# Biochar promotes methane production during anaerobic digestion of organic waste

Leilei Xiao, Eric Lichtfouse, P. Senthil Kumar, Quan Wang, Fanghua Liu

► To cite this version:

Leilei Xiao, Eric Lichtfouse, P. Senthil Kumar, Quan Wang, Fanghua Liu. Biochar promotes methane production during anaerobic digestion of organic waste. Environmental Chemistry Letters, 2021, 19, pp.3557-3564. 10.1007/s10311-021-01251-6. hal-03324513

HAL Id: hal-03324513

https://hal.inrae.fr/hal-03324513

Submitted on 7 Sep 2021

HAL is a multi-disciplinary open access
archive for the deposit and dissemination of sci-
entific research documents, whether they are pub-
lished or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.

L'archive ouverte pluridisciplinaire HAL, est
destinée au dépôt et à la diffusion de documents
scientifiques de niveau recherche, publiés ou non,
émanant des établissements d'enseignement et de
recherche français ou étrangers, des laboratoires
publics ou privés.

<figure><img src="image_1.png" /></figure>

Distributed under a Creative Commons Attribution 4.0 International License
# Biochar promotes methane production during anaerobic digestion of organic waste

Leilei Xiao<fnref n="1" />,<fnref n="2" />✉ · Eric Lichtfouse<fnref n="3" />,<fnref n="4" />✉ · P. Senthil Kumar<fnref n="5" /> · Quan Wang<fnref n="1" /> · Fanghua Liu<fnref n="1" />,<fnref n="2" />

## Abstract

Climate change and energy demand are calling for more sustainable fuels such as biomethane produced by anaerobic digestion of organic waste. Biochar addition to waste is presumed to enhance the efficiency of methane production, yet individual reports disclose contradictory results. Therefore, we performed a meta-analysis of 27 selected publications containing 156 paired measurements of control and biochar-amended treatments to assess the impact of biochar on the methanogenic performance. Results show that biochar promotes biomethane production substantially with a high Hedge's *d* value of 5.7 ± 1.04, yet sporadic publications report a methane decline. Methanogenic performance is statistically controlled by feedstock type, pyrolysis temperature and biochar concentration, but not controlled by pH, size, surface area and methanogen species. These findings should help to tune the parameters of anaerobic digestion with biochar to optimize biomethane productions. Moreover, our results cast some doubt on the efficiency of adding biochar to soil to sequester carbon in soils because biochar promotes methane generation and, in turn, emissions of methane, a greenhouse gas, to the atmosphere.

**Keywords** Anaerobic digestion · Biochar · Methane · Meta-analysis · Wastewater treatment

## Introduction

Global warming and the rising energy demand are calling for more circular processes where waste is recycled into materials and energy. Biomethane is a carbon-neutral, sustainable fuel produced by anaerobic fermentation of organic matter in natural and anthropic environments, yet the efficiency of actual processes is limited (Chen et al. 2018; Gao et al. 2020; Garcia-Mancha et al. 2017). Strategies have been recently developed to improve anaerobic fermentation by microbial immobilization, pH buffering and enzymatic induction (Gao et al. 2020; Xiao et al. 2020a). Anaerobic degradation and biomethane production are also promoted by electromethanogenesis using electroactive microorganisms and conductive materials such as biochar (Fig. 1; Li et al. 2018; Xiao et al. 2020b; Yuan et al. 2018). Recent research has also focused on the use of nanomaterials to favor methanogenesis (Ma et al. 2020; Xiao et al. 2018, 2019c).

Biochar is carbon negative and comprises a wide variety of complex materials produced by pyrolysis of biomass (Glaser et al. 2009; Gunarathne et al. 2019; Akhil et al. 2021; Fawzy et al. 2021). Biochar has been applied to reduce nutrient leaching from soils, to recover resources from water, to accelerate waste disposal and for biomethane production

<footnote>✉ Leilei Xiao
llxiao@yic.ac.cn</footnote>

<footnote>✉ Eric Lichtfouse
eric.lichtfouse@gmail.com</footnote>

<footnote n="1">¹ Key Laboratory of Coastal Biology and Biological Resources Utilization, Yantai Institute of Coastal Zone Research, Chinese Academy of Sciences, Yantai 264003, People's Republic of China</footnote>

<footnote n="2">² CAS Key Laboratory of Coastal Environmental Processes and Ecological Remediation, Yantai Institute of Coastal Zone Research, Chinese Academy of Sciences, Yantai 264003, People's Republic of China</footnote>

<footnote n="3">³ Aix-Marseille Univ, CNRS, IRD, INRAE, Coll France, CEREGE, Avenue Louis Philibert, 13100 Aix en Provence, France</footnote>

<footnote n="4">⁴ State Key Laboratory of Multiphase Flow in Power Engineering, Xi'an Jiaotong University, Xi'an 710049, Shaanxi, People's Republic of China</footnote>

<footnote n="5">⁵ Department of Chemical Engineering, Sri Sivasubramaniya Nadar College of Engineering, Chennai 603110, India</footnote>
<figure><img src="image_2.png" /><figcaption>**Fig. 1** Transformation of organic waste by anaerobic fermentation is promoted by biochar addition</figcaption></figure>

(Lorenz and Lal 2014; Fagbohungbe et al. 2017; Masebinu et al. 2019; Qiu et al. 2019; Yang et al. 2020). Biochar properties and molecular composition vary widely with the nature of the feedstock and pyrolysis conditions (Keiluweit et al. 2010; Gao et al. 2020). Several biochar properties have been proposed to favor biomethane production, e.g., microbial immobilization, pH buffering, and controlling metal ion availability and enzymatic processes (Yuan et al. 2018; Xiao et al. 2019b; Gao et al. 2020; Huang et al. 2020b; He et al. 2020). Overall, mechanisms fostering methanation by biochar are better understood but individual studies report sometimes contradictory results, e.g., rising or declining biomethane production (Cheng et al. 2018; Luo et al. 2015; Shen et al. 2016). Therefore, we report here a meta-analysis to clarify the impact of biochar properties on methanogenesis during anaerobic digestion of environmental waste.

# Experimental

## Biochar data

We found 105 publications in the Web of Science and Bing search engine for documents on biochar application to methane production during anaerobic digestion (AD) for treating environmental waste and pollution, excluding soil-related research, from January 1 2010 to June 15 2020, using the keywords “biochar” AND “methane” OR “CH₄.” We extracted the following variables: feedstock, pyrolysis temperature, pH, size, surface area, conductivity, methanogen species and methanogenic performance. Variable means, standard deviations and sample replicate number were

extracted from publication tables and text. When data were only reported in image format in graphs, data points were extracted using Plot Digitizer 2.6.8 and Web Plot Digitizer. When relevant data were not present in publications, corresponding authors were contacted to get the data. We first considered the highest rate of methane production, but, if not available, we used the highest yield of methane. When accurate maximum rate of methane production or yield could not be obtained due to too much fluctuation of methane concentrations, publications were excluded from this study.

From this initial pool, we selected only documents reporting three or more replicates for each run, and we found 19 publications containing 105 data pairs of treatment data versus control data (Table S1). Control is defined as runs without biochar. We used the Hedges method, rather than the response ratio, because the Hedges method is adapted to data samples of relatively small size (Jeffery et al. 2016; Larry and Ingram 1985). Therefore, a minimum of two replicates is meeting the analysis standard. Consequently, data on biomethane production were collected from 27 articles containing 156 paired measurements of control and biochar-amended treatments for disposal of environmental waste and pollution.

Biochar variables were grouped to facilitate cross-comparisons, e.g., the nature of biochar feedstock was grouped in ‘wood and sawdust,’ ‘herbaceous and lignocellulosic waste,’ ‘manure,’ and ‘sludges’ (Table S1). Similarly, pyrolysis temperatures were grouped in ‘below 500 °C,’ ‘500–700 °C,’ and ‘above 700 °C.’ Conductivities were grouped in ‘below 450 µS/cm’ and ‘above 450 µS/cm.’ Biochar pH was grouped into ‘acidic below 7,’ ‘weakly alkaline from 7 to 9,’ and ‘alkaline above 9.’ Sizes were grouped in ‘below 1 mm’
and 'above 1 mm.' Brunauer, Emmett and Teller (BET) surface areas were grouped in 'below 100 m² g⁻¹' and 'above 100 m² g⁻¹.' Biochar concentrations were grouped in 'below 10 g dm⁻³,' 'equal to 10 g dm⁻³' and 'above 10 g dm⁻³.' Two types of methanogenic archaea were distinguished: acetoclastic methanogens and hydrogenotrophic methanogens.

## Meta-analysis

We used the standardized mean difference metric Hedge's *d* in Eq. 1, which induces less biases that the Hedge's *g* factor in Eq. 2 (Larry and Ingram 1985):

$$d = \left( 1 - \frac{3}{4(n-2) - 1} \right) g \qquad (1)$$

$$g = \frac{X_1 - X_2}{S_p} \qquad (2)$$

$$S_p = \sqrt{\frac{(n_1 - 1)s_1^2 + (n_2 - 1)s_2^2}{(n_1 - 1) + (n_2 - 1)}} \qquad (3)$$

where *n* denotes the total sample size, and $\bar{x}_1$ and $\bar{x}_2$ depict the means of experimental and control treatments. Experimental data refer to the treatment with biochar, whereas control data refer to the treatment without biochar. A categorical random effect model was applied to *d*, with means weighted by the inverse of the variance. Here, $S_p$ is the pooled standard deviation in Eq. 3, where $n_1$ and $n_2$ are the number of repetitions in the control and experimental groups, and $s_1$ and $s_2$ depict the standard deviations of control and experimental groups.

Contrary to the response ratio commonly used in ecological research, the standardized mean effect sizes are probabilistic (Hedges et al. 1999; Larry and Ingram 1985). That is, the mean effect sizes describe the probability that a sample would fall between the experimental mean and the control mean, assuming a normal distribution (Hedges et al. 1999). Consequently, confidence intervals of 95% were generated based on a normal distribution. When the 95% confidence interval of the parameter does not overlap with Hedge's *d* of 0, this implies that the variable promotes biomethane production, which suggests the promotion of anaerobic digestion of environmental waste. When the 95% confidence interval of a biochar parameter does not overlap with that of another variable, there is a statistically significant difference.

By convention, for variables that do not overlap with the Hedge's *g* of 0, a *d* value higher than 0.8 indicates a large effect, *d* of 0.2–0.8 shows a moderate effect, and *d* of 0.0–0.2 displays a small effect (Hedges et al. 1999; Jeffery et al. 2016). A key point is that, using the Hedge's *d* metric,

an effect size of a variable analysis does not equate to an effect size of others in independent analyses presented in this study. As a consequence, only categories within individual analyses, e.g., feedstocks, as differentiated by the horizontal dotted bars, can be compared. The effect sizes do not mean that the extent of the actual biomethane production increase or decrease. Small effect sizes may indicate significant value in biomethane production in absolute terms. For instance, in small effect sizes, the actual biomethane parameter may be several times larger than that of the large effect sizes.

