducing gaseous VOCs 3.3 Effect of biochar on the microbial community – nitrogen during the composting Absorption of biochar increases the activity of nitrifying bacteria, reduces methanogens, and increases heavy metal fixation The surfaces of the biochar particles are partly attributed to the presence of functional – O, C–O, N for high cation ex groups such as Si–O–Si, OH, COOH, C– change capacity and adsorption (Agegnehu et al., 2017) As a result, biochar aids in absorbing both NH3 and greenhouse gas released from the compost pile, increasing oxygen diffusion to reduce the anaerobic zone (Xiao et al., 2017) After the NH3 is adsorbed by biochar, this is reported to benefit the growth of nitrifying bacteria, which convert the ammonium to nitrate and thus retain nitrogen in the compost products (Godlewska et al., 2017; Li et al., 2022) Consequently, it tends to in crease N–NO3− concentration while decreasing the volatilization of NH3 (Chen et al., 2017a) The concentration of NO3− was increased twice ´pez-Cano et al., 2016) In addi compared to conventional compost (Lo tion, biochar also affects the genus denitrification population, such as N2O− producing bacteria, by improving oxygen diffusion into the compost pile, so less N2O is produced (Wang et al., 2013b) The pre dominant aerobic zone is responsible for a decrease in the population of anaerobic species such as denitrifying bacteria and methanogens By contrast, biochar likely increased the activity of methane-oxidizing bacteria that convert CH4 to CO2 (Liu et al., 2017a) In addition, when the compost pile has few anaerobic zones, it results in a limited number of Methanogens and Methanotrophs present to directly reduce CH4 emission (He et al., 2019a; Mao et al., 2018) In summary, a proper biochars dosage could reduce organic and inorganic pollutants by altering the profile and activity of the microbial community A higher rate of biochar addition will increase pollutant uptake and carbon sequestration but be more expensive and might hurt biodiversity Biochar not only helps to create a distinct microbial pop ulation in the compost pile, but its high adsorption capacity is beneficial for heavy metal fixation, improved nitrification, and reduced GHGs emissions To put such results into practice, it is necessary to carry out large-scale experiments 4.2 Greenhouse gases (N2O, CO2, CH4) During composting several major greenhouse gas compounds such as M.K Nguyen et al Chemosphere 299 (2022) 134488 Table Effects of biochar on VOCs and GHGs emissions during composting Composting Biochar Characteristics Effects and benefits Mechanism and limitation References Green waste + bagasse + chicken manure Green waste + bagasse + chicken manure Waste willow wood (Salix spp.) Pyrolysis: 550 ◦ C Waste willow wood (Salix spp.) Pyrolysis: 550 ◦ C Lowered N2O emissions Improved soil quality Yield increase Reduced N2O emissions Increased Na, K, Mg, P, NO3–, NH4+ and soil carbon Biochars are more stable in soil, it is resulting in lower CO2 loss (Agegnehu et al., 2016) Garden peat Pyrolyzer: 450 ◦ C Biochar may be less suited for reducing N2O flux in some agricultural soils, at least on shorter temporal scales Biochar could act as an electron shuttle, which enhances the last step from N2O to N2 Effect of biochar mixed composts illustrated stabilization of native and labile organic carbon present in farm manure (FM), resulting in carbon stabilization in the soil (Agegnehu et al., 2016) Farm manure Compost windrows Field-scale 9% w/w Field-scale Period: 98 days Amendment windrows Rate: 2% w/w Laboratory scale Periods: months Moisture: 60% Pig manure Rice straw Pyrolysis: 450 ◦ C Poultry manure + wheat straw Wood woodchips Pyrolysis: 350 ◦ C Chicken mortality Woodchips Gasification: 520 ◦ C Sewage sludge + zeolite + lime Wheat straw Chicken manure + straw Holm oak Pyrolysis: 650 ◦ C Swine manure Non-activated biochar Pyrolysis: 495–505 ◦ C Biochar made from charcoal at 400 ◦ C Surface area: 35.48 m2 g–1 Fresh chicken manure + peanut straw Reactors: 120 L cylindrical plastic Period: 84 days Passive aeration composting 5% and 10% Laboratory scale Reactors: 165 L Period: 42 days 1, 5, 10, and 15% Period: 11 weeks Pilot-scale Composting test: 32-gallon bins Aerated rate: 1.5 L min–1 Bench-scale PVC: 130 L Period: 56 days 12% biochar 3% biochar addition Trapezoidal piles: 1.5 m × × Pilot scale Period: 20 weeks Pilot-scale Non-activated 10% biochar Reactors: 60 L Aeration rate: L min–1 Period: 40 days Reduced CO2 emission, especially