Influence of low-phosphorus compost and vegetation in bioretention for nutrient and sediment control in runoff from a dairy farm production area

We evaluated the effects of a low-phosphorus compost and vegetation (Panicum virgatum) on the ability of three bioretention cells to remove nutrients and total suspended solids (TSS) from storm runoff originating in a dairy farm production area. The three experimental treatments included a bioretention cell without compost or vegetation (C[sbnd]V[sbnd]), a cell with no compost but vegetation (C-V+), and a cell with vegetation and compost (C + V+); the experimental design allowed for isolation of the variables' (i.e., compost and vegetation) effects on pollutant concentration and mass removal. Influent and effluent were monitored for dissolved and total phosphorus (P) and nitrogen (N), and TSS throughout the first growing season post installation. All bioretention cells significantly reduced pollutant concentrations and loads of TSS, total and dissolved P, and NH₄ ⁺ in the effluent, but significantly exported NO_x ⁻ to the effluent. All cells reduced total N (TN) concentrations, with significant reduction observed only in the C-V+ cell, which was the only treatment that retained TN load. Compost treatment significantly increased P. virgatum biomass, but not survival during the first year. P. virgatum coverage in compost and no-compost treatments were similar the second year. While low-P compost is still preferable to other composts higher in P content, we cannot conclude that any compost is necessary in the soil media for plant establishment. Due to anticipated nutrients inputs in a setting like this dairy operation site, omitting compost and avoiding nutrient leaching while waiting for plants to grow should be considered for better nutrient removal performance, even if plants may take more time to establish.