## Results and discussion

### Overall effect of biochar addition

We assessed the global effect of biochar addition on anaerobic methanogenesis by calculating the grand mean of the Hedge's *d* for 156 published data pairs of treatment versus control without biochar (Fig. 2). Results show a *d* value of 5.70 ± 1.04, which evidences a large effect size and implies that the presence of biochar statistically induces an increase in biomethane in most investigations. Yet sporadic studies have also shown the inhibitory effect of biochar or no effect (Cheng et al. 2018; Shen et al. 2016). This discrepancy is probably due to the high heterogeneous nature of biochar (Diao et al. 2020; Gao and Goldfarb 2019), suggesting that biomethane production may be enhanced by specific biochar properties, as discussed below.

### Effect of biochar feedstock

We calculated *d* values of feedstock including sludges, manure, herbaceous and lignocellulosic waste, and wood and sawdust (Fig. 2). All feedstock types show high *d* values from 4.71 ± 1.72 to 7.99 ± 1.51, implying that biochar addition improves biomethane generation whatever the type of feedstock. Furthermore, there is no statistical difference within feedstock types, sludges displaying the highest *d* of 7.99. Manure, plant waste and woody materials appear equally competitive with Hedge's *d* values around 5.0. High *d* values for sludges are supported by the fact that sludge biochar provides more nutrients for fermentative bacteria and methanogens (Wang et al. 2020). Moreover, biochar from sludge has also induced better pollutant removal and heavy metal adsorption (Diao et al. 2020; Regkouzas and Diama-dopoulos 2019; Singh et al. 2020), which may be explained by a more favorable living environment for microorganisms. Overall, the slight advantage of sludge biochar in terms of methanogenesis is likely due to its ability to adsorb and store nutrients for activating methanogens. We conclude that biochar improves methanogenesis for all biochar feedstocks, but there is no statistical advantage of the feedstock type.
<figure><img src="image_3.png" /><figcaption>**Fig. 2** Forest plot of Hedge's *d* calculated from published literature (Table S1). Top: grouping by feedstock, pyrolysis temperature and conductivity of biochar. Bottom: grouping by biochar concentration, size, BET surface area and pH. Points show means, bars show 95% confidence intervals. The numbers in parentheses indicate the number of pairwise comparisons of treatment with biochar versus control without biochar. BET: Brunauer, Emmett and Teller</figcaption></figure>

## Effect of pyrolysis temperature

We calculated *d* values of biochar produced by pyrolysis below 500 °C, of 6.72 ± 1.86, between 500 and 700 °C, of 6.40 ± 1.11, and above 700 °C, of 0.840 ± 1.50 (Fig. 2). Results imply that biochar favors biomethane generation below 700 °C. There is no significant difference between 500 and 700 °C-produced biochar and biochar produced below 500 °C. On the other hand, pyrolysis above 700 °C induces a drastic decline of biomethane promotion. These findings may be explained by changes of the biochar molecular

structure with temperature (Hao et al. 2018). Indeed, Keilweit et al. (2010) observed a gradual change in the molecular structure of plant biomass-derived biochar with temperature.

High-temperature biochar is characterized by fewer labile compounds at the surface of biochar particles, and therefore less microbial substrates for fermentative bacteria and methanogenic archaea (Bruun et al. 2011). This explanation is strengthened by the declining CH₄ and N₂O emissions from soils amended with high-temperature biochar, which are thus better suited for mitigation of greenhouse gas emissions (Cayuela et al. 2015). This scenario is also supported
by the biochar release of less degradable organic compounds when the pyrolysis temperature increases (Ji et al. 2020). By contrast, slow pyrolysis at low temperature yields more biochar with diverse chemical groups (Chen et al. 2019; Sohi et al. 2010), which are likely to promote methane production in anaerobic digesters, or methane emissions from soils (Jeffery et al. 2016). Overall, our findings show that biochar produced below 700 °C improves methanogenesis. Pyrolysis at lower temperature is also saving energy.

## Effect of biochar conductivity

Biochars having low conductivity, below 450 µS/cm, show a much higher *d* value, of 7.58 ± 1.79, than high-conductivity biochar, displaying a *d* value of 2.06 ± 1.29 (Fig. 2). Low conductivity biochar is therefore statistically more effective at accelerating biomethane production. This finding is unexpected because recent research suggests that biochar acts as an electron shuttle, which should favor microbial activity (Viggi et al. 2017; Xiao et al. 2019b; Yuan et al. 2018). Nonetheless, a recent report explains that electrical conductivity of biochar is controlling only the rate of anaerobic degradation, not the yield of biogas (Rasapoor et al. 2020). Moreover, conductivity does not appear as a relevant factor for choosing which biochar should be used for degrading environmental waste (Lu et al. 2020a), and some studies suggest that attributing rising biomethane production to high material conductivity requires caution (Martins et al. 2018; Van Steendam et al. 2019; Wang et al. 2021). Overall, our findings show that low conductivity biochar favors methanogenesis, yet underlying mechanisms are unclear.

## Effect of biochar pH

Figure 2 displays the effect of biochar of different pH on biomethane production. Results show that varying the biochar pH induces no statistical difference in biomethane production, despite the fact that pH is known to modify fermentation rates (Begum et al. 2018; Feng et al. 2020; Mao et al. 2017). Yet, most investigations included in this meta-analysis did not report the pH of the system before and after biochar application, though pH is expected to vary widely because some biochar contains oxygen-containing organic anions and carbonates that increase alkalinity (Fidel et al. 2017; Yuan et al. 2011; Meng et al. 2020). Overall, varying the pH of biochar does not statistically improve methanogenesis.

## Effect of surface area and biochar size

Values of *d* for biochar with BET surface area above 100 m²/g, of 5.06 ± 1.83, are not statistically different from those of biochar with surface area below 100 m²/g,

of 4.45 ± 1.06 (Fig. 2). Similarly, the size to biochar particles does not appear to modify biomethane generation, yet a trend for higher *d* value is observed for particle size below 1 mm. This implies that smaller particles of biochar may be beneficial to the degradation of environmental waste. For instance, the addition of powdered biochar to a pig manure/wheat straw aerobic compost increased biomethane emissions by 57%, whereas granular biochar decreased biomethane emissions by 22% (He et al. 2018). On the contrary, other investigations have shown that large biochar particles promote methanogenesis (Cheng et al. 2018; Viggi et al. 2017). Overall, there is no clear global effect of surface area and size on anaerobic degradation of waste and pollutant and on biomethane production.

## Effect of biochar concentration

Biochar concentration caused a strong and statistically significant difference in the strength of biomethane production, with a maximal impact for concentrations exceeding 10 g/L and a *d* value of 7.87 ± 0.35 (Fig. 2). Increasing biochar concentration is therefore an efficient means to improve methanogenesis, which may further result in a promotion of waste degradation. This finding is supported by biochar properties that are likely to stabilize anaerobic digestion and rise biomethane yield (Gao et al. 2020; Lim et al. 2020; Ma et al. 2021). For instance, providing immobilization sites for microorganisms could explain the higher anaerobic degradation and methanogenic performance (Zhang et al. 2018).

Moreover, even though biochar itself is not a substantial source of labile carbon, biochar is a sponge-like material able to adsorb and store organo-mineral nutrients for further microbial feeding (Cross and Sohi 2011; Demisie et al. 2014). In this line, elevated biochar concentrations have been shown to increase the availability of organic carbon for fermentation bacteria and methanogenic archaea (Lu et al. 2020b; Jiang et al. 2020; Zhang et al. 2020). Based on this, environmental waste and pollution can be degraded more easily, which in turn is more conducive to biological activities (Xiao et al. 2021a; b). Overall, high biochar concentrations foster methanogenesis, yet underlying mechanisms remain undeciphered.

## Methanogenic species

Values of *d* for acetoclastic methanogens, of 5.19 ± 2.06, and hydrogenotrophic methanogens, of 3.08 ± 1.4, are not statistically different, implying a similar contribution of these species to biomethane production (Fig. 3). These high *d* values also reveal that both acetoclastic and hydrogenotrophic methanogens produce more biomethane following biochar addition. This finding is strengthened by an investigation revealing that *Methanosarcina*, *Methanosaeta* and
<figure><img src="image_4.png" /><figcaption>**Fig. 3** Forest plot of Hedge's *d* calculated from published data grouped by 'Methanogens with the highest abundance' and 'Methanogens with the highest increase in abundance.' Points show means, bars show 95% confidence intervals. The numbers in parentheses indicate the number of pairwise comparisons on which the statistic is based. 'Methanogens with the highest abundance' means the most abundant methanogens in samples. "Methanogens with the highest increase in abundance' means methanogens showing the highest changes in abundance</figcaption></figure>

*Methanobacterium* methanogens predominate in paddy soil-amended biochar during the anaerobic decomposition of rice straw (Huang et al. 2020a). Trophic methanogens, hydrogenotrophic and acetoclastic methanogens may actively participate in the methane production process. Indeed, reports have shown that methanogens that use acetate and hydrogen as substrates coexist in the anaerobic fermentation system (Madigou et al. 2019; Zhang et al. 2019). Compared to hydrogenotrophic methanogens, acetoclastic methanogens should contribute more to methane production with sufficient organic substrates (Garcia-Mancha et al. 2017; Lim et al 2020; Xiao et al. 2019a). Overall, biochar addition improves biomethane production by methanogens, yet acetoclastic and hydrogenotrophic methanogens display similar performances.

## Conclusion

Our findings show that, on the average, biochar addition is favoring biomethane generation, whereas this was not clear in previous individual reports. Our identification of biochar properties that favor or do not favor methanogenesis will be helpful for basic research to decipher underlying mechanisms, and for applied research to improve biomethane production as a sustainable fuel and benefit perfection of environmental waste and pollution control measures. Last, the fact that biochar globally promotes biomethane generation in anaerobic media is casting some doubt on the use of biochar to sequester carbon in soils. Indeed, our findings suggest that soils amended with biochar may accelerate methane emissions in the atmosphere, notably in anaerobic soils where fermentation of organic matter and pollution takes place, thus counteracting the sequestrating effect of biochar.

**Supplementary Information** The online version contains supplementary material available at https://doi.org/10.1007/s10311-021-01251-6.

**Acknowledgements** This research was financially supported by the National Natural Science Foundation of China (no. 42077025) and Youth Innovation Promotion Association, CAS (2021213). We thank the editors and reviewers for helpful comments.

**Author contributions** LX and EL designed the research. LX and QW collected the data. LX, EL, SK, FL analyzed the data. LX and EL wrote the article.

