Biochar improves N cycling during composting of olive mill wastes and sheep manure.

•Biochar acted as an effective additive for olive mill waste composting.•Biochar favoured the initial activation of the composting process.•4% of biochar decreased N losses and doubled the mineral N content of compost.•Biochar did not affect the mobilisation of heavy metals in compost.•Biochar did not have a clear impact on CO2, CH4, and N2O emissions.