Increasing the ability of a green roof to provide ecosystem services by adding organic matter and earthworms

Abstract

The aim of this study was to increase the ecosystem services provided by a green roof by ameliorating the biological and chemical properties of a commercial green roof substrate through the addition of earthworms and compost. We conducted a mesocosm (120 × 80 × 33 cm) experiment with a plant community comprising Hylotelephium maximum,Centaurea jacea, Lotus corniculatus, Koeleria glauca, and Dianthus carthusianorum. Two substrates were compared (i) a low complexity commercial green roof substrate with no alterations and (ii) a high complexity substrate with a layer of locally produced vermicompost and earthworms (Lumbricus terrestris) placed on top of the commercial substrate. The mesocosms were placed on the roof of a 20 m high building and at ground level.Results show that the percent of vegetative cover on both substrates was lower in the roof mesocoms than on those placed at ground level. This is explained by climatic conditions, such as higher light intensity at roof level. The substrate with earthworms and vermicompost had significantly higher enzyme activity, microbial biomass, and metabolic activity. This resulted in more available nitrogen and phosphorus for plants, increased the plant biomass, floral nectar volume and sugar concentration which resulted in the flowers receiving more diverse and abundant insect pollinators. This is the first time that adding earthworms and compost to a green roof substrate has been shown to have a significant effect on plant growth and plant-pollinator interactions. These findings will help in designing green roof systems that are more efficient at preserving ecosystem services in urban areas.