## 2 Biochar--Preparation and Unique Physiological Characteristics
The primary surface functional groups of biochar include aromatic and heterocyclic carbon, which play an important role in determining the properties of biochar [52]. Consequently, biochar with a large surface area and high cation exchange capability enhances soil productivity and helps reduce groundwater contamination [53]. Due to the presence of alkali and alkaline-earth metals in biochar, soil pH is also increased, which is advantageous particularly for acidic soils [39]. An increase in soil pH affects the amounts of soluble essential nutrients and chemicals, thereby enabling the growth of plants and microbial communities [54; 55]. Biochar itself also hasnutrients whose availability depends on the feedstock and pyrolysis temperature, further contributing to the nutrient availability in the soil [50]. Figure 4 provides a summary of the practical applications of biochar in GRs. Figure 5 presents the mechanisms by which biochar alters the structure of GR substrates and improves its water-holding capacity. The aforementioned physiochemical parameters of biochar are crucial for determining its effectiveness in green roofs. Optimal parameters can be achieved by carefully selecting the raw biomass and adjusting the pyrolysis conditions, such as the temperature, residence time, and heating rate. Figure 4: A flow chart illustrating the benefits of biochar in green roofs. Figure 5: Mechanisms by which biochar increases soil porosity and improves water-holding capacity: (**a**) low-porosity soil structure without biochar, (**b**) high-porosity soil structure due to inter-pores between soil and biochar and intra-pores within biochar particles [56]. Comparisons were made in previous studies between the performance of biochar and other substrate additives. In the study by [PERSON], et al. [57], biochar surpassed other organic wastes such as sawdust, wood bark, coir, and compost in enhancing plant drought resistance and promoting vegetation growth. [PERSON], et al. [58] evaluated the effects of different biochar and organic additives, including dried sewage sludge and compost containing sewage sludge, on runoff quality. Although all substrate additives demonstrated positive impacts, the addition of wood biochar resulted in the most significant benefit. [PERSON], et al. [59] investigated the performance of three different GR substrates including coconut-shell-based biochar, fiber, and biochar-fiber substrates. Biochar was observed to outperform fiber in different benefit categories such as soil thermal conductivity and water-holding capacity. However, the addition of both biochar and fiber was still suggested to enhance air entry, allowing fast drainage and mitigating crack propagation in substrates. It is also worth noting that the application of other substrate additives also led to improvements when compared to bare substrates. Although biochar offers many of the same advantages as other organic materials, it serves as a more durable substitute that lasts in the soil for many years, thus providing a long-term beneficial impact [60]. Biochar can be applied to the GR as either a top layer, middle layer, or bottom layer of the GR substrate, or it could be thoroughly mixed with the substrate. Figure 6 illustrates the structure of commonly used application methods of biochar in green roofs. Table 1 provides some noteworthy case studies of biochar-amended GRs involving single or multiple key benefits of biochar along with the application methods used in those case studies. This review has identified that thoroughly mixing biochar with other GR substrate components is the most commonly used biochar application method.