Solid digestate is often stocked or composted before its utilization due to the short legal period for soil spreading in the EU. These practices generate environmental pollution through nitrogen emissions. Biochar-supplemented composting has proven to be a promising strategy to address this issue. However, the main parameters to minimize nitrogen losses during digestate composting with biochar have not been identified. In this view, the current work studied the influence of biochar ratio (0, 5, and 10%), biochar type (produced at 450 and 900 degrees C), and digestate type (produced from agricultural waste and organic fraction of municipal solid waste) over NH3 emissions and Total Kjeldahl Nitrogen (TKN) losses during aerobic biodegradation. The results show that biochar and digestate type significantly affected TKN losses (p-values = 0.038 and 0.0007, respectively) and NH3 emissions (p-values = 0.07 and 0.002, respectively). Regarding biochar type, chemical characteristics (total oxygen and acid functions) play a more significant role in the biochar impact on the composting nitrogen cycle than the physical characteristics (surface area). Regarding digestate type, it was hypothesized that biochar addition to organic wastes with limited intrinsic nitrogen immobilization capacity could yield a higher percentage of NH3 emissions reduction compared to wastes with high nitrogen immobilization potential.