with a higher proportion of biochar in the compost Increased SOC, yield, N and K contents in plant Significantly reduced N2O emissions 5% and 10% of biochar can reduce NH3 emission by 30% and 44%, respectively Biochar amendment at 10 and 15% could reduce the cumulative NH3 emissions up to 40% and 57% 12% biochar + zeolite could significantly reduce the CH4 58.03–65.17% and N2O 92.85–95.34% Biochar efficiently reduced the levels of VOC during the thermophilic phase The most efficient VOC reduction was observed in OVOCs compounds (e.g., ketones, phenols and organic acids) Biochar can be a promising and comparably-priced option for reducing NH3 emissions from swine manure NH3, H2S, and TVOCs emission decreased by 20.04%, 16.18%, and 17.55% in the experiment (Qayyum et al., 2017) The low degradability of biochar could be led to the core reason for these findings (Vu et al., 2015) Biochar addition to poultry manure contributes to nitrogen retention in the solid Biochar’s beneficial effect on nitrogen loss is due mainly to its adsorption properties and the presence of surface acid groups The potential to decrease NH3 release is due to the adsorption of NH3/NH4+ by biochar pores Acid functional groups on biochar surfaces can trap NH4+ and prevent their volatilization (Janczak et al., 2017) Biochar enhanced the gaseous NH3 adsorption and as a potential additive Rapid mineralization of total organic matter (TOM) Biochar dramatically reduced the amounts of volatile nitrogen compounds, which are produced by microbial modification of Ncompounds Biochar’s strong sorption capacity may serve as a mechanism for VOCs removal, which is aided by the original biochar’s surface area The biochar’s NH3 mitigation is likely related to creating a semi-porous crust layer over the surface of the manure Biochar could absorb TVOCs based on distribution mechanisms and surface adsorption (Wang et al., 2018) (Awasthi et al., 2016) (S´ anchez-Monedero et al., 2019) (Maurer et al., 2017) (Li et al., 2021) Remarks: PVC: Polyvinyl chloride, VOCs: Volatile organic compounds, TVOCs: Total volatile organic compounds, OVOCs: Oxygenated volatile organic compounds, SOC: and soil organic carbon nitrous oxide (N2O), carbon dioxide (CO2) and methane (CH4) can be emitted due to organic matter degradation (Chowdhury et al., 2014) Adding biochar to composts increases carbon stabilization, which raises the potential benefits of employing co-composting, such as minimizing nutrient losses through leaching and lowering GHG emissions Biochar addition to the co-composting is beneficial approach in the reduction of CH4 emissions, that is due to the better aeration, reduced bulk density, gas diffusion, and creating suitable conditions for methanotrophs can consume CH4 (Vandecasteele et al., 2016) Illustration of the GHG emissions rates measured during the composting process by amending biochar resulted in a clear reduction in CH4 The average emission rates for the whole process of the control and the biochar mixed compost were 8.1 and 1.5 g CH4 m2 d− 1, respectively Biochar addition in the feedstock mixture of green and organic wastes enhance the composting process and shows the different effects before and after adding biochar 10% into the composting piles The removal efficiency of CH4 emissions has been reported with a wide range from 10.8% up to greater than 80% (Agyarko-Mintah et al., 2017b; Chen et al., 2017a; Vandecasteele et al., 2016) Also, Steiner et al (2010) found improved aeration when biochar addition at doses of 5% and 20% reduced emissions by 58% and 71%, respectively He et al (2019b) showed that granular bamboo biochar has a high potential for reducing CO2 emissions The positive effect of additives during co-composting may have significant potential to reduce CO2 emissions In another study, biochar addition led to a reduction of CO2 emissions up to 44% compared to the control (Barthod et al., 2016) This can be explained by the adsorption of organic matters on the biochar surface, and biochar has been used as a co-composting agent to reduce carbon emissions The major mechanism can trap of CO2 during com posting related to biochars addition, leading to decreased CO2 emis sions, with biochar as the alkaline agent In addition, it is seen that the organic matter decomposition during the composting process (bio- oxidative phase) were partially limited by the biochar adsorption for CO2 reduction The average CO2 emission rates for the whole com posting process of the control and the biochar