While the use of bioretention for stormwater management is widespread, data about the impacts of plants and microorganisms on long-term treatment efficacy remain region-specific. To help address this knowledge gap for the Pacific Northwest region of the United States, we installed twelve under-drained bioretention mesocosms built to Washington State Department of Ecology stormwater management standards in an urban watershed in Seattle, WA that included a busy portion of Interstate 5. Six mesocosms were planted with Pacific ninebark (Physocarpus capitatus
) and six were inoculated with the wine cap mushroom (Stropharia rugoso-annulata
) resulting in four replicated factorial treatments. Because region-specific studies must be mindful of the prevailing regulatory framework, all mesocosms used the Washington State Department of Ecology design standard soil: a blend of 60% sand and 40% compost by volume, despite the known leaching problems with high compost volume fraction soils. Five water quality sampling events over 15 months of continuous stormwater loading were analyzed for dozens of water quality parameters. Multiple linear regression analyses of treatment differences over the 400-day loading period illustrate that incorporating fungi into the wood mulch slowed the release of total and ortho-phosphorus from the bioretention soil; however net export of phosphorus from this compost rich media continued through 400 days of loading for all treatments. Multivariate ordination methods illustrate that time and temperature dramatically affect performance of this media, but the impact of planting and fungal inoculation had marginal detectible effects on overall water quality during the study timeframe. These results demonstrate that future studies of this media blend must plan for at least one year of nutrient and metal leaching before the time-dependent heterogenous variance introduced by these exports will no longer pose an obstacle to analysis of other performance changing factors. The results highlight important physical and chemical considerations for this media blend, and the opportunity for continued research on the use of fungal inoculated mulch application as a new ecological engineering tool for reducing phosphorus leaching from soils.
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