## Declarations

**Conflict of interest** Authors declare no competing financial interest.

## References

Akhil D, Lakshmi D, Kartik A, Vo DVN, Arun J, Gopinath K (2021) Production, characterization, activation and environmental applications of engineered biochar: a review. *Environ Chem Lett.* https://doi.org/10.1007/s10311-020-01167-7

Begum S, Anupoju G, Sridhar S, Bhargava S, Jegatheesan V, Eshtiaghi N (2018) Evaluation of single and two stage anaerobic digestion of landfill leachate: effect of pH and initial organic loading rate on volatile fatty acid (VFA) and biogas production. *Bioresour Technol* 251:364–373

Bruun E, Hauggaard-Nielsen H, Ibrahim N, Egsgaard H, Ambus P, Jensen P, Dam-Johansen K (2011) Influence of fast pyrolysis temperature on biochar labile fraction and short-term carbon loss in a loamy soil. *Biomass Bioenergy* 35:1182–1189

Cayuela M, Jeffery S, van Zwieten L (2015) The molar H: corg ratio of biochar is a key factor in mitigating N₂O emissions from soil. *Agr Ecosyst Environ* 202:135–138

Chen R, Jiang H, Li Y (2018) Caffeine degradation by methanogenesis: efficiency in anaerobic membrane bioreactor and analysis of kinetic behavior. *Chem Eng J* 334:444–452

Chen W, Wei R, Yang L, Yang Y, Li G, Ni J (2019) Characteristics of wood-derived biochars produced at different temperatures
before and after deashing: their different potential advantages in environmental applications. Sci Total Environ 651:2762-2771

Cheng Q, Call D, Reyes de los F (2018) Amending anaerobic bio-reactors with pyrogenic carbonaceous materials: the influence of material properties on methane generation. Environ Sci-Wat Res 4:1794-1806

Cross A, Sohi S (2011) The priming potential of biochar products in relation to labile carbon contents and soil organic matter status. Soil Biol Biochem 43:2127-2134

Demisie W, Liu Z, Zhang M (2014) Effect of biochar on carbon fractions and enzyme activity of red soil. Catena 121:214-221

Diao Z, Dong F, Yan L, Chen Z, Qian W, Kong L, Zhang Z, Zhang T, Tao X, Du J, Jiang D, Chu W (2020) Synergistic oxidation of bisphenol A in a heterogeneous ultrasound-enhanced sludge biochar catalyst/persulfate process: reactivity and mechanism. J Hazard Mater 384:121385

Fagbohungbe M, Herbert B, Hurst L, Ibeto C, Li H, Usmani S, Semple K (2017) The challenges of anaerobic digestion and the role of biochar in optimizing anaerobic digestion. Waste Manage 61:236-249

Fawzy S, Osman A, Yang H, Doran J, Rooney D (2021) Industrial biochar systems for atmospheric carbon removal: a review. Environ Chem Lett. https://doi.org/10.1007/s10311-021-01210-1

Feng K, Wang Q, Li H, Zhang Y, Deng Z, Liu J, Du X (2020) Effect of fermentation type regulation using alkaline addition on two-phase anaerobic digestion of food waste at different organic load rates. Renew Energ 154:385-393

Fidel R, Laird D, Thompson M, Lawrinenko M (2017) Characterization and quantification of biochar alkalinity. Chemosphere 167:367-373

Gao L, Goldfarb J (2019) Heterogeneous biochars from agriculture residues and coal fly ash for the removal of heavy metals from coking wastewater. RSC Adv 9:16018-16027

Gao M, Zhang L, Liu Y (2020) High-loading food waste and black-water anaerobic co-digestion: maximizing bioenergy recovery. Chem Eng J 394:124911

Garcia-Mancha N, Monsalvo V, Puyol D, Rodriguez J, Mohedano A (2017) Enhanced anaerobic degradability of highly polluted pesticides-bearing wastewater under thermophilic conditions. J Hazard Mater 339:320-329

Glaser B, Parr M, Braun C, Kopolo G (2009) Biochar is carbon negative. Nature Geosci 2:2

Gunalathne V, Ashiq A, Ramanayaka S, Wijekoon P, Vithanage M (2019) Biochar from municipal solid waste for resource recovery and pollution remediation. Environ Chem Lett 17:1225-1235

Hao S, Zhu X, Liu Y, Qian F, Fang Z, Shi Q, Zhang S, Chen J, Ren Z (2018) Production temperature effects on the structure of hydrochar-derived dissolved organic matter and associated toxicity. Environ Sci Technol 52:7486-7495

He X, Yin H, Sun X, Han L, Huang G (2018) Effect of different particle-size biochar on methane emissions during pig manure/wheat straw aerobic composting: insights into pore characterization and microbial mechanisms. Bioresour Technol 268:633-637

He P, Zhang H, Duan H, Shao L, Lu F (2020) Continuity of biochar-associated biofilm in anaerobic digestion. Chem Eng J 390:124605

Hedges L, Gurevitch J, Curtis P (1999) The meta-analysis of response ratios in experimental ecology. Ecology 80:1150-1156

Huang J, Ma K, Xia X, Gao K, Lu Y (2020a) Biochar and magnetite promote methanogenesis during anaerobic decomposition of rice straw. Soil Biol Biochem 143:107740

Huang W, Lee D, Huang C (2020b) Modification on biochars for applications: a research update. Bioresour Technol 319(4):124100

Jeffery S, Verheijen F, Kammann C, Abalos D (2016) Biochar effects on methane emissions from soils: a meta-analysis. Soil Biol Biochem 101:251-258

Ji M, Zhou L, Zhang S, Luo G, Sang W (2020) Effects of biochar on methane emission from paddy soil: focusing on dom and microbial communities. Sci Total Environ 743:140725

Jiang Q, Chen Y, Yu S, Zhu R, Zhong C, Zhou H, Gu L, He Q (2020) Effects of citrus peel biochar on anaerobic co-digestion of food waste and sewage sludge and its direct interspecies electron transfer pathway study. Chem Eng J 398:125643

Keiluweit M, Nico P, Johnson M, Kleber M (2010) Dynamic molecular structure of plant biomass-derived black carbon (biochar). Environ Sci Technol 44:1247-1253

Larry H, Ingram O (1985). Statistical Methods for Meta-analysis. In Academic Press, Elsevier: New York 20: 369. https://www.elsevier.com/books/statistical-methods-for-meta-analysis/hedges/978-0-08-057065-5

Li J, Xiao L, Zheng S, Zhang Y, Luo M, Tong C, Xu H, Tan Y, Liu J, Wang O, Liu F (2018) A new insight into the strategy for methane production affected by conductive carbon cloth in wetland soil: beneficial to acetoclastic methanogenesis instead of CO2 reduction. Sci Total Environ 643:1024-1030

Lim E, Tian H, Chen Y, Ni K, Zhang J, Tong Y (2020) Methanogenic pathway and microbial succession during start-up and stabilization of thermophilic food waste anaerobic digestion with biochar. Bioresour Technol 314:123751

Lorentz K, Lal R (2014) Biochar application to soil for climate change mitigation by soil organic carbon sequestration. J Plant Nutr Soil Sci 177:651-670

Lu C, Shen Y, Li C, Zhu N, Yuan H (2020a) Redox-active biochar and conductive graphite stimulate methanogenic metabolism in anaerobic digestion of waste activated sludge: beyond the direct interspecies electron transfer. ACS Sustain Chem Eng 8(33):12626-12636

Lu J, Chen C, Huang C, Zhuang H, Lee D (2020b) Dark fermentation production of volatile fatty acids from glucose with biochar amended biological consortium. Bioresour Technol 303:122921

Luo C, Lu F, Shao L, He P (2015) Application of eco-compatible biochar in anaerobic digestion to relieve acid stress and promote the selective colonization of functional microbes (vol 68, pg 710, 2014). Water Res 70:496-496

Ma W, Li H, Zhang W, Shen C, Wang L, Li Y, Li Q, Wang Y (2020) TiO₂ nanoparticles accelerate methanogenesis in mangrove wetlands sediment. Sci Total Environ 713:136602

Ma J, Chen F, Xue S, Pan J, Khoshnevisan B, Yang Y, Liu H, Qiu L (2021) Improving anaerobic digestion of chicken manure under optimized biochar supplementation strategies. Bioresour Technol 325(1):124697

Madigou C, Le Cao K, Bureau C, Mazeas L, Dejean S, Chapleur O (2019) Ecological consequences of abrupt temperature changes in anaerobic digesters. Chem Eng J 361:266-277

Mao C, Zhang T, Wang X, Feng Y, Ren G, Yang G (2017) Process performance and methane production optimizing of anaerobic co-digestion of swine manure and corn straw. Sci Rep-Uk 7:9379

Martins G, Salvador A, Pereira L, Alves M (2018) Methane production and conductive materials: a critical review. Environ Sci Technol 2:10241-10253

Masebinu S, Akinlabi E, Muzenda E, Aboyade A (2019) A review of biochar properties and their roles in mitigating challenges with anaerobic digestion. Renew Sust Energ Rev 103:291-307

Meng L, Xie L, Suenaga T, Riya S, Terada A, Hosomi M (2020) Eco-compatible biochar mitigates volatile fatty acids stress in high load thermophilic solid-state anaerobic reactors agricultural treating waste. Bioresour Technol 309:123366

Qiu L, Deng Y, Wang F, Davaritouchae M, Yao Y (2019) A review on biochar-mediated anaerobic digestion with enhanced methane recovery. Renew Sust Energ Rev 115:109373

Rasapoor M, Young B, Asadov A, Brar R, Sarmah A, Zhuang W, Baroutian S (2020) Effects of biochar and activated carbon on
biogas generation: a thermogravimetric and chemical analysis approach. *Energ Convers Manag* 203:112221

Regkouzas P, Diamadopoulos E (2019) Adsorption of selected organic micro-pollutants on sewage sludge biochar. *Chemosphere* 224:840–851

Shen Y, Linville J, Ignacio-de Leon P, Schoene R, Urgun-Demirtas M (2016) Towards a sustainable paradigm of waste-to-energy process: enhanced anaerobic digestion of sludge with woody biochar. *J Clean Prod* 135:1054–1064

Singh S, Kumar V, Dhanjal D, Datta S, Bhatia D, Dhiman J, Samuel J, Prasad R, Singh J (2020) A sustainable paradigm of sewage sludge biochar: valorization, opportunities, challenges and future prospects. *J Clean Prod* 269:122259

Sohi S, Krull E, Lopez-Capel E, Bol R (2010) A review of biochar and its use and function in soil. *Adv Agron* 105:47–82

Tang S, Wang Z, Liu Z, Zhang Y, Si B (2020) The role of biochar to enhance anaerobic digestion: a review. *J Renew Mater* 8:1033–1052

Van Steendam C, Smets I, Skerlos S, Raskin L (2019) Improving anaerobic digestion via direct interspecies electron transfer requires development of suitable characterization methods. *Curr Opin Biotechnol* 57:183–190

Viggi C, Simonetti S, Palma E, Pagliaccia P, Braguglia C, Fazi S, Baronti S, Navarra M, Pettiti I, Koch C, Harnisch F, Aulenta F (2017) Enhancing methane production from food waste fermentate using biochar: the added value of electrochemical testing in pre-selecting the most effective type of biochar. *Biotechnol Biofuels* 10:303

Wang W, Lee D (2021) Direct interspecies electron transfer mechanism in enhanced methanogenesis: a mini-review. *Bioresour Technol* 330:124980

Wang H, Xiao K, Yang J, Yu Z, Yu W, Xu Q, Wu Q, Liang S, Hu J, Hou H, Liu B (2020) Phosphorus recovery from the liquid phase of anaerobic digestate using biochar derived from iron-rich sludge: a potential phosphorus fertilizer. *Water Res* 174:115629

Xiao L, Liu F, Liu J, Li J, Zhang Y, Yu J, Wang O (2018) Nano-Fe₃O₄ particles accelerating electromethanogenesis on an hour-long timescale in wetland soil. *Environ Sci-Nano* 5:436–445