mixed experiments were 401 and 195 g m2 d− 1, respectively (Vandecasteele et al., 2016) M.K Nguyen et al Chemosphere 299 (2022) 134488 Fig Effect of biochar on VOCs and GHGs emissions during composting (Org-N: Organic nitrogen) Chowdhury et al (2014) also reported that the addition of biochar during barley straw with hen manure co-composting reduced by 27–32% total GHGs emissions (CO2 equivalents) compared to barley straw addition alone However, some cases illustrated that biochar might facilitate CO2 emissions due to increasing the temperature factor during composting (Guo et al., 2020) As mentioned above, declined CO2 emission was a result of absorption by biochar properties, while increase in their emission was related to strongly enhanced composting substrate degradation and improved aeration environment during composting Thus, the effect of biochar on CO2 emission should be clarified in further studies (Fan et al., 2008; Steiner et al., 2010, 2011) Meanwhile, nitrous oxide (N2O) is also currently examined as the third most important long-lived GHG (i.e., after CO2 and CH4) ´nchez-García et al (2015) found N2O emissions in both thermophilic Sa and mesophilic phases, which they attributed to the intensive nitrifica tion occurring in these composting piles, where hydroxylamine was Fig Effects of biochar addition on nitrogen transformation during the composting process 10 M.K Nguyen et al Chemosphere 299 (2022) 134488 likely formed as a nitrification intermediary (Maeda et al., 2011) Reduced emission of N2O was commonly confirmed during the com ´pez-Cano et al., posting, which is enhanced with biochar addition (Lo 2016; Vandecasteele et al., 2016) In the case of the average N2O emission levels for the biochar blended compost and the control were 182 and 213 mg N2O m2 d− 1, respectively According to Janczak et al (2017), adding biochar (10%) to poultry manure composting reduced nitrogen loss by 38.3% when compared to the control Furthermore, the effect of added biochar on nitrogen transformation during the com posting process is shown in Fig Biochar’s potential to reduce N2O emissions during composting is dependent on how it affects some pro cesses Wang et al (2013a) illustrated that biochar increased the gene expression of N2O-reductase (nosZ), and the last enzyme in the deni trifier reduction chain from N2O to N2 The nitrogen transformation and N2O reduction mechanism include (i) first, biochar absorbs NO3− , reducing the amount of substrate accessible to denitrifying bacteria; (ii) second, metals bonded to the biochar surface, such as iron and manga nese, aid in the chemical reduction from NO3− to N2; (iii) third, improved aeration aids nitrification while suppressing denitrification; and (iv) finally, denitrifying bacteria of N2O-reductase (nosZ) with high expression through converting N2O to N2 are enriched during the composting process (Antonangelo et al., 2021; Guo et al., 2020) enhancing the structure, reducing contaminant bioavailability, improving the nutrient status and increasing efficiency of composting (humification) and these effects vary with the nature of the biochar (feedstock, pyrolysis, etc.) (Godlewska et al., 2017; Guo et al., 2020; Xiao et al., 2017) Thus, the beneficiation of organic wastes by adding biochar has been investigated in recent years to gain insight into the process Of interest is the effect of different biochar on various organic waste composts (e.g., food waste, green waste, non-hazardous wood waste, biowastes, biomass, manures, etc.) (Al-Gheethi et al., 2021; Godlewska et al., 2017; Steiner et al., 2011; Tran et al., 2021b) and the optimal biochar addition ratio with respect to the composting process (the effect on organic matter), heavy metals availability, and compost nutrient status The ratio of biochar addition examined has generally been between 2% and 20% by weight, and the addition of biochar at different times in the composting process has been investigated (Xiao et al., 2017) 5.1 Heavy metals Whilst biochar may contain small amounts of heavy metals depending on the feedstock, it’s role in reducing the availability of metals is well documented (Godlewska et al., 2017; Guo et al., 2020; Xiao et al., 2017) This role of biochar is therefore highly applicable to the use of organic waste composts due to the tendency of metals to be sorbed and concentrated in organic matter Recent research has affirmed the role of biochar in immobilizing metals during composting These studies have investigated several