Greenhouse gas emissions from stormwater bioretention basins

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 (N₂O), methane (CH₄) and carbon dioxide (CO₂) 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 CH₄ (−16 and − 2 μg CH₄ − C m⁻² h⁻¹) while being a high source of CO₂ (> 520 × 10³ μg CO₂ − C m⁻² h⁻¹). In comparison, the wet basin was a source of CO₂ and CH₄ with a mean value of 123× 10³ μg CO₂ − C m⁻² h⁻¹ and 2405 μg CH₄ − C m⁻² h⁻¹, respectively. The dry and wet basins were a slight source of N₂O emissions and were positively driven by precipitation. The presence of C. appressa plant increased CH₄ consumption and N₂O 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 CO₂ and N₂O fluxes from the soils and potentially improve water quality performance of these basins. However, an increase in CH₄ fluxes should be the expected by-product.