Xiao L, Liu F, Xu H, Feng D, Liu J, Han G (2019a) Biochar promotes methane production at high acetate concentrations in anaerobic soils. *Environ Chem Lett* 17:1347–1352

Xiao L, Sun R, Zhang P, Zheng S, Tan Y, Li J, Zhang Y, Liu F (2019b) Simultaneous intensification of direct acetate cleavage and CO₂ reduction to generate methane by bioaugmentation and increased electron transfer. *Chem Eng J* 378:122229

Xiao L, Wei W, Luo M, Xu H, Feng D, Yu J, Huang J, Liu F (2019c) A potential contribution of a Fe(III)-rich red clay horizon to methane

release: biogenetic magnetite-mediated methanogenesis. *CAT-ENA* 181:104081

Xiao L, Zheng S, Lichtfouse E, Luo M, Tan Y, Liu F (2020a) Carbon nanotubes accelerate acetoclastic methanogenesis: from pure cultures to anaerobic soils. *Soil Biol Biochem* 150:107938

Xiao L, Liu F, Lichtfouse E, Zhang P, Feng D, Li F (2020b) Methane production by acetate dismutation stimulated by Shewanella oneidensis and carbon materials: an alternative to classical CO₂ reduction. *Chem Eng J* 389:124469

Xiao L, Li J, Lichtfouse E, Li Z, Wang Q, Liu F (2021a) Augmentation of chloramphenicol degradation by Geobacter-based biocatalysis and electric field. *J Hazard Mater* 410:124977

Xiao L, Wang Y, Lichtfouse E, Li Z, Kumar S, Liu J, Feng D, Yang Q, Liu F (2021b) Effect of antibiotics on the microbial efficiency of anaerobic digestion of wastewater: a review. *Front Microbiol* 11:611613

Yang H, Ye S, Zeng Z, Zeng G, Tan X, Xiao R, Wang J, Song B, Du L, Qin M, Yang Y, Xu F (2020) Utilization of biochar for resource recovery from water: a review. *Chem Eng J* 397:125502

Yuan J, Xu R, Zhang H (2011) The forms of alkalis in the biochar produced from crop residues at different temperatures. *Bioresour Technol* 102:3488–3497

Yuan H, Ding L, Zama E, Liu P, Hozzein W, Zhu Y (2018) Biochar modulates methanogenesis through electron syntrophy of microorganisms with ethanol as a substrate. *Environ Sci Technol* 52:12198–12207

Zhang J, Zhao W, Zhang H, Wang Z, Fan C, Zang L (2018) Recent achievements in enhancing anaerobic digestion with carbon-based functional materials. *Bioresour Technol* 266:555–567

Zhang L, Loh K, Zhang J (2019) Jointly reducing antibiotic resistance genes and improving methane yield in anaerobic digestion of chicken manure by feedstock microwave pretreatment and activated carbon supplementation. *Chem Eng J* 372:815–824

Zhang L, Lim E, Loh K, Ok Y, Lee J, Shen Y, Wang C, Dai Y, Tong Y (2020) Biochar enhanced thermophilic anaerobic digestion of food waste: focusing on biochar particle size, microbial community analysis and pilot-scale application. *Energ Convers Manag* 209:112654
## Supplementary Material

# Biochar promotes methane production during anaerobic digestion of organic waste

Leilei Xiao<sup>1,2*</sup>, Eric Lichtfouse<sup>3,4*</sup>, P. Senthil Kumar<sup>5</sup>, Quan Wang<sup>1</sup>, Fanghua Liu<sup>1,2</sup>

Number of pages: 13

Number of Tables: 1

Table SI 1 Literature data and references of the effect of biochar addition on biomethane production during anaerobic disposal of solid waste and wastewater.
<table><caption>**Table S1** Literature data and references of the effect of biochar addition on biomethane production during anaerobic disposal of solid waste and wastewater.</caption><thead><tr><th rowspan="2">Feedstock</th><th rowspan="2">Pyrolysis temperature (°C)</th><th rowspan="2">pH</th><th rowspan="2">Conductivity (mS/m)</th><th rowspan="2">Size (mm)</th><th rowspan="2">BET (m²/g)</th><th rowspan="2">Doze</th><th rowspan="2">Methanogens <sup>3</sup></th><th rowspan="2">Methanogens <sup>4</sup></th><th colspan="2">Control group</th><th colspan="4">Experimental group</th><th rowspan="2">Hedge's d</th><th rowspan="2">References</th></tr><tr><th>Max rate or yield</th><th>SD</th><th>Number</th><th>Max rate or yield</th><th>SD</th><th>Number</th></tr></thead><tbody><tr><td>Pine wood</td><td>980</td><td>NA<sup>1</sup></td><td>2.2 ±<br>0.46</td><td>2.0-<br>2.4</td><td>451 ±<br>11</td><td>NU<sup>2</sup></td><td>NA</td><td>NA</td><td>0.87</td><td>0.018</td><td>2</td><td>1.06</td><td>0.0792</td><td>2</td><td>1.89</td><td>(Cheng et al., 2018)</td></tr><tr><td>Pine wood</td><td>980</td><td>NA</td><td>2.2 ±<br>0.46</td><td>2.0-<br>2.4</td><td>451 ±<br>11</td><td>NU</td><td>NA</td><td>NA</td><td>0.935</td><td>0.015</td><td>2</td><td>1.3</td><td>0.0532</td><td>2</td><td>5.33</td><td>(Cheng et al., 2018)</td></tr><tr><td>Pine wood</td><td>980</td><td>NA</td><td>2.2 ±<br>0.46</td><td>2.0-<br>2.4</td><td>451 ±<br>11</td><td>NU</td><td>NA</td><td>NA</td><td>1.16</td><td>0.145</td><td>2</td><td>1.11</td><td>0.0166</td><td>2</td><td>-0.277</td><td>(Cheng et al., 2018)</td></tr><tr><td>Pine wood</td><td>980</td><td>NA</td><td>2.2 ±<br>0.46</td><td>0.21-<br>0.25</td><td>531 ±<br>31</td><td>NU</td><td>NA</td><td>NA</td><td>1.08</td><td>0.037</td><td>2</td><td>0.741</td><td>0.0748</td><td>2</td><td>-3.28</td><td>(Cheng et al., 2018)</td></tr><tr><td>Pine wood</td><td>980</td><td>NA</td><td>2.2 ±<br>0.46</td><td>0.21-<br>0.25</td><td>531 ±<br>31</td><td>NU</td><td>NA</td><td>NA</td><td>1.11</td><td>0.014</td><td>2</td><td>0.748</td><td>0.184</td><td>2</td><td>-1.59</td><td>(Cheng et al., 2018)</td></tr><tr><td>Pine wood</td><td>980</td><td>NA</td><td>2.2 ±<br>0.46</td><td>0.21-<br>0.25</td><td>531 ±<br>31</td><td>NU</td><td>NA</td><td>NA</td><td>1.08</td><td>0.014</td><td>2</td><td>0.413</td><td>0.0317</td><td>2</td><td>-15.54</td><td>(Cheng et al., 2018)</td></tr><tr><td>Fruitwoods</td><td>800</td><td>0.13 ±<br>8.63</td><td>NA</td><td>NA</td><td>NA</td><td>2</td><td>NA</td><td>NA</td><td>2.81</td><td>0.624</td><td>3</td><td>2.25</td><td>0.32909</td><td>3</td><td>-0.899</td><td>(Luo et al., 2015)</td></tr><tr><td>Fruitwoods</td><td>800</td><td>0.13 ±<br>8.63</td><td>NA</td><td>NA</td><td>NA</td><td>4</td><td>NA</td><td>NA</td><td>1.12</td><td>0.087</td><td>3</td><td>2.09</td><td>0.12124<br>4</td><td>3</td><td>7.37</td><td>(Luo et al., 2015)</td></tr><tr><td>Fruitwoods</td><td>800</td><td>0.13 ±<br>8.63</td><td>NA</td><td>0.5-1</td><td>NA</td><td>6</td><td>Methanobacterium</td><td>Methanobacterium</td><td>1.26</td><td>0.104</td><td>3</td><td>1.53</td><td>0.12124<br>4</td><td>3</td><td>1.91</td><td>(Luo et al., 2015)</td></tr><tr><td>Fruitwoods</td><td>800</td><td>0.13 ±<br>9.15</td><td>NA</td><td>NA</td><td>NA</td><td>8</td><td>Methanobacterium</td><td>Methanobacterium</td><td>0.97</td><td>0.104</td><td>3</td><td>1.02</td><td>0.10392<br>3</td><td>3</td><td>0.385</td><td>(Luo et al., 2015)</td></tr><tr><td>Sawdust</td><td>500</td><td>0.12</td><td>NA</td><td>0.25-<br>1</td><td>248 ±<br>34</td><td>10</td><td>NA</td><td>NA</td><td>1.349</td><td>0.095</td><td>2</td><td>1.351</td><td>0.0843</td><td>2</td><td>0.0127</td><td>(Li et al., 2018)</td></tr></tbody></table>
<table><tbody><tr><td></td><td></td><td>9.15 ± 0.12</td><td>NA</td><td>0.25-1</td><td>248 ± 34</td><td>10</td><td>NA</td><td>NA</td><td>2.606</td><td>0.058</td><td>2</td><td>2.619</td><td>0.0635</td><td>2</td><td>0.123</td><td>(Li et al., 2018)</td></tr><tr><td>Sawdust</td><td>500</td><td>9.15 ± 0.12</td><td>NA</td><td>0.25-1</td><td>248 ± 34</td><td>10</td><td>NA</td><td>NA</td><td>3.502</td><td>0.163</td><td>2</td><td>5.482</td><td>0.192</td><td>2</td><td>6.36</td><td>(Li et al., 2018)</td></tr><tr><td>Sawdust</td><td>500</td><td>9.15 ± 0.12</td><td>NA</td><td>0.25-1</td><td>248 ± 34</td><td>10</td><td>NA</td><td>NA</td><td>4.675</td><td>0.075</td><td>2</td><td>8.473</td><td>0.114</td><td>2</td><td>22.5</td><td>(Li et al., 2018)</td></tr><tr><td>Sawdust</td><td>500</td><td>9.15 ± 0.12</td><td>NA</td><td>0.25-1</td><td>248 ± 34</td><td>10</td><td>Methanosaeta</td><td>Methanosaeta</td><td>5.696</td><td>0.2</td><td>2</td><td>10.21</td><td>0.534</td><td>2</td><td>6.40</td><td>(Li et al., 2018)</td></tr><tr><td></td><td></td><td>7.9 ± 0.3</td><td>14 ± 10</td><td>NA</td><td>NA</td><td>5</td><td>NA</td><td>NA</td><td>1.813</td><td>0.142</td><td>3</td><td>2.872</td><td>0.245</td><td>3</td><td>4.23</td><td>(Paritosh and Vivekanand, 2019)</td></tr><tr><td>Hardwood</td><td>NA</td><td>7.9 ± 0.3</td><td>14 ± 10</td><td>NA</td><td>NA</td><td>10</td><td>NA</td><td>NA</td><td>1.813</td><td>0.142</td><td>3</td><td>3.662</td><td>0.254</td><td>3</td><td>7.19</td><td>(Paritosh and Vivekanand, 2019)</td></tr><tr><td>Hardwood</td><td>NA</td><td>7.9 ± 0.3</td><td>14 ± 10</td><td>NA</td><td>NA</td><td>15</td><td>NA</td><td>NA</td><td>1.813</td><td>0.142</td><td>3</td><td>2.993</td><td>0.238</td><td>3</td><td>4.82</td><td>(Paritosh and Vivekanand, 2019)</td></tr><tr><td>Hardwood</td><td>NA</td><td>7.9 ± 0.3</td><td>14 ± 10</td><td>NA</td><td>NA</td><td>20</td><td>NA</td><td>NA</td><td>1.813</td><td>0.142</td><td>3</td><td>2.611</td><td>0.12</td><td>3</td><td>4.86</td><td>(Paritosh and Vivekanand, 2019)</td></tr><tr><td>Hardwood</td><td>NA</td><td>7.9 ± 0.3</td><td>14 ± 10</td><td>NA</td><td>NA</td><td>25</td><td>NA</td><td>NA</td><td>1.813</td><td>0.142</td><td>3</td><td>2.463</td><td>0.214</td><td>3</td><td>2.86</td><td>(Paritosh and Vivekanand, 2019)</td></tr><tr><td>Hardwood</td><td>NA</td><td>7.9 ± 0.3</td><td>14 ± 10</td><td>NA</td><td>NA</td><td>30</td><td>NA</td><td>NA</td><td>1.813</td><td>0.142</td><td>3</td><td>2.49</td><td>0.228</td><td>3</td><td>2.85</td><td>(Paritosh and Vivekanand, 2019)</td></tr><tr><td>Rice straw</td><td>600</td><td>10.4 ± 0.02</td><td>2.853 ± 0.086</td><td>&lt;2</td><td>38.8 ± 0.99</td><td>20</td><td>Methanosarcina</td><td>Methanosarcina</td><td>0.025 ± 0.025</td><td>7</td><td>0.031</td><td>3</td><td>0.347</td><td>0.217</td><td>3</td><td>1.66</td><td>(Yuan et al., 2018)</td></tr><tr><td>Wood chips cow</td><td>600</td><td>9.60 ± 0.03</td><td>0.225 ± 0.006</td><td>&lt;2</td><td>14.75 ± 0.95</td><td>20</td><td>Methanosarcina</td><td>Methanosarcina</td><td>0.025 ± 0.025</td><td>7</td><td>0.031</td><td>3</td><td>0.0019</td><td>0.0014</td><td>3</td><td>-0.868</td><td>(Yuan et al., 2018)</td></tr><tr><td>Manure biomasses</td><td>600</td><td>10.2 ± 0.06</td><td>0.277 ± 0.007</td><td>&lt;2</td><td>16.91 ± 0.73</td><td>20</td><td>Methanobacterium</td><td>Methanobacterium</td><td>0.025 ± 0.025</td><td>7</td><td>0.031</td><td>3</td><td>0.825</td><td>0.238</td><td>3</td><td>3.77</td><td>(Yuan et al., 2018)</td></tr><tr><td>Pine sawdust</td><td>650</td><td>9.6 ± 0.6</td><td>NA</td><td>5-</td><td>130</td><td>8.3</td><td>NA</td><td>NA</td><td>113</td><td>8.66</td><td>3</td><td>156</td><td>6</td><td>3</td><td>3.27</td><td>(Sunyoto et al., 2016)</td></tr></tbody></table>
Sewage 6.4 ± 44.22 ± NA NA 0.5 NA NA 162.2 4.196 3 274.5 3.148 3 24.2 (Ambaye et al., 2020)

