Tools to Quantify the Potential for Phosphorus Loss from Bioretention Soil Mixtures

Abstract

This study was conducted to determine if soil indices for phosphorus availability can be applied to bioretention systems. We tested a broad range of composts and biosolids in combination with three water treatment residuals (WTRs) as components of bioretention systems in batch incubation studies. Combinations of organics and WTRs were tested and observed P release was compared to expected release predicted by the phosphorus saturation ratio (PSR) and phosphorus saturation index (PSI). Phosphorus release ranged from 326 mg P·kg−1 in a food/yard compost to 7,227±1,277 mg P·kg−1 in anaerobically digested biosolids. Adsorption capacity of the WTRs ranged from 56 to 94% of P added. Specific surface area and carbon content of the WTRs had minimal predictive ability. Across a wide range of PSI (0.046 to 1.4) and PSR (0.13 to 6.5) values, these ratios were highly significant in predicting P release with an R2 value of 0.68 and 0.55, respectively. The adjusted R2 when a reactivity term was included was 0.78 and 0.68 for PSI- and PSR-extractable measures, respectively. Our work suggests both extracts are appropriate tools for predicting P release from bioretention mixes and that their predictability can be significantly improved by considering the reactivity term.