## 1 Introduction  
Green roofs (GRs) are a well-established green infrastructure (GI) strategy to effectively tackle numerous social and environmental challenges caused by rapid urbanization and climate change. GRs have garnered significant attention from researchers and have been studied for decades. The ecosystem services offered by GRs include runoff retention; enhancement of runoff water quality; urban heat island effect reduction; lowered building energy costs; improved air quality; and various other economic, social, and environmental benefits [1; 2]. Despite its huge potential, GR remains restricted in its widespread application due to several reasons. While a considerable number of GR studies have been undertaken by using various approaches, there are still inconsistent findings and research gaps that necessitate further investigation [3]. For instance, [PERSON] and [PERSON] [4] and [PERSON], et al. [5] observed positive results in GR runoff quality, while contrasting findings were found in the studies of [PERSON] and [PERSON] [6] and [PERSON], et al. [7]. Besides runoff quality, the performance of GRs in temperature and runoff reduction has also been reported inconsistently. The reduction in runoff volume by GRs was less significant in regions with high annual precipitation, with only an 11.9% reduction observed in a humid subtropical climate [8] and 11% under 3110 mm of rainfall per year in the city of Bergen, Norway [9]. In addition, the benefits of GRs related to temperature and energy savings are also highly dependent on climatic conditions. Exceptional performance of GRs was observed in a hot climate in the city of Shanghai, China [10; 11; 12]. In contrast, GRs did not reduce energy consumption for cooling in several Mexican cities, where they actually led to increased energy use for heating [13]. Additionally, challenges in installing GRs such as high investment costs and added structural loads on existing buildings that were not initially designed for GR systems pose significant barriers [14; 15]. Therefore, further research is essential to fully understand the benefits of GRs and thus facilitate their widespread adoption. There are an increasing number of studies that have employed modeling software to further understand the effectiveness and benefits of GRs [16; 17; 18; 19]. Furthermore, new technologies such as hybrid GR systems and innovative substrate materials have also been applied to enhance the multiple benefits of GR. For instance, GRs have been integrated with photovoltaic (PV) systems, resulting in enhanced thermal benefits for buildings and increased electricity production due to the cooling effect of GRs on PV panels [20; 21; 22]. The effectiveness of GRs in temperature reduction was also improved by integrating them with a radiant cooling system [23; 24; 25]. Additionally, an improved GR system called \"Blue-green roof\", which features a large water storage layer, has significantly reduced runoff [26; 27; 28]. Furthermore, new GR materials have also been utilized to enhance the effectiveness of GR systems. For instance, [PERSON], et al. [29] obtained an impressive average runoff retention rate of 96.8%, surpassing commonly reported values, due to the use of an effective drainage layer. Another innovative material, \"biochar\" has emerged as a promising solution to enhance GR benefits by modifying the characteristics of GR substrates. The term \"biochar\" refers to a carbon-rich material that is manufactured by burning biomass in an oxygen-deficient environment [30]. Biochar has garnered significant attention from researchers due to its positive effects on soil structure, along with its physiochemical properties such as high porosity and large surface area, which enable biochar to enhance soil water-holding capacity, nutrient retention, soil fertility, plant performance, and carbon sequestration, among other benefits [31]. Figure 1 illustrates an increasing trend in the number of studies on both biochar and biochar-based GRs from 2011 to 2023, based on data from the Scopus database.