An investigation of the potential to use street trees and their root zone soils to remove nitrogen from urban stormwater

Abstract

Eutrophication in coastal ecosystems is often attributed to elevated nitrogen levels. Reducing the amount of nitrogen entering Port Phillip Bay is therefore an important objective for Melbournians. This article details the performance of a pilot scale street tree bioretention system in reducing nitrogen loads in urban stormwater. Three tree species (Eucalyptus polyanthemos, Lophostemon confertus and Platanus orientalis) and three sandy soils of different hydraulic conductivity (4, 95 and 170 mm/hr) were tested in a randomised block design. Over the course of the experiment, all species displayed seasonal height growth patterns with maximum growth rates occurring in late Spring and Summer. Applications of stormwater increased height growth and root length density compared with tapwater applications. Tree growth was similar in the three soils studied. During the warmer months, leachate volumes from the planted systems were significantly less than the unplanted systems due to transpirational drying. Leached nitrogen loads were significantly reduced in systems with a tree. Whilst there were some statistically significant differences in ammonium, oxidised nitrogen and organic nitrogen removal between species, these were not large in practical terms. In December, total nitrogen load removal from stormwater was 82 to 95% in planted systems (L. confertus), compared to 36 to -7% in the unplanted systems

Keywords:

• Bioretention
• leachate
• nitrate
• ammonium
• total nitrogen
• Lophostemon confertus
• Eucalyptus polyanthemos
• Platanus orientalis

Additional information

Notes on contributors

L Denman

Liz Denman is a horticulturist currently undertaking a PhD in the Faculty of Land and Food Resources at The University of Melbourne. The PhD research investigates the potential to use street tree bioretention systems to remove nitrogen and phosphorus from urban stormwater. Her major interests include the establishment and management of vegetation in urban areas and also the environmental benefits afforded by plants in the landscape.

P B May

Peter May is a horticultural consultant specializing in urban and landscape soils and vegetation. Prior to establishing his consultancy business he was for 30 years a member of the academic staff of the Burnley Campus of The University of Melbourne where he taught across a range of urban horticulture subject areas. He has particular interest in urban soils and their management and also in the establishment and management of urban vegetation.

P F Breen

Peter Breen is a founding partner and director of Ecological Engineering. He has extensive experience in water resources management and the impact of landuse change on aquatic ecosystems. This experience has been gained in both the rural and urban environments with appointments in industry, academia, research and consulting.

Peter has published on aquatic botany, wetland, stream and lake ecology, stormwater and wastewater treatment, water quality management and restoration ecology and has authored or co-authored over a 100 papers and delivered numerous presentations. Peter’s research and design expertise has contributed to: urban stream ecology in Australia (Peter established and led the urban ecology group in the Cooperative Research Centre for Freshwater Ecology, Monash University from 1992-2001), best practice stormwater management objectives, and guidelines on constructed wetland design, waterway design, bioretention system design, lake design, vegetation design for constructed wetlands and bioretention systems.