Presentation Title

Characterization of Soil Phosphorus and its Potential for Mobilization in Stormwater Bioretention Facilities

Start Date

February 2018

End Date

February 2018

Abstract

Urban bioretention facilities (BRF) are designed to reduce peak discharges to streams, while also retaining pollutants in soils and plants. While these structures have been shown to function well hydrologically, the nutrient retention aspects are poorly understood. The target pollutant in this study was soil phosphorus (P), with the objective of understanding whether P is vulnerable to mobilization in BRF soils. Low-oxygen conditions during flooding can cause P release from soil minerals, while oxidizing conditions during drier periods enables microbial mineralization of organic P, with either scenario potentially releasing soluble phosphate. We sampled 16 bioretention soils from municipally managed “Green Streets” facilities in Portland, OR, across an infiltration rate gradient. Sequential P extractions and tests for degree of P saturation (DPS) and P sorption index (PSI) were performed alongside other soil characteristics. We assessed the effects of drying and flooding on P mobilization from soils with experimental soil incubations. The 16 sites showed a wide range in total P (TP), organic matter (OM) and P forms. The finding of a strong relationship between water-extractable inorganic P and DPS suggests that phosphate is being released from associations with iron and aluminum minerals. Flooding and drying treatments showed evidence of phosphate release, particularly in a soil with greater TP and mineral-associated P forms. These findings show that hydrological patterns might alter profiles of P forms, and stress the P retention capacity of some bioretention soils that experience extended drying and flooding cycles.

Subjects

Soil science, Sustainable development, Water quality

Persistent Identifier

https://archives.pdx.edu/ds/psu/25627

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Characterization of Soil Phosphorus and its Potential for Mobilization in Stormwater Bioretention Facilities

Urban bioretention facilities (BRF) are designed to reduce peak discharges to streams, while also retaining pollutants in soils and plants. While these structures have been shown to function well hydrologically, the nutrient retention aspects are poorly understood. The target pollutant in this study was soil phosphorus (P), with the objective of understanding whether P is vulnerable to mobilization in BRF soils. Low-oxygen conditions during flooding can cause P release from soil minerals, while oxidizing conditions during drier periods enables microbial mineralization of organic P, with either scenario potentially releasing soluble phosphate. We sampled 16 bioretention soils from municipally managed “Green Streets” facilities in Portland, OR, across an infiltration rate gradient. Sequential P extractions and tests for degree of P saturation (DPS) and P sorption index (PSI) were performed alongside other soil characteristics. We assessed the effects of drying and flooding on P mobilization from soils with experimental soil incubations. The 16 sites showed a wide range in total P (TP), organic matter (OM) and P forms. The finding of a strong relationship between water-extractable inorganic P and DPS suggests that phosphate is being released from associations with iron and aluminum minerals. Flooding and drying treatments showed evidence of phosphate release, particularly in a soil with greater TP and mineral-associated P forms. These findings show that hydrological patterns might alter profiles of P forms, and stress the P retention capacity of some bioretention soils that experience extended drying and flooding cycles.