sources of biochars and composts and the effect on availability of a range of metals Zhou et al (2018) found after composting pig manure with sawdust, wheat straw biochar and humic acid exchangeable Cu, Pb and Zn was reduced between 60 and 95% with addition of 5–7.5% biochar and 2.5% humic acid Owing to biochar being generally alkaline, the pH value in the environment is increased and leading to heavy metal ions could be converted to hy droxide (–OH) and absorbed on the biochar’s surface The incorporation of biochar increased the formation of surface complexes between func tional groups and Pb ions on their surface (Jiang et al., 2012) Liu et al (2017b) reported a range of reduction in metal bioaccessibilities for Pb, Ni, Cu, Zn, As, Cr and Cd (1–59%), with a variable effect of the amendment rate of biochar to compost Analysis with principal component analysis (PCA) identified three principal components influ encing the effect of biochar compost on the bioavailability of heavy metals, with PC1 accounting for 47.1% of the variability, which was mostly related to the passivation of Pb The impact of biochar mixing compost on heavy metal bioavailability illustrated that Cr accounted for the most significant contributions to PC2 (24.75%), and As mainly contributed to the PC3 (20.57%) The three PCs results revealed an understanding of the passivation effect of biochar amendments on the bioavailability related to heavy metals (i.e., Pb, Ni, Cu, Zn, As, Cr and Cd) during the composting process The surface Oxygen-functional groups on biochar could adsorb and immobilize heavy metals based on the mechanisms such as cation and anion metal attraction, ion exchange, reduction, precipitation, electron shuttling, physisorption, and so on (Ahmad et al., 2014; Ding et al., 2014; Zheng et al., 2021) The decreasing heavy metals concentration has been linked to immobilization and accumulation of mobile metal fractions in the biochar pores and organic substances from compost piles Biochar could change the physico-chemical profiles, cation ex ´ndez et al., change capacity, and microbial community activities (Herna 2022; Wu et al., 2017a) Therefore, compost-added biochar can adsorb and immobilize the various heavy metals from organic waste sources The interaction mechanism between biochar and compost stabilizes heavy metals contaminants through adsorption, ion exchange, binding, reduction, co-precipitation, electron shuttling, and physisorption are shown in Fig Studies have also linked the role of biochar in reducing heavy metal availability to organic matter humification and the effect of biochar on 4.3 Other gaseous emissions The properties of biochar have facilitated the adsorption of odorous gaseous emissions (e.g., NH3, H2S) during composting (Antonangelo et al., 2021; Vandecasteele et al., 2016) Biochar can absorb large quantities of NH3/NH4+ on the surfaces areas and biochar pores could be ´ ska led to significantly decreased NH3 emission (Dias et al., 2010; Malin et al., 2014) Most of the experiments demonstrated that the biochars could decrease NH3 emissions in a range from 7% to 98%, with biochar used rates of range from 2% to 28% (Sanchez-Monedero et al., 2018) This capacity can be explained by relation to the internal pore volume, micropores structure, large SSA, the occurrence of several surface acidic – O, amino group, etc.), and also the cation function groups (i.e., OH•, C– exchange process that can reduce the NH3 emissions during composting ´ ska et al., 2014; Steiner (Chowdhury et al., 2014; Khan et al., 2014; Malin et al., 2010) Compared to composting without biochar, composting with biochar produced lower NH3 emissions and more significant NO3− concentra tions (Chen et al., 2010; Mali´ nska et al., 2014) NH4+/NH3 is converted − to NO3 by nitrifying bacteria from nitrification process that was improved due to biochar addition, whereas the reverse pathway (i.e., the ´pez-Cano et al., 2016) Biochar ammonification process) was reduced (Lo addition to composting has been frequently investigated to reduce NH3 ´ ska et al., 2014) Similarly, emissions and prevent nitrogen losses (Malin Maurer et al (2017) used inactive biochar on the surface of pig manure to minimize odor and found that NH3 emissions were reduced by 13–23% Most biochars, on the other hand, are alkaline and have a highly catalytic center dispersion throughout the pore system, making them as excellent ideal for successful H2S oxidation (Agyarko-Mintah et al., 2017b) During chicken litter