Sewage 6.4 ± 44.22 ± NA NA 1 NA NA 162.2 4.196 3 227.3 4.196 3 12.4 (Ambaye et al., 2020)

Sewage 6.4 ± 44.22 ± NA NA 1.5 NA NA 162.2 4.196 3 210.49 3.146 3 10.4 (Ambaye et al., 2020)

Sewage 6.4 ± 44.22 ± NA NA 2 NA NA 162.2 4.196 3 202.1 3.147 3 8.61 (Ambaye et al., 2020)

Sewage 9.5 ± 21.07 ± NA NA 0.5 NA NA 155.9 3.147 3 172.7 3.147 3 4.27 (Ambaye et al., 2020)

Sewage 9.5 ± 21.07 ± NA NA 1 NA NA 155.9 3.147 3 222 5.245 3 12.2 (Ambaye et al., 2020)

Sewage 9.5 ± 21.07 ± NA NA 1.5 NA NA 155.9 3.147 3 204.2 2.098 3 14.4 (Ambaye et al., 2020)

Sewage 9.5 ± 21.07 ± NA NA 2 NA NA 155.9 3.147 3 189.5 2.098 3 10.1 9 (Huang et al., 2020)

NA 1.59 ± NA NA NA 20 Methanosarcina Methanosarcina 4 2E-04 3 0.0068 0.00024 3 5.60 (Martinez et al., 2018)

Vineyard 1.59 ± NA NA NA 10 Methanosaeta Methanobacterium 10.89 0.21 3 14.27 0.28 3 10.9 (Martinez et al., 2018)

Vineyard 1.59 ± NA NA NA 30 Methanosaeta Methanofollis 10.89 0.21 3 14.15 0.28 3 10.5 (Martinez et al., 2018)

Vineyard 1.59 ± NA NA NA 10 Methanosaeta Methanosphaerula 18.73 0.37 3 23.13 0.46 3 8.43 (Martinez et al., 2018)

Vineyard 1.59 ± NA NA NA 30 Methanosaeta Mathanolinea 18.73 0.37 3 33.39 0.66 3 21.9 (Martinez et al., 2018)

Vineyard 1.59 ± NA NA NA 10 NA NA 14.35 0.28 3 66.34 1.15 3 49.7 (Martinez et al., 2018)

Vineyard 9.4 ± NA NA NA 30 NA NA 14.35 0.28 3 75.53 3.2 3 21.5 (Martinez et al., 2018)

