Biochar as an additive has been used in different research to mitigate the emissions resulting from composting processes
[33][36][58][72][73]. This additive has been used as a sole material or mixed with other additives
[74]. Noteworthy, under almost all studied conditions, promising results were obtained, despite the lack of clarity regarding its mechanism on promoting nitrogen assimilation and nitrification
[5][31][38][75]. The change in nitrogen functional groups on the biochar surface was evidence for adsorption and microbial transformation of NH
3/NH
4+ [76]. As indicated in several works, the biochar promoted microbial activity during the composting process, as it increases the nitrogen source and decreases toxicity of free NH
3 on the microbial activity
[77]; hence, a high respiration rate as well as a fast decomposition of organic matter were recorded
[75][77][78]. Additionally, this was associated with an increase in the temperature and NO
3 concentration along with a decrease in the pH and NH
4+ concentrations
[71][73]. Emissions of NH
3 and nitrogen losses were reduced by 64% and 52%, respectively, when biochar was mixed with poultry litters
[37]. Similar results were observed when cornstalk biochar was used where cumulative NH
3 emissions were reduced by 24.8%
[79]. The presence of the biochar boosted the activity of nitrifiers due to its high sorption capacity for gases and the high cation exchange capacity. According to Zhou et al.
[80], adding modified biochar could significantly reduce NH
3 emissions by increasing the number of ammonia-oxidizing bacteria (AOB), inhibiting urease activity, and decreasing the abundance of nitrogen functional genes such as narG and nirS, facilitating the conversion of NH
+4-N into NO
−3-N and decreasing nitrogen loss. These conditions were responsible for promoting N
2O reduction up to 59.8%
[81]. The effects of bamboo charcoal (BC) and bamboo vinegar (BV) on lowering NH
3 and N
2O emissions during aerobic composting (Wheat straw and pig manure) revealed that both BC and BV enhanced nitrogen conversion and compost quality, with the combination BC + BV treatment achieving the greatest results. The BC, BV, and BC + BV treatments decreased NH
3 emissions by 14.35%, 17.90%, and 29.83%, respectively, and the N
2O emissions by 44.83%, 55.96%, and 74.53%. BC and BV reduced the NH
3 and N
2O emissions during composting
[82]. Similarly, Biochar (BC) and bean dregs’ (BD) effects on nitrifiers and denitrifiers, as well as contributions to NH
3 and N
2O emissions, were investigated by Yang et al.
[83]. When comparing the BD + BC treatment to the BD treatment, the highest value of NH
3 and N
2O emission was reduced by 32.92% and 46.61%, respectively. The number and structure of nitrogen functional genes were shown to be closely related to the synthesis of NH
3 and N
2O in the study. In this case, it was discovered that BD + BC enhanced the abundance of the AOB amoA gene, resulting in a reduction in NH
3 emission. The presence of nirS was more closely linked to the presence of N
2O. When compared to the BD treatment, the abundance of nirS in the BD + BC treatment was reduced by 18.93%, lowering N
2O emissions after composting. Furthermore, the nosZ-type gene was the most functional denitrification bacterial community to influence N
2O emissions.
[83]. Noteworthy, when biochar is to be used, it is important to keep in mind that its characteristics have a major role on its efficiency.