composting, biochar (5% and 20% w/w) reduced H2S emissions by 58–71%, respectively (Steiner et al., 2010) Also, Vandecasteele et al (2016) has demonstrated that the biochar’s ability to absorb hazardous chemicals (e.g., H2S) aims to reduce their negative effects on organic matter (OM) degradation during composting Furthermore, biochar used during composting can enhance aeration, thus reducing toxic gas (i.e., H2S) emission from the decomposition of OM (e.g., sulfur-containing organic compounds, protein, etc.) (Steiner et al., 2010) Effect of biochar on compost quality and maturity Biochar is known to improve the quality of compost through 11 M.K Nguyen et al Chemosphere 299 (2022) 134488 Fig Interaction between biochar and composting in stabilizing heavy metals (M: Heavy metals) microbial community Cui et al (2020) reported the beneficial effects of biochar in promoting certain bacteria, microbial decomposition contributed to reduction of heavy metal (Pb, Zn, Cd and Cu) bio accessibility by up to 44% using maize straw biochar with swine manure during composting The higher amendment rate (10%) was more effective than a lower amendment rate (5%) In a study which compared application of corn stalk biochar and montmorillonite for reduction of bioavailability of heavy metals in chicken manure, the benefit of biochar to bacterial diversity was linked to the heavy metal fractions and bio accessibility reduction (Hao et al., 2019) Reduction of Cu by up to 90% and 15% for Zn (Hao et al., 2019) Similarly, Song et al (2021) reported that the key factor in reducing the availability of heavy metals during composting chicken manure and rice hull with biochar was the inter action of bacteria and organic components through the transformation of organic matter by heavy metal resistant bacteria (63% and 73% contribution to Cu and Zn bioavailability respectively) Other studies by Awasthi et al (2021) reported the addition of bio char affected the community distribution of metal resistant bacterial Chen et al (2017b) reported reduced metal availability (DTPA extract able) by rice straw biochar addition to sediment and agricultural waste compost and a significant influence on the bacterial community di versity However, the main drivers of change in bacterial community composition were related to pile temperature, C/N ratio, organic matter content, and water soluble carbon rather than DTPA extractable heavy metals on the dynamics and retention of nutrients during composting (Joseph et al., 2018; Lee et al., 2018; Wang et al., 2021) Lee et al (2018) re ported increased soil pH, organic carbon and exchangeable K, reduced loss of ammonium and nitrate from soils (by ~40% relative to control and ~20% relative to compost only) and avoided loss of phosphorus relative to compost only (similar to control) with the application of wood biochar at 4% to green dreg waste compost The addition of biochar to compost plays an important role in ni trogen dynamics Fig demonstrates a comparison between biochar application aims to improve N cycling and reduce N losses during the composting process Biochar addition significantly impacted mineral N dynamics, improved N cycling by increasing NO3− concentrations due to biochar creating a suitable microenvironment for nitrifying bacteria ´pez-Cano et al., 2016) The beneficial activity (Godlewska et al., 2017; Lo effects during the composting has enhanced nitrification with the pres ence of high concentrations of NO3− , leading to decreasing the amount of N2O emissions released compared to without biochar L´ opez-Cano et al (2016) reported an addition of 4% oak wood biochar to composting of olive mill waste with sheep manure aided nitrification by inhibition of ammonification and increased total nitrogen content Liu et al (2017b) reported addition of 0–7% wheat straw biochar to sewage sludge resulted in ammonia loss of 22.4–35.6% of the control and proliferation of nitrifying bacteria increased nitrate substantially (62–310%) In contrast, Plaimart et al (2021) reported 10% coconut husk biochar amended to anaerobic dairy/pig slurry digestate increased nutrient adsorption (N and P) and reduced abundance of nitrifying bacteria and as a result reducing nitrate leaching Ammonia volatilization is also reduced with increasing biochar application rates (Janczak et al., 2017; Mandal et al., 2019) Though a meta-analysis by Sha et al (2019) found that biochar and soil pH were 5.2 Nutrients Due to the properties of biochar (including surface area, pore vol