Wood pellet 0.2 ± NA NA NA NA Methanosaetaceae NA 1.4 0.1 2 1.6 0.04 2 1.50 (Indren et al., 2020)
<table><tbody><tr><td>Wheat straw</td><td>NA</td><td>10.2 ± 0.03</td><td>NA</td><td>NA</td><td>NA</td><td>NA</td><td>Methanosaetace ae</td><td>NA</td><td>1.4</td><td>0.1</td><td>2</td><td>1.2</td><td>0.1</td><td>2</td><td>-1.14</td><td>(Indren et al., 2020)</td></tr><tr><td>Sheep manure</td><td>NA</td><td>11 ± 0.1</td><td>NA</td><td>NA</td><td>NA</td><td>NA</td><td>Methanosaetace ae</td><td>NA</td><td>1.4</td><td>0.1</td><td>2</td><td>1.2</td><td>0.1</td><td>2</td><td>-1.14</td><td>(Indren et al., 2020)</td></tr><tr><td>Dry dairy manure</td><td>350</td><td>NA</td><td>NA</td><td>0.6</td><td>6.3</td><td>1</td><td>NA</td><td>NA</td><td>20.19</td><td>0.96</td><td>2</td><td>21.4</td><td>1.09</td><td>2</td><td>0.673</td><td>(Jang et al., 2018)</td></tr><tr><td>Dry dairy manure</td><td>350</td><td>NA</td><td>NA</td><td>0.6</td><td>6.3</td><td>10</td><td>NA</td><td>NA</td><td>20.19</td><td>0.96</td><td>2</td><td>24.32</td><td>1.12</td><td>2</td><td>2.26</td><td>(Jang et al., 2018)</td></tr><tr><td>Dry dairy manure</td><td>350</td><td>NA</td><td>NA</td><td>0.6</td><td>6.3</td><td>1</td><td>NA</td><td>NA</td><td>28.23</td><td>0.64</td><td>2</td><td>29.85</td><td>0.62</td><td>2</td><td>1.47</td><td>(Jang et al., 2018)</td></tr><tr><td>Dry dairy manure</td><td>350</td><td>NA</td><td>NA</td><td>0.6</td><td>6.3</td><td>10</td><td>NA</td><td>NA</td><td>28.23</td><td>0.64</td><td>2</td><td>37.35</td><td>0.67</td><td>2</td><td>7.96</td><td>(Jang et al., 2018)</td></tr><tr><td>Dry dairy manure</td><td>350</td><td>NA</td><td>NA</td><td>0.6</td><td>6.3</td><td>1</td><td>NA</td><td>NA</td><td>25.58</td><td>0.87</td><td>2</td><td>31.07</td><td>1.01</td><td>2</td><td>3.33</td><td>(Jang et al., 2018)</td></tr><tr><td>Dry dairy manure</td><td>350</td><td>NA</td><td>NA</td><td>0.6</td><td>6.3</td><td>10</td><td>NA</td><td>NA</td><td>25.58</td><td>0.87</td><td>2</td><td>38.49</td><td>1.44</td><td>2</td><td>6.20</td><td>(Jang et al., 2018)</td></tr><tr><td>Corn stover</td><td>500</td><td>NA</td><td>NA</td><td>841 to &lt;74</td><td>NA</td><td>NA</td><td>NA</td><td>NA</td><td>620</td><td>10.39</td><td>3</td><td>611</td><td>46.7653 7 27.7128</td><td>3</td><td>-0.152</td><td>(Achi et al., 2020)</td></tr><tr><td>Corn stover</td><td>500</td><td>NA</td><td>NA</td><td>841 to &lt;74</td><td>NA</td><td>NA</td><td>NA</td><td>NA</td><td>620</td><td>10.39</td><td>3</td><td>611</td><td>1</td><td>3</td><td>-0.246</td><td>(Achi et al., 2020)</td></tr><tr><td>Douglas fir wood</td><td>400</td><td>5.62 ± 0.01 5.77</td><td>46.8±1.9</td><td>NA</td><td>16.99 ± 0.58</td><td>10</td><td>Methanothermob acter</td><td>Methanosaeta</td><td>19.3</td><td>1.4</td><td>2</td><td>31</td><td>1.9</td><td>2</td><td>4.01</td><td>(Wang et al., 2020b)</td></tr><tr><td>Douglas fir wood</td><td>500</td><td>5.77 ± 0.00 6.08</td><td>49.9±1.2</td><td>NA</td><td>13.17 ± 4.05</td><td>10</td><td>Methanothermob acter</td><td>Methanosarcina</td><td>19.3</td><td>1.4</td><td>2</td><td>38</td><td>2.2</td><td>2</td><td>5.80</td><td>(Wang et al., 2020b)</td></tr><tr><td>Douglas fir wood</td><td>600</td><td>6.08 ± 0.04 6.13</td><td>51.2±5.7</td><td>NA</td><td>18.36 ± 2.61</td><td>10</td><td>Methanothermob acter</td><td>Methanosarcina</td><td>19.3</td><td>1.4</td><td>2</td><td>33.5</td><td>1.6</td><td>2</td><td>5.40</td><td>(Wang et al., 2020b)</td></tr><tr><td>Douglas fir wood</td><td>730</td><td>6.13 ± 0.08 5.62</td><td>63.6±3.4</td><td>NA</td><td>18.39 ± 0.29</td><td>10</td><td>Methanothermob acter</td><td>Methanosarcina</td><td>19.3</td><td>1.4</td><td>2</td><td>25.6</td><td>1.8</td><td>2</td><td>2.23</td><td>(Wang et al., 2020b)</td></tr><tr><td>Douglas fir wood</td><td>400</td><td>5.77 ± 0.01 5.77</td><td>46.8±1.9</td><td>NA</td><td>16.99 ± 0.58</td><td>10</td><td>Methanothermob acter</td><td>Methanothermobacter</td><td>3.9</td><td>1.8</td><td>2</td><td>7.4</td><td>0.5</td><td>2</td><td>1.51</td><td>(Wang et al., 2020b)</td></tr><tr><td>Douglas fir wood</td><td>500</td><td>6.08 ± 0.00 6.08</td><td>49.9±1.2</td><td>NA</td><td>13.17 ± 4.05</td><td>10</td><td>Methanothermob acter</td><td>Methanobacterium</td><td>3.9</td><td>1.8</td><td>2</td><td>6.7</td><td>0.2</td><td>2</td><td>1.25</td><td>(Wang et al., 2020b)</td></tr><tr><td>Douglas fir wood</td><td>600</td><td>6.13 ± 0.04 6.13</td><td>51.2±5.7</td><td>NA</td><td>18.36 ± 2.61</td><td>10</td><td>Methanothermob acter</td><td>Methanobacterium</td><td>3.9</td><td>1.8</td><td>2</td><td>6.8</td><td>1</td><td>2</td><td>1.14</td><td>(Wang et al., 2020b)</td></tr><tr><td>Douglas fir wood</td><td>730</td><td>6.13 ± 0.08 5.62</td><td>63.6±3.4</td><td>NA</td><td>18.39 ± 0.29</td><td>10</td><td>Methanothermob acter</td><td>Methanothermobacter</td><td>3.9</td><td>1.8</td><td>2</td><td>4</td><td>0.7</td><td>2</td><td>0.0418</td><td>(Wang et al., 2020b)</td></tr><tr><td>Douglas fir wood</td><td>400</td><td>5.62 ± 0.01 5.62</td><td>46.8±1.9</td><td>NA</td><td>16.99 ± 0.58</td><td>10</td><td>Methanobrevibacter</td><td>Methanobrevibacter</td><td>30.7</td><td>1.4</td><td>2</td><td>35.8</td><td>1.9</td><td>2</td><td>1.75</td><td>(Wang et al., 2020b)</td></tr></tbody></table>
<table><tbody><tr><td>Douglas fir wood</td><td>500</td><td>5.77 ± 0.00 6.08</td><td>49.9±1.2 NA</td><td>13.17 ± 4.05 10</td><td>Methanothermob acter</td><td>Methanomassiliicocc us</td><td>30.7</td><td>1.4</td><td>2</td><td>35.5</td><td>2.2</td><td>2</td><td>1.49</td><td>(Wang et al., 2020b)</td></tr><tr><td>Douglas fir wood</td><td>600</td><td>± 0.04 6.13</td><td>51.2±5.7 NA</td><td>18.36 ± 2.61 10</td><td>Methanothermob acter</td><td>Methanomassiliicocc us</td><td>30.7</td><td>1.4</td><td>2</td><td>36.2</td><td>2.2</td><td>2</td><td>1.70</td><td>(Wang et al., 2020b)</td></tr><tr><td>Douglas fir wood</td><td>730</td><td>± 0.08 5.62</td><td>63.6±3.4 NA</td><td>18.39 ± 0.29 10</td><td>Methanothermob acter</td><td>Methanomassiliicocc us</td><td>30.7</td><td>1.4</td><td>2</td><td>33.8</td><td>2.1</td><td>2</td><td>0.992</td><td>(Wang et al., 2020b)</td></tr><tr><td>Douglas fir wood</td><td>400</td><td>± 0.01 5.77</td><td>46.8±1.9 NA</td><td>16.99 ± 0.58 10</td><td>Methanothermob acter</td><td>Methanothermobacte r</td><td>17.6</td><td>1.6</td><td>2</td><td>25</td><td>2.8</td><td>2</td><td>4.44</td><td>(Wang et al., 2020b)</td></tr><tr><td>Douglas fir wood</td><td>500</td><td>± 0.00 6.08</td><td>49.9±1.2 NA</td><td>13.17 ± 4.05 10</td><td>Methanothermob acter</td><td>Methanothermobacte r</td><td>17.6</td><td>1.6</td><td>2</td><td>24.6</td><td>1</td><td>2</td><td>2.0487</td><td>(Wang et al., 2020b)</td></tr><tr><td>Douglas fir wood</td><td>600</td><td>± 0.04 6.13</td><td>51.2±5.7 NA</td><td>18.36 ± 2.61 10</td><td>Methanothermob acter</td><td>Methanothermobacte r</td><td>17.6</td><td>1.6</td><td>2</td><td>24.2</td><td>1.7</td><td>2</td><td>1.28</td><td>(Wang et al., 2020b)</td></tr><tr><td>Douglas fir wood</td><td>730</td><td>± 0.08</td><td>63.6±3.4 NA</td><td>18.39 ± 0.29 10</td><td>Methanothermob acter</td><td>Methanothermobacte r</td><td>17.6</td><td>1.6</td><td>2</td><td>22.6</td><td>1.9</td><td>2</td><td>0.723</td><td>(Wang et al., 2020b)</td></tr><tr><td>Sewage sludge</td><td>300</td><td>6.78</td><td>64.1 NA</td><td>21.319 ± 6</td><td>Methanosaeta</td><td>NA</td><td>111.4</td><td>0.87</td><td>3</td><td>132.1</td><td>4.775</td><td>3</td><td>4.83</td><td>(Wu et al., 2019)</td></tr><tr><td>Sewage sludge</td><td>500</td><td>7.5</td><td>27.4 NA</td><td>39.757 ± 3</td><td>Methanosaeta</td><td>NA</td><td>111.4</td><td>0.874</td><td>3</td><td>123.4</td><td>2.581</td><td>3</td><td>4.98</td><td>(Wu et al., 2019)</td></tr><tr><td>Sewage sludge</td><td>700</td><td>7.92</td><td>24.85 NA</td><td>31.774 ± 6</td><td>Methanosaeta</td><td>NA</td><td>111.4</td><td>0.874</td><td>3</td><td>114.7</td><td>0.735</td><td>3</td><td>3.27</td><td>(Wu et al., 2019)</td></tr><tr><td>Cow manure</td><td>500</td><td>8.5</td><td>NA</td><td>1.0 ± 0.5–</td><td>Methanospirillum</td><td>Methanospirillum</td><td>220.1</td><td>7.7</td><td>2</td><td>263.6</td><td>8.9</td><td>2</td><td>2.99</td><td>(Sun et al., 2019)</td></tr><tr><td>Cow manure</td><td>500</td><td>8.5</td><td>NA</td><td>1.0 ± 0.5–</td><td>Methanospirillum</td><td>Methanospirillum</td><td>220.1</td><td>7.7</td><td>2</td><td>341.5</td><td>3.6</td><td>2</td><td>11.5</td><td>(Sun et al., 2019)</td></tr><tr><td>Cow manure</td><td>500</td><td>8.5</td><td>NA</td><td>1.0 ± 0.5–</td><td>Methanospirillum</td><td>Methanospirillum</td><td>220.1</td><td>7.7</td><td>2</td><td>401.8</td><td>7.7</td><td>2</td><td>13.5</td><td>(Sun et al., 2019)</td></tr><tr><td>Cow manure</td><td>500</td><td>8.5</td><td>NA</td><td>1.0 ± 0.5–</td><td>Methanospirillum</td><td>Methanospirillum</td><td>220.1</td><td>7.7</td><td>2</td><td>358.1</td><td>4.2</td><td>2</td><td>12.7</td><td>(Sun et al., 2019)</td></tr><tr><td>Cow manure</td><td>500</td><td>8.5</td><td>NA</td><td>1.0 ± 0.5–</td><td>Methanosaeta</td><td>Methanosaeta</td><td>310.4</td><td>9.2</td><td>2</td><td>350</td><td>3.9</td><td>2</td><td>3.20</td><td>(Sun et al., 2019)</td></tr><tr><td>Cow manure</td><td>500</td><td>8.5</td><td>NA</td><td>1.0 ± 0.5–</td><td>Methanosaeta</td><td>Methanosaeta</td><td>310.4</td><td>9.2</td><td>2</td><td>391.8</td><td>5.5</td><td>2</td><td>6.14</td><td>(Sun et al., 2019)</td></tr><tr><td>Cow manure</td><td>500</td><td>8.5</td><td>NA</td><td>1.0 ± 0.5–</td><td>Methanosaeta</td><td>Methanosaeta</td><td>310.4</td><td>9.2</td><td>2</td><td>456.8</td><td>7.7</td><td>2</td><td>9.86</td><td>(Sun et al., 2019)</td></tr><tr><td>Cow manure</td><td>500</td><td>8.5</td><td>NA</td><td>1.0 ± 0.5–</td><td>Methanosaeta</td><td>Methanosaeta</td><td>310.4</td><td>9.2</td><td>2</td><td>416.9</td><td>8.9</td><td>2</td><td>6.72</td><td>(Sun et al., 2019)</td></tr><tr><td>Corn straw</td><td>400</td><td>8.2</td><td>NA</td><td>29.8</td><td>NA</td><td>NA</td><td>117.2</td><td>8.244</td><td>3</td><td>184.2</td><td>9.178</td><td>3</td><td>6.14</td><td>(Zhang et al., 2019)</td></tr><tr><td>Corn straw</td><td>500</td><td>8.3</td><td>NA</td><td>32.8</td><td>NA</td><td>NA</td><td>117.2</td><td>8.244</td><td>3</td><td>196.1</td><td>6.859</td><td>3</td><td>8.32</td><td>(Zhang et al., 2019)</td></tr></tbody></table>
