The influence of vegetation type on runoff nutrient concentrations from an extensive green roof system

Abstract

Green roofs, also known as vegetated roofs, have been shown to provide a wide range of environmental benefits in urban areas. However, runoff from green roofs may contain soil nutrients like nitrogen and phosphorus, as well as dissolved carbon, which are contaminants if present in excess in water. These contaminants degrade water quality, and as a result can lead to nutrient enrichment problems downstream. This study compares concentrations of total nitrogen, nitrate, and dissolved carbon in the runoff from a modular, extensive green roof system among different vegetation types. Additionally, a supplementary analysis of the growing media was assessed for nitrate, phosphorus and potassium levels. Given that plant species differ in their nutrient requirements, identifying species with high nutrient demands could help reduce runoff nutrient concentrations. Analysis of runoff was done both prior to, and following the addition of a controlled-release fertilizer. Results from runoff nutrient analysis show that, for both total nitrogen and nitrate, vegetation type had a strong effect. Treatments with overall higher amounts of living biomass, such as species mixtures and monocultures of Sedum acre, resulted in better runoff quality (i.e. lower nitrogen concentrations) than those with little to no biomass. A regression between biomass and runoff nutrient concentrations suggests that nutrient uptake is affected in part by biomass production in green roof systems. Selection of higher biomass species or species mixtures can help improve the environmental performance of green roofs.