umes, functional groups, CEC) it has been found to play a beneficial role 12 M.K Nguyen et al Chemosphere 299 (2022) 134488 Fig Biochar improved N cycling and reduced N losses during composting (Org-N: Organic nitrogen, AOB: Ammonium oxidizing bacteria, NOB: Nitrite oxidizing bacteria) important considerations as biochar applied to acidic soil could stimu late ammonia volatilization In general, the acidic functional groups of biochar were a factor in reducing NH3 and CH4 volatilization from compost Other studies have also reported reduced nitrogen loss by biochar addition to compost (Hestrin et al., 2020; Qu et al., 2020; Wang et al., 2021) Biochar oxidation was reported as an important factor for reducing loss of NH3 (Hestrin et al., 2020) Qu et al (2020) reported biochar in combination with gypsum reduced both NH3 and CO2 loss A 10% rate of biochar addition was reported as optimal for reducing ni trogen loss with a reduction relative ton control of 15% (Wang et al., 2021) Biochar also plays a role in phosphorus availability with com posting Biochar derived from wheat straw added to pig manure was reported to increase the concentrations of water-soluble nutrients, including PO43− (5.6–7.4%), K+ (14.2–58.6%), and Ca2+ (0–12.5%) and was correlated with biochar amendment rate (at a rate of 10–15%) (Zhang et al., 2016) Conversely, Xu et al (2019) reported biochar decreased the risk of eutrophication by enhancing P retention important role with a large increase in total porosity and decrease in bulk density mainly related to the decomposition of organic matter The degree of humification was enhanced by up to 31% with biochar addition 5.4 Phytotoxicity The application of immature compost to the soil can be toxic and inhibit plant growth This is due to N–NH4+ and hazardous wastes, which inhibit germination and root elongation Commonly, N–NH4+ will increase rapidly in the early stages of composting due to ammoni fication In addition, some hazardous wastes contain active compounds harmful to germination, such as heavy metals, phenols, ethylene com pounds, salts, and organic acids Therefore, to assess the quality of whether the compost is ready for use, several parameters could be considered to track potential phytotoxicity such as temperature, NH4+/ NO3− ratio, C/N ratio, germination capacity, the ratio of humic acids to fulvic acids (HA/FA), SUV254/DOC trend, oxygen-uptake rate (OUR) and CO2 and biochemical composition (Godlewska et al., 2017; Xiao et al., 2017) However, some parameters such as C/N ratios show un certain results corresponding to different types of feedstocks and addi tives (Godlewska et al., 2017; Xiao et al., 2017) To date, the germination index (GI) is by far the most practical parameter to check the phytotoxicity in compost This is because the GIrelated method is reliable when directly related to the seed germination event (Chen et al., 2017b; Liu et al., 2017a; Wei et al., 2014) Moreover, this method can be used to test the degree of phytotoxicity on germi nation caused by salinity, soil pathogens, toxic substances, some other physical and chemical properties of the compost (Selim et al., 2012) According to Zucconi (1981), compost samples at different phase treatments were extracted by shaking fresh samples with distilled water at a ratio of 1:10 w/v for h Then, 10–20 mL of the water sample was applied to filter paper in a Petri dish containing 20 seeds, such as radish or cucumbers (Liu et al., 2017a; Wei et al., 2014) Each Petri dish was incubated at room temperature in the dark for 48–72 h Seed germina tion rate and root length were then estimated and compared with the control condition (distilled water only) The presence of components such as NH3, organic acids, and phenols can reduce GI value (80%), which indicates that the compost is virtually free of phytotoxins (Chen et al., 2017b; Du et al., 2019b) Recent results indicate that biochar in compost can further reduce phytotoxicity By adding biochar, the GI value at the early stage can start at less than 20% and reach 128% at the mature stage (Awasthi et al., 2020; Chen et al., 2017b; Wei et al., 2014) The addition of biochar (10%) to compost (i.e., chicken manure and peanut straw) helped to achieve a GI of 97% vs 92% as control (without biochar) (Li et al., 2022) Similarly, the addition of bamboo biochar (3.8%) to the compost (i,e, sewage sludge and rice straw) achieved a GI of 116% vs 101% as control (Xue et al., 2021) The proposed mechanism is that the added biochar can reduce