Greenhouse gas and ammonia emissions during composting with different biochar types and particle sizes were investigated. Compared with powder-biochar, granular-biochar improved pore connectivity and was benefit to methanotrophs activities, like Methylococcaceae, reducing CH4 emissions. At the same particle size, bamboo biochar (BB) had a higher pore volume and more aerobic microenvironment within the compost than rice straw biochar (RSB), reducing GHG emissions. Bamboo biochar had high aromatic compound and NO3- concentrations and therefore surface pi-pi electron donor/acceptor interactions, causing low N2O emissions and inhibiting denitrifying bacteria (e.g., Bacteroidales). More C-O and C=O bonds in rice straw biochar than bamboo biochar caused lower NH3 emissions using rice straw than bamboo biochar. Powdered biochar had more exposed reactive functional groups and decreased NH3 production better than granular biochar. Powdered bamboo biochar controls gaseous emissions better than other biochars during aerobic pig manure/wheat straw composting.