Biochar Improves Sustainability of Green Roofs via Regulate of Soil Microbial Communities

Green roofs are an important part of urban green spaces. A good roof soil system contributes to the stability of the green roof ecosystem in harsh environments. Biochar as a soil additive can improve soil nutrients, although the mechanism of improvement on the roof substrate is still unclear. This research studied the effects on the physical and chemical properties of green roof soil and analyzed the biological characteristics of green roofs at five gradient ratios of biochar addition (0%, 5%, 10%, 15% and 20% biochar; v/v). The results indicated that biochar could improve the soil porosity (5.3-9.3%) and reduce the bulk density (3.9-10.8%); increase the soil moisture (14.0-37.2%); adjust the temperature, available nutrients and cation exchange capacity (38.1-75.9%) and regulate the soil pH values of the green roof. The biomass of soil microbes, eukaryotes and plants were increased by 75.3%, 199.2% and 57.5%, respectively. Meanwhile, the correlation between microbial diversity and soil nutrients was more significant due to the addition of biochar, and the increase of the phosphorus (P) and carbon (C) contents was the main factor affecting the microbial community. The structural equation model showed that biochar has a direct impact on the microbial diversity by improving the soil moisture, temperature and available nutrients, and the increase of the microbial diversity is conducive to plant growth. Summarily, biochar can be considered as a potential additive for roof soil amendment and promoting the growth of plants and microbes, which is beneficial to the development of a roof ecosystem.