# Agricultural Waste-Based Biochar for Agronomic Applications
## 4 Effect of Biochar on Greenhouse Gas Emissions  
Agricultural activities contribute significantly to the release of greenhouse gases into the atmosphere. Natural processes through photosynthesis can participate in CO\({}_{2}\) sequestration; however, the long-term efficiency of this process is limited, as the unstable fraction of C can be decomposed and returned to the atmosphere as CO\({}_{2}\) within months to years [188]. An efficient long-term C sequestration could be only possible by converting biomass into more stable forms of carbon [188; 189]. Biochar as a carbon material has been widely reported for its high potential for long-term carbon sequestration. The average residence time of biochar in soil can reach thousands of years [190; 191]. This is due to its recalcitrant character, which makes its organic fraction unavailable for soil microorganisms and allows for the long-term sequestration of carbon in soils. The results of a life cycle assessment for CO\({}_{2}\) sequestration in Miyako Island revealed that if all the available sugarcane bagasse, which is around 12,000 t/year, is converted to biochar, a total of 1200-1800 t CO\({}_{2}\) can be sequestrated each year [154]. [PERSON] et al. [192] showed a reduction in CO\({}_{2}\) emission in a silt loam soil after the addition of biochar derived from wood chip at a rate of 20% (_w/w_). Similar results were also found by [PERSON] et al. [193] in a biochar amended paddy soil. Contrary to this, [PERSON] et al. [194] claimed to find an increase in the CO\({}_{2}\) emissions after addition of biochar derived from mango (_Mangifera indica_ L.) trees. The same tendency was also reported by [PERSON] and [PERSON] [195], who observed increased CO\({}_{2}\) emissions from unvegetated arable plots amended with lump-wood-derived biochar; however, no significant increase was recorded for planted plots. Biochar was also reported for its ability to mitigate CH\({}_{4}\) and N\({}_{2}\)O emissions from soils [196; 197; 198]. The efficiency of biochar in mitigating CH\({}_{4}\) and N\({}_{2}\)O emissions is strongly related to soil type, cultivated crops and origin of biochar [124; 199]. In a 2-year experiment performed by [PERSON] et al. [200], biochar produced from rice straw was reported to be more capable to reduce CH\({}_{4}\) emissions from a paddy field than bamboo-derived biochar. The results show a reduction in CH\({}_{4}\) emissions of up to 86.43% during the rice growing cycle and an annual decrease between 20 and 51% after four years. Similar results were also described by [PERSON] et al. (2011). [PERSON] et al. (2022) recorded an increase in CH\({}_{4}\) emissions with a simultaneous decrease in CO\({}_{2}\) release after application of peanut shell and sewage sludge-derived biochars in bean plant growth. Biochar can also affect the denitrification process as reported by [PERSON] et al. (2003). The results show a reduction in N\({}_{2}\)O emission by about 10-90% in 14 different tested soils (2003). Biochar addition promotes the transfer of electrons to denitrifying microorganisms, which accelerates the process of N\({}_{2}\)O reduction. According to [PERSON] et al. (2004), the efficiency of biochar in mitigating N\({}_{2}\)O emission is related to its ability in affecting the relative abundance and the taxonomic composition of N\({}_{2}\)O-reducing functional microbial in soil. In the study of [PERSON] et al. (2005), biochars with relatively low N concentrations were reported as being more favorable in reducing N\({}_{2}\)O emissions. In the same line, [PERSON] et al. (2006) observed an increase in the transcription of bacterial N\({}_{2}\)O reductase following the addition of biochar in water-saturated soils, with a simultaneous increase in the abundance of N\({}_{2}\) fixation bacteria. Many investigations addressed their research to study the impact of biochar on greenhouse gas emissions.