Variability of Potential Soil Nitrogen Cycling Rates in Stormwater Bioretention Facilities

Erin N Rivers,Jennifer L Morse

doi:10.3390/su14042175

Abstract

Low-impact development (LID) is a common management practice used to infiltrate and filter stormwater through vegetated soil systems. The pollutant reduction potential of these systems is often characterized by a single pollutant removal rate; however, the biophysical properties of soils that regulate the removal of pollutants can be highly variable depending on environmental conditions. The goal of this study was to characterize the variability of soil properties and nitrogen (N) cycling rates in bioretention facilities (BRFs). Soil properties and potential N cycling processes were measured in nine curbside bioretention facilities (BRFs) in Portland, OR during summer and winter seasons, and a subset of six sites was sampled seasonally for two consecutive years to further assess temporal variability in soil N cycling. Potential N cycling rates varied markedly across sites, seasons, and years, and higher variability in N cycling rates was observed among sites with high infiltration rates. The observed seasonal and annual changes in soil parameters suggest that nutrient removal processes in BRFs may be highly variable across sites in an urban landscape. This variability has important implications for predicting the impacts of LID on water quality through time, particularly when estimated removal rates are used as a metric to assess compliance with water quality standards that are implemented to protect downstream ecosystems.

Highlights

Urbanization is a rapidly growing form of land-use change occurring throughout the world [1]
To characterize soil properties and potential N cycling under contrasting seasonal conditions, nine curbside bioretention facilities (BRFs) were sampled in Portland, OR, USA during August and December 2015 to assess the responses of DEA, net nitrification (Nnit), N mineralization (Nmin), and microbial biomass N to seasonal differences in soil conditions
To estimate implications of potential denitrification rate differences at the site scale, we modeled two scenarios using the 10-year design storm used by the City of Portland to develop drainage standards (86.4 mm in 24 h) with average NO3 -N concentrations in stormwater runoff measured in Portland in 2020 (0.07 mg-N/L [22]) and the national average stormwater NO3 -N concentration (1.0 mg-N/L [45])

Summary

Introduction

Urbanization is a rapidly growing form of land-use change occurring throughout the world [1]. To mitigate the damaging effects of stormwater on urban water bodies, cities are increasingly adopting low-impact development (LID) approaches to manage stormwater runoff [3]. Types of LID span a wide range of designs, from completely engineered systems, such as porous pavement, to intact natural systems such as wetlands [4]. Engineered, vegetated LID systems such as wet basins, infiltration basins, stormwater planters, and bioswales are designed to mimic natural hydrologic processes and utilize retention, infiltration, and evapotranspiration processes in soil media and/or vegetation to manage urban stormwater runoff [5]. Based LID, such as bioretention facilities (BRFs), are designed to infiltrate a specified volume of runoff within a given period

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