Biochar effect on greenhouse gases mitigation was mainly studied in amended mineral soils and little information is available for peat-based growing media (PBGMs) used in greenhouse production. We established a 58-d PBGM microcosm incubation study to investigate the effect of different biochars and a compost on the mineralization of carbon (C) and nitrogen (N) and on mitigation of CO 2 , CH 4 and N 2 O. Biochars and compost were added respectively at a rate of 15% and 4% (v/v) and a weekly mineral N fertilizer was applied. Five biochars were produced from three feedstocks at three pyrolysis temperatures: maple bark 400 °C (M400), 550 °C (M550) and 700 °C (M700); willow chips 400 °C (W400); and pine chips 700 °C (P700). Biochar addition increased C and decreased N availability in fertigated PBGMs. The highest microbial biomass C and N were obtained (p < 0.05) with M400 rich in volatile matter, and with compost treatments. Microbial enzymatic activity was lower (p < 0.05) in maples and W400 treated PBGMs than in P700 and control. Addition of compost and maple biochars produced at 400 °C and 550 °C lowered the microbial metabolic quotient. The highest mitigation efficiency of CO 2 was obtained with M550 which released in 58-d, 50% less than other treatments. The total cumulative CH 4 emission was also 21% lower in maple PBGMs than in control or P700. The total cumulative N 2 O emission was reduced by 66% by the addition of alkaline biochars (M400, M550, M700 and W400) in the PBGM without compost. This mitigation can be associated with the important ammonium adsorption by these biochars which can slow the nitrification process, or to a probable increase in the N 2 /N 2 O ratio. Compost addition had no effect on the N 2 O fluxes from all treatments. This work indicates that biochars can be favorably used as an amendment to PBGMs for the development of a sustainable greenhouse production.