Effects of Biochar and Sludge on Carbon Storage of Urban Green Roofs

Green roofs can improve urban ecological conditions by mitigating the heat island effect and absorbing harmful gases. Soil additives can improve roof soil properties and promote the stability of the urban ecosystem. As soil additives, biochar and sludge are widely used in the ground, but their application on roofs is still scarce. This study examined the carbon storage potential of green roofs amended with sludge and biochar. The roof soil was composed of soil and varying proportions of the additives (0%, 5%, 10%, 15%, and 20%, v/v); Sedum lineare was then planted. The carbon contents of the soils and plants were measured for one year. The influence of biochar or sludge on the carbon content of the roof soil and the factors affecting the roof carbon storage potential were analyzed. The results showed that the carbon storage potential of a biochar green roof (9.3 kg C m(-2)) was significantly higher than that of a sludge green roof (7.9 kg C m(-2)). Biochar increased the carbon content of the green roof by improving the physical properties of the roof soil and promoting plant growth, whereas sludge increased the carbon content of the green roof by improving the chemical properties of the roof soil. Moreover, biochar could not only store large amounts of stable carbon, but also reduce the weight of the green roof, improve soil moisture, and provide a resourceful utilization of municipal sludge. Thus, biochar can be considered a material for promoting carbon storage on green roofs.