The influence of green roof features on their carbon and nutrient stocks

Extensive green roofs can be implemented to counteract the adverse effects of nature loss in urban areas by providing ecosystem services, such as biodiversity increase, storm water management and temperature regulation. Green roofs also sequester carbon (C) and retain other nutrients, improving urban air and rainwater quality. This study examined how green roof characteristics (i.e. green roof age, substrate depth, fertilization, vegetation type and composition) affect total carbon (TC), total nitrogen (TN) and total phosphorus (TP) stocks in the green roof substrate, as well as two important nitrogen fluxes (mineralization and nitrification). We hypothesized that vegetation type ( Sedum -only vs. diverse vegetation), substrate depth and fertilization would be the main characteristics affecting TC, TN and TP substrate stocks and nitrogen-fluxes.Twelve extensive green roofs in three cities in Flanders, Belgium, were sampled across four seasons. Results showed that green roofs have a low C sequestration potential compared to natural ecosystems. Roofs with diverse vegetation had higher TC and TN substrate stocks, particularly those with mosses and herbs. Fertilization had no significant effect on vegetation or TC, TN, and TP substrate stocks, while substrate depth had a significant effect only on TP substrate stocks. Overall, green roofs offer limited C sequestration and nutrient retention, although optimizing substrate composition and increasing plant species richness could enhance these benefits. This study highlights the potential for improving green roof performance through better design and management practices.