<p>Biochar has intrinsic and nascent structural and sorption properties that may alter the physical and chemical properties of a composting mixture thus influencing the production of greenhouse gases [GHGs" nitrous oxide (N₂O) and methane (CH₄)]. In this study, contrasting biochars produced from green-waste (GWB) or poultry litter (PLB) were incorporated into a composting mixture containing poultry litter and straw, and GHG emissions were measured in situ during composting using Fourier Transform Infrared Spectroscopy (FTIR). Emissions of N₂O from the biochar-amended composting mixtures decreased significantly (<i>P</i> < 0.05) soon after commencement of the composting process compared with the non-amended control. The cumulative emissions of N₂O over 8 weeks in the GWB composting mixture (GWBC), PLB composting mixture (PLBC) and control (no biochar) were 4.2, 5.0 and 14.0 g N₂O-N kg⁻¹ of total nitrogen (TN) in composting mixture, respectively (<i>P</i> < 0.05). The CH₄ emissions were significantly (<i>P</i> < 0.05) lower in the GWBC and PLBC treatments than the control during the period from day 8 to day 36, when anaerobic conditions likely prevailed. The cumulative CH₄ emissions were 12, 18 and 80 mg CH₄-C kg⁻¹ of total carbon (TC) for the GWBC, PLBC and control treatments, respectively, though due to wide variation between replicates this difference was not statistically significant. The cumulative N₂O and CH₄ emissions were similar between the GWBC and PLBC despite differences in properties of the two biochars. X-ray Photoelectron Spectroscopy (XPS) analysis and SEM imaging of the composted biochars indicated the presence of iron oxide compounds and amine-NH₃ on the surface and pores of the biochars (PLB > GWB). The change in nitrogen (N) functional groups on the biochar surface after composting is evidence for sorption and/or reaction with N from labile organic N, mineral N, and gaseous N (e.g. N₂O). The concentration of NH₄₊ increased during the thermophilic phase and then decreased during the maturation phase, while NO₃₋ accumulated during the maturation phase. Total N retained was significantly (<i>P</i> < 0.05) higher in the PLBC (740 g) and the GWBC (660 g) relative to the control (530 g). The TC retained was significantly higher in the GWBC (10.0 kg) and the PLBC (8.5 kg) cf. the control (6.0 kg). Total GHG emissions across the composting period were 50, 63 and 183 kg CO₂-eq t⁻¹ of initial mass of GWBC, PLBC and control (dry weight basis) respectively. These results support the co-composting of biochar to lower net emissions of GHGs while increasing N retention (and fertiliser N value) in the mature compost.</p>