<table><tbody><tr><td>Corn straw</td><td>600</td><td>8.3</td><td>NA</td><td>NA</td><td>56.6</td><td>8</td><td>NA</td><td>NA</td><td>117.2</td><td>8.244</td><td>3</td><td>218.4</td><td>9.352</td><td>3</td><td>9.18</td><td>(Zhang et al., 2019)</td></tr><tr><td>Sewage sludge</td><td>400</td><td>9.3</td><td>NA</td><td>NA</td><td>16.1</td><td>8</td><td>NA</td><td>NA</td><td>117.2</td><td>8.244</td><td>3</td><td>155.9</td><td>8.986</td><td>3</td><td>3.59</td><td>(Zhang et al., 2019)</td></tr><tr><td>Sewage sludge</td><td>500</td><td>9.5</td><td>NA</td><td>NA</td><td>18.9</td><td>8</td><td>NA</td><td>NA</td><td>117.2</td><td>8.244</td><td>3</td><td>207.6</td><td>8.092</td><td>3</td><td>8.85</td><td>(Zhang et al., 2019)</td></tr><tr><td>Sewage sludge</td><td>600</td><td>9.7</td><td>NA</td><td>NA</td><td>26.3</td><td>8</td><td>NA</td><td>NA</td><td>117.2</td><td>8.244</td><td>3</td><td>165.8</td><td>7.662</td><td>3</td><td>4.89</td><td>(Zhang et al., 2019)</td></tr><tr><td>Coconut shell</td><td>400</td><td>8.7</td><td>NA</td><td>NA</td><td>2.32</td><td>8</td><td>NA</td><td>NA</td><td>117.2</td><td>8.244</td><td>3</td><td>143.8</td><td>8.923</td><td>3</td><td>2.48</td><td>(Zhang et al., 2019)</td></tr><tr><td>Coconut shell</td><td>500</td><td>9.5</td><td>NA</td><td>NA</td><td>1.92</td><td>8</td><td>NA</td><td>NA</td><td>117.2</td><td>8.244</td><td>3</td><td>125.4</td><td>9.005</td><td>3</td><td>0.760</td><td>(Zhang et al., 2019)</td></tr><tr><td>Coconut shell</td><td>600</td><td>11.1</td><td>NA</td><td>NA</td><td>12.7</td><td>8</td><td>NA</td><td>NA</td><td>117.2</td><td>8.244</td><td>3</td><td>173.7</td><td>5.964</td><td>3</td><td>6.28</td><td>(Zhang et al., 2019)</td></tr><tr><td>Corn straw</td><td>600</td><td>8.3</td><td>NA</td><td>NA</td><td>56.6</td><td>6.2</td><td>NA</td><td>NA</td><td>126.9</td><td>1.997</td><td>3</td><td>149</td><td>3.472</td><td>3</td><td>6.24</td><td>(Zhang et al., 2019)</td></tr><tr><td>Corn straw</td><td>600</td><td>8.3</td><td>NA</td><td>NA</td><td>56.6</td><td>15.9</td><td>NA</td><td>NA</td><td>126.9</td><td>1.997</td><td>3</td><td>185.3</td><td>5.015</td><td>3</td><td>12.2</td><td>(Zhang et al., 2019)</td></tr><tr><td>Corn straw</td><td>600</td><td>8.3</td><td>NA</td><td>NA</td><td>56.6</td><td>26.1</td><td>NA</td><td>NA</td><td>126.9</td><td>1.997</td><td>3</td><td>199.2</td><td>5.016</td><td>3</td><td>15.2</td><td>(Zhang et al., 2019)</td></tr><tr><td>Corn straw</td><td>600</td><td>8.3</td><td>NA</td><td>NA</td><td>56.6</td><td>34.2</td><td>NA</td><td>NA</td><td>126.9</td><td>1.997</td><td>3</td><td>137.5</td><td>3.858</td><td>3</td><td>2.76</td><td>(Zhang et al., 2019)</td></tr><tr><td>Sewage sludge</td><td>600</td><td>9.7</td><td>NA</td><td>NA</td><td>26.3</td><td>6.2</td><td>NA</td><td>NA</td><td>126.9</td><td>1.997</td><td>3</td><td>140.2</td><td>4.659</td><td>3</td><td>2.97</td><td>(Zhang et al., 2019)</td></tr><tr><td>Sewage sludge</td><td>600</td><td>9.7</td><td>NA</td><td>NA</td><td>26.3</td><td>15.9</td><td>NA</td><td>NA</td><td>126.9</td><td>1.997</td><td>3</td><td>155.5</td><td>1.997</td><td>3</td><td>11.5</td><td>(Zhang et al., 2019)</td></tr><tr><td>Sewage sludge</td><td>600</td><td>9.7</td><td>NA</td><td>NA</td><td>26.3</td><td>26.1</td><td>NA</td><td>NA</td><td>126.9</td><td>1.997</td><td>3</td><td>168.5</td><td>3.661</td><td>3</td><td>11.3</td><td>(Zhang et al., 2019)</td></tr><tr><td>Sewage sludge</td><td>600</td><td>9.7</td><td>NA</td><td>NA</td><td>26.3</td><td>34.2</td><td>NA</td><td>NA</td><td>126.9</td><td>1.997</td><td>3</td><td>124.9</td><td>2.663</td><td>3</td><td>-0.680</td><td>(Zhang et al., 2019)</td></tr><tr><td>Coconut shell</td><td>600</td><td>11.1</td><td>NA</td><td>NA</td><td>12.7</td><td>6.2</td><td>NA</td><td>NA</td><td>126.9</td><td>1.997</td><td>3</td><td>142.7</td><td>3.802</td><td>3</td><td>4.16</td><td>(Zhang et al., 2019)</td></tr><tr><td>Coconut shell</td><td>600</td><td>11.1</td><td>NA</td><td>NA</td><td>12.7</td><td>15.9</td><td>NA</td><td>NA</td><td>126.9</td><td>1.997</td><td>3</td><td>154.1</td><td>2.662</td><td>3</td><td>9.25</td><td>(Zhang et al., 2019)</td></tr><tr><td>Coconut shell</td><td>600</td><td>11.1</td><td>NA</td><td>NA</td><td>12.7</td><td>26.1</td><td>NA</td><td>NA</td><td>126.9</td><td>1.997</td><td>3</td><td>169.7</td><td>2.661</td><td>3</td><td>14.6</td><td>(Zhang et al., 2019)</td></tr><tr><td>Coconut shell</td><td>600</td><td>11.1</td><td>NA</td><td>NA</td><td>12.7</td><td>34.2</td><td>NA</td><td>NA</td><td>126.9</td><td>1.997</td><td>3</td><td>184.9</td><td>3.042</td><td>3</td><td>18.0</td><td>(Zhang et al., 2019)</td></tr><tr><td>Bamboo biochar</td><td>NA</td><td>0.02</td><td>0.024 ± 0.002</td><td>2-3</td><td>54.5</td><td>NA</td><td>NA</td><td>NA</td><td>9.01</td><td>0.96</td><td>3</td><td>4.11</td><td>0.678</td><td>3</td><td>-4.72</td><td>(Mao et al., 2018)</td></tr><tr><td>Pine</td><td>800</td><td>NA</td><td>NA</td><td>0.5–1 < 0.00</td><td>5.21</td><td>NA</td><td>Methanothermobacter</td><td>Methanothermobacter</td><td>9.11</td><td>0.53</td><td>3</td><td>7.06</td><td>0.66</td><td>3</td><td>-2.74</td><td>(Lu et al., 2019)</td></tr><tr><td>Pine</td><td>800</td><td>NA</td><td>NA</td><td>5</td><td>210.78</td><td>NA</td><td>Methanolinea</td><td>Methanolinea</td><td>9.11</td><td>0.53</td><td>3</td><td>9.55</td><td>0.53</td><td>3</td><td>0.664</td><td>(Lu et al., 2019)</td></tr><tr><td>Pine</td><td>800</td><td>NA</td><td>NA</td><td>0.5–1</td><td>5.21</td><td>NA</td><td>Methanomicrobia</td><td>Methanomicrobia</td><td>9.08</td><td>0.69</td><td>3</td><td>10.56</td><td>0.53</td><td>3</td><td>1.92</td><td>(Lu et al., 2019)</td></tr></tbody></table>
<table><tbody><tr><td>Pine</td><td>800</td><td>NA<br>7.3<br>±</td><td>NA</td><td>5</td><td>210.78</td><td>NA</td><td>Methanomicrobia</td><td>Methanomicrobia</td><td>9.08</td><td>0.69</td><td>3</td><td>9.44</td><td>0.75</td><td>3</td><td>0.400</td><td>(Lu et al., 2019)</td></tr><tr><td>Sawdust</td><td>300</td><td>0.1<br>9.2<br>±</td><td>NA</td><td>0.25-<br>1</td><td>53.2 ±<br>2.8</td><td>15</td><td>NA</td><td>NA</td><td>115.9</td><td>2.362</td><td>3</td><td>159.7</td><td>2.362</td><td>3</td><td>14.8</td><td>(Wang et al., 2020a)</td></tr><tr><td>Sawdust</td><td>500</td><td>0.1<br>10.0<br>±</td><td>NA</td><td>0.25-<br>1</td><td>248.6<br>± 3.5</td><td>15</td><td>NA</td><td>NA</td><td>115.9</td><td>2.362</td><td>3</td><td>168.7</td><td>5.196</td><td>3</td><td>10.5</td><td>(Wang et al., 2020a)</td></tr><tr><td>Sawdust</td><td>700</td><td>0.2<br>±</td><td>NA</td><td>0.25-<br>1</td><td>511.3<br>± 3.8</td><td>15</td><td>NA</td><td>NA</td><td>115.9</td><td>2.362</td><td>3</td><td>150.7</td><td>1.89</td><td>3</td><td>13.0</td><td>(Wang et al., 2020a)</td></tr><tr><td>Straw</td><td>350-550</td><td>NA<br>8.01<br>±</td><td>143<br>46.6 ±</td><td>NA</td><td>NA</td><td>1</td><td>Methanosaetaceae</td><td>Methanobacteriaceae</td><td>0.756</td><td>0.092</td><td>3</td><td>0.811</td><td>0.094</td><td>3</td><td>0.0744</td><td>(Xiao et al., 2019)</td></tr><tr><td>Sludge</td><td>500</td><td>0.05<br>±</td><td>2.8<br>±</td><td>NA</td><td>NA</td><td>NA</td><td>Methanothermobacter</td><td>NA</td><td>20.14</td><td>1.49</td><td>3</td><td>30.06</td><td>2.87</td><td>3</td><td>3.47</td><td>(Yin et al., 2019)</td></tr><tr><td>Sawdust</td><td>500</td><td>9.2<br>±</td><td>NA</td><td>0.25-1</td><td>248.6</td><td>2</td><td>NA</td><td>NA</td><td>6.7</td><td>0.1</td><td>2</td><td>8.7</td><td>0.2</td><td>2</td><td>7.23</td><td>(Wang et al., 2018)</td></tr><tr><td>Sawdust</td><td>500</td><td>9.2<br>±</td><td>NA</td><td>0.25-1</td><td>248.6</td><td>6</td><td>NA</td><td>NA</td><td>6.7</td><td>0.1</td><td>2</td><td>9.4</td><td>0.2</td><td>2</td><td>9.76</td><td>(Wang et al., 2018)</td></tr><tr><td>Sawdust</td><td>500</td><td>9.2<br>±</td><td>NA</td><td>0.25-1</td><td>248.6</td><td>10</td><td>NA</td><td>NA</td><td>6.7</td><td>0.1</td><td>2</td><td>8.2</td><td>0.2</td><td>2</td><td>5.42</td><td>(Wang et al., 2018)</td></tr><tr><td>Sawdust</td><td>500</td><td>9.2<br>±</td><td>NA</td><td>0.25-1</td><td>248.6</td><td>15</td><td>Methanosaeta</td><td>Methanosaeta</td><td>6.7</td><td>0.1</td><td>2</td><td>7.8</td><td>0.2</td><td>2</td><td>3.98</td><td>(Wang et al., 2018)</td></tr><tr><td>Red spruce woodchips</td><td>NA</td><td>8.4<br>±</td><td>NA</td><td>NA</td><td>327</td><td>NA</td><td>NA</td><td>NA</td><td>486.9</td><td>13.12</td><td>3</td><td>640.6</td><td>16.88</td><td>3</td><td>8.13</td><td>(Mainardis et al., 2019)</td></tr><tr><td>Red spruce woodchips</td><td>NA</td><td>8.4<br>±</td><td>NA</td><td>NA</td><td>327</td><td>NA</td><td>NA</td><td>NA</td><td>291</td><td>7.693</td><td>3</td><td>402.6</td><td>8.974</td><td>3</td><td>10.7</td><td>(Mainardis et al., 2019)</td></tr><tr><td>Pine biochar</td><td>NA</td><td>NA</td><td>NA</td><td>NA</td><td>310.19</td><td>NU</td><td>NA</td><td>NA</td><td>72.45</td><td>1.11</td><td>3</td><td>82.9</td><td>0.28</td><td>3</td><td>10.3</td><td>(Mainardis et al., 2019)</td></tr><tr><td>Pine biochar</td><td>NA</td><td>NA</td><td>NA</td><td>NA</td><td>310.19</td><td>NU</td><td>NA</td><td>NA</td><td>72.45</td><td>1.11</td><td>3</td><td>71.35</td><td>1.19</td><td>3</td><td>-0.765</td><td>(Shen et al., 2016)</td></tr><tr><td>White oak biochar</td><td>NA</td><td>NA</td><td>NA</td><td>NA</td><td>296.81</td><td>NU</td><td>NA</td><td>NA</td><td>72.45</td><td>1.11</td><td>3</td><td>83.22</td><td>0.28</td><td>3</td><td>10.6</td><td>(Shen et al., 2016)</td></tr><tr><td>White oak biochar</td><td>NA</td><td>NA</td><td>NA</td><td>NA</td><td>296.81</td><td>NU</td><td>NA</td><td>NA</td><td>72.45</td><td>1.11</td><td>3</td><td>79.41</td><td>1.27</td><td>3</td><td>4.67</td><td>(Shen et al., 2016)</td></tr><tr><td>Pine biochar</td><td>NA</td><td>NA</td><td>NA</td><td>NA</td><td>310.19</td><td>NU</td><td>NA</td><td>NA</td><td>101.8</td><td>6.46</td><td>3</td><td>107.1</td><td>3.35</td><td>3</td><td>0.824</td><td>(Shen et al., 2016)</td></tr><tr><td>Pine biochar</td><td>NA</td><td>NA</td><td>NA</td><td>NA</td><td>310.19</td><td>NU</td><td>NA</td><td>NA</td><td>101.8</td><td>6.46</td><td>3</td><td>103.9</td><td>2.81</td><td>3</td><td>0.337</td><td>(Shen et al., 2016)</td></tr><tr><td>White oak biochar</td><td>NA</td><td>NA</td><td>NA</td><td>NA</td><td>296.81</td><td>NU</td><td>NA</td><td>NA</td><td>101.8</td><td>6.46</td><td>3</td><td>106.1</td><td>3.59</td><td>3</td><td>0.658</td><td>(Shen et al., 2016)</td></tr><tr><td>White oak biochar</td><td>NA</td><td>NA</td><td>NA</td><td>NA</td><td>296.81</td><td>NU</td><td>NA</td><td>NA</td><td>101.8</td><td>6.46</td><td>3</td><td>101.5</td><td>3.04</td><td>3</td><td>0.0475</td><td>(Shen et al., 2016)</td></tr><tr><td>Wheat bran pellets</td><td>800</td><td>NA</td><td>49900<br>±</td><td>1.7-2</td><td>55 ± 1</td><td>25</td><td>Methanosarcina</td><td>Methanosarcina</td><td>14.15<br>±</td><td>4.001</td><td>2</td><td>58.308</td><td>2.308</td><td>2</td><td>4.17</td><td>(Viggi et al., 2017)</td></tr></tbody></table>
<table><tr><td>Coppiced woodlands</td><td>500</td><td>NA</td><td>1600</td><td>1.7-2</td><td>61 ± 1</td><td>25</td><td>Methanosarcina</td><td>Methanosarcina</td><td>14.15</td><td>4</td><td>4.001</td><td>2</td><td>66.923</td><td>2.461</td><td>2</td><td>4.28</td><td>(Viggi et al., 2017)</td></tr><tr><td>Orchard pruning</td><td>500</td><td>NA</td><td>500</td><td>1.7-2</td><td>13.7 ± 0.5</td><td>25</td><td>Methanosarcina</td><td>Methanosarcina</td><td>14.15</td><td>4</td><td>4.001</td><td>2</td><td>72.154</td><td>1.539</td><td>2</td><td>4.12</td><td>(Viggi et al., 2017)</td></tr></table>