N–NH4+ through high nitrification activity, while the reduction of heavy metals such as Cu, Zn may be due to the complexation of metals of this type with chelating organic compounds (Selim et al., 2012) Overall, the difference in feedstocks and biochar dosage is considered the main factor affecting the final GI value during composting With biochar, the richness and diversity of the mi crobial community were observed to improve, leading to enhanced microbial activities, thereby promoting nutrient contents and humifi cation during composting To achieve the maximum efficiency for bio char application during composting, some of the commonly referenced ranges should be considered such as pH (6.5–7.5), moisture content (50–60%), initial C/N ratio (20–25:1), biochar doses (1–20% w/w) aim to improve oxygen content availability, enhancing the performance and humification, accelerating organic matter decomposition and growth of the microbial community Also, need to select the biochar materials that – O, contain significantly functional groups, e.g., Si–O–Si, OH, COOH, C– C–O, N for enhancing remediation mechanisms such as adsorption, distribution, ion exchange, π− π interaction, etc Our review has pro vided current knowledge on the role of biochar during the organic waste composting process; however, many aspects are still ambiguous in this area Therefore, we highly recommended these knowledge gaps following for further research: • Many studies have reported on the effectiveness of biochar addition to the composting process However, the effect of biochar properties on composting is still not fully understood Biochar types (feedstock, pyrolysis conditions), dosage, and particle size need to be further studied to promote efficiency and improve the quality of the end product • The effect of biochar on the nitrogen cycle has been evaluated through biochar increasing the activity of nitrifying bacteria, reducing methanogens However, how it affects to carbon cycle needs to be investigated to clarify the role and function of microbial community and biomass balance during the composting process • The interaction of specific microorganisms (bacteria and fungi) with biochar and its correlation with composting performance (e.g., biodegradation, VOCs and odors emission, maturity, and nutrients) should be investigated to fill up the research gaps in this area • Biochar addition to the composting process has been successfully applied at lab-scale with short period through a number of studies However, field scale applications should be conducted to evaluate the efficiency and optimal conditions Furthermore, long-term studies need to be focused on providing an in-depth understanding of its effects on biodegradation during the composting period • Porous biochar (mesopores, and macropores) will be dramatically statured by organic matter of compost mixture, leading to reduced performance and its efficiency Further studies should be conducted to address this matter to extend the life cycle of biochar • Heavy metal contaminants have adverse effects on the microbial community of compost mixture Biochar addition to composting can reduce the availability of heavy metals; however, heavy metal inactivation mechanisms should be clarified to elucidate this concern 5.5 Enzymatic activity There are many enzymes involved in catalyzing the reactions of the composting process, such as Amylase, Aryl-sulfatase, Beta-glucosidase, Cellulase, Dehydrogenases, Protease, Phosphatase, Peroxidase, Xylanase, Urease, etc (Awasthi et al., 2020; Gong et al., 2021; Sun et al., 2016) Cellulase, Protease, and Phosphatase are essential enzymes that act as catalysts to disintegrate cellulose, proteins, and phosphorous, respec tively Besides, Dehydrogenases are intracellular enzymes that catalyze the respiration of organic compounds It was reported that the Dehy drogenase and Xylanase activities were enhanced with a higher added biochar dosage (10%) during the composting of poultry manure (Awasthi et al., 2020) These results were attributed to the proliferation of enzyme-producing microorganisms thanks to biochar increased porosity, surface area, nutrients, and temperature However, mixed poultry manure, straw, and biochar can reduce Beta-glucosidase and Phosphatase activity at mesophilic and thermophilic phases (Sun et al., 2016) According to this study, the use of rice straw biochar (2%) did not promote enzyme activity because the small particle size (
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