Environmental Advances (Jul 2023)
What happens to nitrogen and phosphorus nutrient contributions from green roofs as they age? A review
Abstract
Green roofs (GR) have gained widespread popularity in cities due to their multiple benefits. This review synthesizes the latest knowledge around GR ageing and the associated changes in nutrient runoff quality from GRs. Most GR studies have focused on water quality trends for the first 10 years. Information about nutrient levels in runoff, vegetation, and substrate for older GRs (>10 years) is extremely limited and based on monitoring at snapshot points in time, rather than continuously over time, mainly for GRs in Europe, UK and the USA. Nitrogen (N) and phosphorus (P) levels in GR runoff are initially high, especially in the first-year post-installation (NO3: 0.08-100 mg/L, PO4: 0.01-13 mg/L) and subsequently decline to lower concentrations over time (NO3: 0.05-7 mg/L, PO4: 0.01-1.2 mg/L for GRs over 10 years old). Green roof substrate received most research attention but there is growing interest in how vegetation and microbiome characteristics change with time. The review provides a conceptual model of GR ageing that includes the impacts of seasonal changes and other disturbances (e.g., fertilizer applications, extreme events) on nutrient runoff quality. Recommendations for GR design and management are also provided. Future areas of research should focus on long-term holistic monitoring of all GR components across more climate zones, especially arid and tropical climate zones. Studies comparing GRs of different ages are encouraged. Ageing experiments conducted under controlled laboratory conditions complement field monitoring studies and provide a continuous timeline of changes in GR components and runoff quality as they age.
