Role of Mycelium in Bioretention Systems: Evaluation of Nutrient and Metal Retention in Mycorrhizae-Inoculated Mesocosms

Bioretention systems have become an increasingly common method for treating stormwater in urban areas, which help reduce peak flows and remove contaminants from stormwater. However, nutrients often leach out of the bioretention soil mix, which can contribute to the degradation of receiving waters in bioretention systems with underdrains. Development of mycelium may improve retention of nutrients and increase the water-holding capacity. To evaluate the impact of mycelium on nutrient leaching from bioretention systems, ectomycorrhizal and endomycorrhizal fungi were added to the bioretention soil mix to promote mycelium growth. A proprietary mix with bacteria and mycorrhizal fungi also was tested. Mesocosms were planted with Carex stipata, a native sedge with endomycorrhizal associations. Four tests were conducted with collected stormwater. Lower rates of phosphorus export were observed in mescocosms with mycorrhizal fungi; the export of total phosphorus was reduced by 13-48%, and the export of phosphate was reduced by 14-60%. There also was evidence of additional copper and nitrate uptake in mesocosms with mycorrhizal fungi. Retention of total phosphorus and phosphate, rather than export, was observed in mesocosms with the proprietary mix, but export rates of nitrate were high. This study indicates that mycelium may help reduce phosphorus export from bioretention systems.