Notes: 1, NA, not available; 2, Not used; 3, The predominant methanogens; 4, Methanogens with the highest increase in abundance. These paired measurements of control and biochar-amended treatments was in duplicate with a gray background.
## References

Achi C, Hassanein A, Lansing S (2020) Enhanced Biogas Production of Cassava Wastewater Using Zeolite and Biochar Additives and Manure Co-Digestion. *Energies* 13:491.

Ambaye T, Rene E, Dupont C, Wongrod S, van Hullebusch E (2020) Anaerobic Digestion of Fruit Waste Mixed With Sewage Sludge Digestate Biochar: Influence on Biomethane Production. *Front Energy Res* 8, 31.

Cai J, He P, Wang Y, Shao L, Lu F (2016) Effects and optimization of the use of biochar in anaerobic digestion of food wastes. *Waste Manage Res* 34:409-416.

Cheng Q, de los Reyes F, Call D (2018) Amending anaerobic bioreactors with pyrogenic carbonaceous materials: the influence of material properties on methane generation. *Environ Sci-Wat Res* 4:1794-1806.

Huang J, Ma K, Xia X, Gao K, Lu Y (2020) Biochar and magnetite promote methanogenesis during anaerobic decomposition of rice straw. *Soil Biol Biochem* 143, 107740.

Indren M, Birzer C, Kidd S, Hall T, Medwell P (2020) Effects of biochar parent material and microbial pre-loading in biochar-amended high-solids anaerobic digestion. *Bioresour. Technol* 298, 122457.

Jang H, Choi Y, Kan E (2018) Effects of dairy manure-derived biochar on psychrophilic, mesophilic and thermophilic anaerobic digestions of dairy manure. *Bioresour. Technol* 250:927-931.

Li Q, Xu M, Wang G, Chen R, Qiao W, Wang X. (2018) Biochar assisted thermophilic co-digestion of food waste and waste activated sludge under high feedstock to seed sludge ratio in batch experiment. *Bioresour. Technol* 249:1009-1016.

Lu F, Liu Y, Shao L, He, P (2019) Powdered biochar doubled microbial growth in anaerobic digestion of oil. *Appl Energ* 247:605-614.

Luo C, Lu F, Shao L, He P (2015) Application of eco-compatible biochar in anaerobic digestion to relieve acid stress and promote the selective colonization of functional microbes (vol 68, pg 710, 2014). *Water Res* 70:496-496.

Mainardis M, Flaibani S, Mazzolini F, Peressotti A, Goi D (2019) Techno-economic analysis of anaerobic digestion implementation in small Italian breweries and evaluation of biochar and granular activated carbon addition effect on methane yield. *J Environ Chem Eng* 7, 103184.

Mao H, Lv Z, Sun H, Li R, Zhai B, Wang Z, Awasthi M, Wang Q, Zhou L (2018) Improvement of biochar and bacterial powder addition on gaseous emission and bacterial community in pig manure compost. *Bioresour. Technol* 258:195-202.

Martinez E, Rosas J, Sotres A, Moran A, Cara J, Sanchez M, Gomez X (2018) Codigestion of sludge and citrus peel wastes: Evaluating the effect of biochar addition on microbial communities. *Biochem Eng J* 137: 314-325.

Paritosh K, Vivekanand V (2019) Biochar enabled syntrophic action: Solid state anaerobic digestion of agricultural stubble for enhanced methane production. *Bioresour. Technol* 289, 121712.

Shanmugam S, Adhikari S, Nam H, Sajib S (2018) Effect of bio-char on methane generation from glucose and aqueous phase of algae liquefaction using mixed anaerobic cultures. *Biomass Bioenerg* 108:479-486.

Shen Y, Linville J, Ignacio-de Leon P, Schoene R, Urgun-Demirtas M (2016) Towards a sustainable paradigm of waste-to-energy process: Enhanced anaerobic digestion of sludge with woody biochar. *J Clean Prod* 135:1054-1064.

Sun C, Liu F, Song Z, Wang J, Li Y, Pan Y, Sheng T, Li L (2019) Feasibility of dry anaerobic digestion of beer lees for methane production and biochar enhanced performance at mesophilic and thermophilic temperature. *Bioresour. Technol* 276:65-73.

Sunyoto N, Zhu M, Zhang Z, Zhang D (2016) Effect of biochar addition on hydrogen and methane production in two-phase anaerobic digestion of aqueous carbohydrates food waste. *Bioresour. Technol* 219:29-36.

Viggi C, Simonetti S, Palma E, Pagliaccia P, Braguglia C, Fazi S, Baronti S, Navarra M, Pettiti I, Koch C, Harnisch F, Aulenta F (2017) Enhancing methane production from food waste fermentate using biochar: the added value of electrochemical testing in pre-selecting the most effective type of biochar. *Biotechnol Biofuels* 10, 303.

Wang G, Li Q, Gao X, Wang X (2018) Synergetic promotion of syntrophic methane production from anaerobic digestion of complex organic wastes by biochar: Performance and associated mechanisms. *Bioresour. Technol* 250:812-820.

Wang G, Li Q, Li Y, Xing Y, Yao G, Liu Y, Chen R, Wang X (2020a) Redox-active biochar facilitates potential electron transfer between syntrophic partners to enhance anaerobic digestion under high organic loading rate. *Bioresour. Technol* 298, 122524.

Wang P, Peng H, Adhikari S, Higgins B, Roy P, Dai W, Shi X (2020b) Enhancement of biogas production from wastewater sludge via anaerobic digestion assisted with biochar amendment. *Bioresour. Technol* 309, 123368.

Wu B, Yang Q, Yao F, Chen S, He L, Hou K, Pi Z, Yin H, Fu J, Wang D, Li X (2019) Evaluating the effect of biochar on mesophilic anaerobic digestion of waste activated sludge and microbial diversity. *Bioresour. Technol* 294,
122235.

Xiao L, Sun R, Zhang P, Zheng S, Tan Y, Li J, Zhang Y, Liu, F (2019) Simultaneous intensification of direct acetate cleavage and CO₂ reduction to generate methane by bioaugmentation and increased electron transfer. *Chem Eng J* 378, 122229.

Yin C, Shen Y, Yuan R, Zhu N, Yuan H, Lou Z (2019) Sludge-based biochar-assisted thermophilic anaerobic digestion of waste-activated sludge in microbial electrolysis cell for methane production. *Bioresour. Technol* 284:315-324.

Yuan H, Ding L, Zama E, Liu P, Hozzein W, Zhu Y (2018) Biochar Modulates Methanogenesis through Electron Syntrophy of Microorganisms with Ethanol as a Substrate. *Environ Sci Technol* 52:12198-12207.

Zhang M, Li J, Wang Y, Yang C (2019) Impacts of different biochar types on the anaerobic digestion of sewage sludge. *Rsc Adv* 9(72): 42375-42386.