Response mechanism of filler-plant in bioretention systems under different inflow load effects

The response mechanisms and quantitative analysis of runoff pollution reduction, accumulation effect of pollutants in media, and plant physiological characteristics for bioretention systems remain inadequately investigated. In this study, we constructed pot-scale systems with a mixture of [soil + sand + leaf litter compost + Ophiopogon japonicus], and designed the inflow condition by L39 orthogonal combinations under the impact conditions of runoff pollution load from different functional zones/underlying surface. The indicators of inflow/outflow, media, and plant aboveground and underground parts were tested. The results showed that the bioretention systems under different pollutants load impacts had a significant effect on runoff ammonium nitrogen (NH4+-N), total phosphorus (TP), and chemical oxygen demand (COD), with load reduction rate of 11.9%-71.8%, 8.3%-43.7%, 5.1%-30.1%, respectively. Significant leaching was observed with nitrate nitrogen (NO3--N) and total nitrogen (TN). The results of multiple linear regression analysis showed that the load reduction of carbon, nitrogen, and phosphorus (C, N, and P) in runoff is positively correlated with their inflow concentrations (R2 > 0.8), and the interaction between them is not significant. Under different inflow load impacts, the variation ranges for chlorophyll, malondialdehyde, and biomass are 63.4%, 77.4%, and 59.8%, respectively. The growth of plants is mainly influenced by the inflow concentration of C and N. The research results contribute to a deeper understanding of the impact of incoming pollutant concentrations on bioretention systems, and are helpful for the long-term management of bioretention systems and the assessment of potential risks.