Role of biochar as an additive in organic waste composting.

•Biochar enhances environmental conditions for microbial growth in composting piles.•Key biochar properties include porosity, surface area and cation exchange capacity.•Biochar application at 10% is recommended for optimum composting performance.•Biochar at 10–30% rates succeeded in mitigating NH3, N2O and CH4 emissions.•Biochar decreased bioavailability of Cu and Zn in composts.