Methane production from ruminant livestock should be minimised in order to contribute to sustainable agriculture and climate change mitigation. There have been several nutritional strategies to reduce methane production from ruminants. Biochar supplementation to ruminants’ diets has attracted significant concerns recently. Supplementing biochar (with and without biochar, different levels of biochar) has shown to decrease from 5 to 25% methane production, both in vitro and in vivo experiments7–10. Different types of biochar used in different experiments were probably the reason for this high variation of methane reduction. Biochar characteristics can vary with biomass resources and pyrolysis procedures, leading to differences in rumen fermentation and gas and methane production12. Therefore, we hypothesised that biochars produced from different tropical biomass resources and at different processing temperatures have different characteristics, thus manipulating rumen fermentation and methane production.
In this study, biochar produced at higher temperatures had a larger surface area and higher water holding capacity. When processing temperature increased from 300 to 700oC, biochar surface area increased from 2.7 to 211.6; 1.1 to 98.6; and 5.4 to 154.2 m2/g, respectively for rice straw, corncob and bamboo tree; water holding capacity was increased from 4.8 to 6.6; 3.2 to 5.5; and 3.6 to 5.8, respectively (Table 1). This confirms the findings of Bonelli et al.13 .This is most likely due to the decomposition of organic matter and the formation of micropores, as explained by Katyal et al.14. In addition, according to Shaaban et al.15, a higher pyrolysis temperature causes the release of volatile matter and creates more pores. Moreover, Chen and Chen16 declared that the destruction of aliphatic alkyls and ester groups, as well as the exposure of the aromatic lignin core under higher pyrolysis temperatures, may result in increased surface area. According to Ghani et al.17, at lower pyrolysis temperatures, less than 500oC, lignin is not converted into a hydrophobic polycyclic aromatic hydrocarbon, and biochar becomes more hydrophilic. At higher pyrolysis temperatures, more than 650oC, biochar is thermally stable and becomes more hydrophobicity.
Increased processing temperature reduced total gas and methane production at different incubation times (Table 4). This results from increasing surface area and water holding capacity when biochar is produced at a higher temperature. Biochar with a larger surface area absorbs and adsorbs more gasses and/or methane5,7. In addition, methanotrophic proteobacteria and methanogenic archaea are the key bacteria responsible for methane production. Increasing the methanotrophs group increases methane oxidation, thus reducing methane accumulation18. In the rumen, biochar supplementation provides habitat and stimulates methanotrophic growth, thus reducing methane accumulation6. Furthermore, biochar produced from high pyrolysis temperature has high electrical conductivity and electron buffering capacity of fodder decomposing redox reactions11.
Biomass sources affected total gas and methane production. Corncob produced higher total gas and methane production than rice straw and bamboo tree derived biochars (Table 4). This is probably due to the smaller surface area and water-holding capacity of corncob derived biochar than its rice straw and bamboo tree counterparts. Effects of biomass resources on total gas and methane production were not consistent in the literature. Cabeza et al.9 reported that biochar prepared from Miscanthus reduced total gas and methane to the greatest extent and biochar prepared from rice husk and softwood pellets were least effective. Hansen et al.7 reported that straw-derived biochar numerically reduced methane to a greater extent than wood-derived biochar. However, Calvelo Pereira et al.19 did not find any differences in total gas and methane production between wood and crop residue derived biochar (i.e. corn stover and pine wood chips). McFarlane et al.12 found no effects of biomass resources (Chrestnut, Yellow Poplar, White Pine) on gas production. Calvelo Pereira et al.19 and Gurwick20 found no clear relationships between biochar chemical composition and in vitro total gas and methane production. This may explain for non-effects of biomass resources on total gas and methane production. According to Teoh et al.21, the variable success rate of past biochar studies at reproducing significant methane mitigation has largely been attributed to variation in biochar properties such as the particle size, adsorptive potential, electrical conductivity, and ability to act as an electron mediator in redox reactions during digestion.
In this study, biomass sources affected DM digestibility at 4, 24 and 48 hours and OM digestibility at 4 and 24 hours of the incubation. Processing temperature affected DM digestibility at 4, 24 and 48 hours, and OM digestibility at 48 hours of the incubation. Effects of biochar supplementation to diets on DM and OM digestibility were not consistent in the literature. Winders et al.10 could not find differences in DM and OM digestibility between two levels of 0.8 or 3% biochar supplementation. This was also confirmed by Hansen et al.7. According to Teoh et al.21, supplementing up to 800 mg/day hardwood biochar over a 15 day period did not affect DM digestibility. Teoh et al.21 also argued that biochar is 100% inorganic matter and not metabolised by the rumen microbiota. However, Saleem et al.22 found improved DM and OM digestibility when biochar was supplemented to diets. They explained that biochar encourages biofilm creation, which stimulates the growth of desirable microbes by providing a niche for their continued proliferation. On the contrary, McFarlance et al.12 found reduced DM digestibility when biochar was supplemented with the level of 81 g/kg DM. The authors argued that the inconsistent findings were due to differences in biomass sources, particle size, and pyrolysis conditions.
The pH and NH3-N are important parameters regulating rumen fermentation. Therefore it is important to study the effects of biochar supplementation on the pH and NH3-N. Literature shows the effects of biochar supplementation and sources on the pH and NH3-N concentration. For example, Zhang et al.23 declared that biochar supplementation leads to increased pH value due to the alkaline nature of the biochar. Mirheidari et al.24 reported a stable of ruminal pH due to the lack of changes in primary ruminal fluid VFA, acetic and propionic concentrations among treatments of 0 (no added biochar; control), 1% walnut shell biochar (WSB), 1% pistachio by-product biochar (PBB), and 1.5% chicken manure biochar (CMB). Cabeza et al.9 reported a reduction in ruminal pH and NH3-N concentration when biochar was prepared from Miscanthus straw, oilseed rape straw and softwood pellets at 1.16% of feed substrate were added to incubations. However, Mirheidari et al.24 reported that the inclusion of WSB, PBB and CMB increased ruminal NH3-N concentration by 34.5; 25.06 and 18.89%, respectively.
In this study, a higher processing temperature decreased the pH and increased the NH3-N concentration after 24 and 48 hours of incubation. We expected a reduction of NH3-N concentration when biochar was produced at a higher temperature because of its larger surface area, which can adsorb more NH3-N. However, it was not the case in this study. This can be explained. Effects of biochar on the pH and NH3-N concentration also depend on the archaeal and bacterial rumen microbiota, the fungal community structure, VFA concentration. In this study, those criteria were not measured. Future studies should analyse the rumen archaeal, bacterial, and fungal microbiotas, VFA concentration. Another speculation is that NH3-N concentration also depends on (i) proteolysis and deamination of nitrogen constituents in the substrate and (ii) incorporation of NH3-N into microbial protein or combine the two processes. Increased biomass processing temperature resulted in reduced gas production, which is the energy supply for microbial growth. This can be the reason for increasing nitrogen deamination to provide energy for microbial growth. This process releases NH3-N, which results in a higher NH3-N concentration.