Bioretention basins are frequently subjected to anaerobic conditions, which can create an optimum environment for microbial activities to remove nitrogen (N) and sequester carbon (C) in the below-ground filter media. However, these biological processes are associated with the potential production of greenhouse gas (GHG) emissions that need to be measured. In this study, we quantified nitrous oxide (N 2 O), methane (CH 4) and carbon dioxide (CO 2) fluxes from the soil under a transition period from dry to wet conditions in subtropical Australia. The GHG fluxes were measured on the bed of slow (wet basin) and fast (dry basin) draining basins and their embankment area. In addition, the influence of Carex appressa plant on emissions was investigated. Finally, the denitrification potential of the basin soil and their C and N accumulation (over an 18-month interval) were measured. The dry and the embankment soils were both slight sinks of CH 4 (−16 and − 2 μg CH 4 − C m−2 h−1) while being a high source of CO 2 (> 520 × 103 μg CO 2 − C m−2 h−1). In comparison, the wet basin was a source of CO 2 and CH 4 with a mean value of 123× 103 μg CO 2 − C m−2 h−1 and 2405 μg CH 4 − C m−2 h−1, respectively. The dry and wet basins were a slight source of N 2 O emissions and were positively driven by precipitation. The presence of C. appressa plant increased CH 4 consumption and N 2 O generation. The results suggest that adopting a slow-draining design for bioretention systems, such as lowering the hydraulic conductivity and/or provision of a saturation zone higher in the soil profile, can reduce CO 2 and N 2 O fluxes from the soils and potentially improve water quality performance of these basins. However, an increase in CH 4 fluxes should be the expected by-product. Greenhouse gas emissions (μg m−2 h−1) from a dry and a wet bioretention basin and their embankment area under a dry and a wet condition. Unlabelled Image • Dry basin and embankment areas were both sinks of CH 4 but a large source of CO 2. • Slow draining design for bioretention basins can reduce CO 2 and N 2 O fluxes. • The dry and wet bioretention basins were a slight source of N 2 O emissions. • N 2 O fluxes were positively driven by rainfall in the dry and wet basins. • C. appressa plant can increase CH 4 consumption and N 2 O production. [ABSTRACT FROM AUTHOR]