Factors affecting the ability of extensive green roofs to reduce nutrient pollutants in rainfall runoff

Green roofs can retain urban rainfall runoff, but there are doubts about whether they can reduce urban nonpoint source pollution. To explore the factors affecting the ability of green roofs to reduce nutrients in rainfall runoff, nine types of extensive green roofs (EGRs) were analysed during 38 natural rainfall events and two early spring irrigation runoff events from 1 March to 30 November 2019 in Beijing. Differences among the module scale, growing medium material, growing medium depth, drainage layer material, planting time, rainfall characteristics and seasonal variation were examined to study their correlation with pollutant event mean concentration (EMC) and the load reduction performance of EGRs. The results showed that EGRs had higher total nitrogen (TN), ammonia nitrogen (NH₄⁺-N) and nitrate nitrogen (NO₃^--N) concentrations than traditional concrete roofs, but total phosphorus (TP) concentrations were similar, and EGRs could reduce some of the nutrient loads. One-way analysis of variance showed that the module scale, growing medium material, growing medium depth, drainage layer material, and planting time had no significant effect on TN and NO₃^--N concentrations (p > 0.05). The growing medium material had a significant effect on the TP concentration (p < 0.05). From the perspective of nutrient load reduction, module scale had a significant effect on TN and NH₄⁺-N loads (p < 0.05). The growing medium depth had a significant effect on NH₄⁺-N loads (p < 0.05). In addition, the growing medium material had a significant effect on TP loads (p < 0.05). When porous wool fibre and a bumpy plastic drainage board were selected as drainage layer materials, the effect on the NO₃^--N load differed significantly. In other situations, there were no factors with significant differences. In addition, the rainfall characteristics and seasonal variation influenced the pollutant concentration and EGR runoff load.