## 3 Benefits of Biochar Amendment in Green Roofs
### Plant Health and Microbial Diversity  
Vegetation is a crucial component of a GR system; hence, plant survival is essential for maintaining the expected serviceability of GRs. Plant performance is understood to strongly depend on the condition of substrates. Biochar is well known as an effective soil additive to improve soil fertility. The porosity of substrates increased by biochar leads to enhancements in water/nutrient retention, water saving for irrigation, plant drought-stress resistance, and plant root development. Additionally, it is noteworthy that microorganisms or microbes exist in GR substrates. Thus, adding biochar makes GR substrates more porous and hence creates more shelters for microorganisms [83]. Interactions between plants and the microbial community are beneficial to the biomass of plants and microorganisms [107]. This microbiome benefits plants through different mechanisms including improving physical and chemical characteristics of substrates; producing and modifying plant hormones; and producing antibiotics, volatile organic compounds, and lytic enzymes against phytopathogenic microbes [108]. GRs are facing challenges in creating an ideal living place for plants and microbes due to thin substrates, limited irrigation, strong wind, extreme solar heat gain, and isolation from habitats at the ground level [107; 109]. Consequently, some preliminary studies [110] have observed a small number of microorganisms in GR substrates, which could lead to a decline in plant performance and limit the coservices of GRs. Biochar, being a carbon-rich material, is utilized to generate a favorable environment for microorganisms and plants. Granulated biochar was studied and compared with traditional biochar in [89] to assess their impacts on plant performance. The results revealed that granulated biochar outperformed traditional biochar by significantly improving plant biomass. Moreover, plant biomass from traditional biochar was even lower than that from non-biochar GRs. Biochar granules were also beneficial in reducing biochar loss caused by water and wind erosion. Biochar also demonstrated effectiveness in supporting the soil bacterial community. Adding biochar to GRs in [109] increased the biomass of soil microbes, eukaryotes, and plants by 75.3%, 199.2%, and 57.5%, respectively. This was due to an increase in soil porosity (by 5.3-9.3%), soil moisture (by 14-37.2%), available nutrients, and cation exchange capacity (by 38.1-75.9%). [PERSON], et al. [111] also recommended the use of biochar to increase the yield of _Lactuca sativa_ L. var. crispa (lettuce). [PERSON], et al. [112] successfully increased the biomass of microorganisms and plants by 63.9-89.6% and 54.0-54.2%, respectively, using a 10-15% (\(v/v\)) sludge biochar application rate. The combined effects of compost and biochar were assessed in the study by [PERSON], et al. [113]. Increases in the tomato size and biomass of leafy vegetables planted on rooftop gardens were identified. In the study by [PERSON], [PERSON] and [PERSON] [57], biochar outperformed other organic wastes such as sawdust, wood bark, coir, and compost in terms of plant drought resistance and vegetation growth. In addition to improved (temperature-reducing) evapotranspiration, and enhanced nutrient and water retention, [PERSON], [PERSON], [PERSON], [PERSON], [PERSON] and [PERSON] [80] confirmed that biochar-amended GRs also supported vegetation growth. Regarding Plant Available Water (PAW) and the Permanent Wilting Point (PWP), the GR substrate amended with 30% (\(v/v\)) urban green waste biochar in [114] improved PAW by 16% and delayed PWP by 2 days. [PERSON], et al. [115] advocated for the utilization of biochar to improve plant performance. Similarly, they found that biochar had a higher PAW and longer PWP, thereby decreasing the irrigation volume and days of plants experiencing water stress. The PWP was delayed by 3 days with the addition rate of 20% (\(v/v\)) biochar. The plant coverage area was investigated in the study by [PERSON] and [PERSON] [78].