Can hydrological function and vegetation cover of failed green roofs be restored with wood-waste-derived biochar?

Green roofs are becoming more necessary due to their multiple functions, like mitigation of runoff, absorption and filtering pollutants, and air temperature moderation in urban areas. Therefore, applying and improving green roofs are currently being investigated by many researchers. Biochar could be used to store carbon and enhance soil properties, and biochar addition is a potentially reliable approach to enhance green roofs’ functions in the future. Our experiment applied different types of biochar (pelletized biochar and unprocessed biochar) and different application methods (surface treatment and mixing treatment) to compare which treatments had positive effects on hydrological function and vegetation growth of green roofs. Results showed that the surface pelletized biochar, surface unprocessed biochar, and mixing unprocessed biochar treatments significantly reduced water discharge volume and peak discharge rate of green roofs under moderate storm (total precipitation depth of 8.33 mm). Vegetation cover was not significantly affected by biochar treatments even if surface pelletized biochar treatment had higher vegetation cover based on two short-term vegetation surveys. Hence, long-term observations on vegetation restoration are required in the further research to explore the effects of biochar addition on vegetation growth. Optimal biochar treatment should be considered from multiple aspects including hydrological function, plants growth, and cost of application. Surface pelletized biochar was the optimal application due to better hydrological performance and vegetation restoration, and lower wind loss