## 6 Conclusions  
GRs are one of the most widely used water-sensitive urban design (WSUD) strategies aimed at addressing a variety of social and environmental challenges caused by urbanization and climate change. GRs have been studied for decades to optimize their ecosystem services and promote their widespread implementation. Biochar, an innovative carbon-rich material, has been recently introduced into GRs and other GI practices. Biochar has physiochemical characteristics that are beneficial to the performance of GRs including water/nutrient retention, plant performance, microbial diversity, carbon sequestration, thermal reduction, and soil erosion control. Despite significant research efforts to investigate the use of biochar across various fields, the potential of biochar in green roofs has not been thoroughly explored. This paper aimed to review the previous studies analyzing the performance of GRs with the application of biochar. Additionally, this review attempted to provide readers with a broad picture of the efficiency of biochar-based GRs by considering different biochar benefits. Furthermore, this paper also presented knowledge gaps, implementation challenges, and future research opportunities. The following points are a summary of important findings and recommendations arising from this study:  
1. There is a clear trend indicating the benefits of biochar amendment in GRs. However, significant variation in parameters across biochar-GR studies make direct comparisons challenging. Additionally, the application of biochar in GRs is relatively new and not yet thoroughly studied, highlighting the need for further research to provide comprehensive insights. 2. Although some studies report contradictory findings, such as higher pollutant concentrations in runoff from biochar-amended GRs, the long-term improvement in runoff quality due to a significant reduction in runoff remains noteworthy. 3. The research on biochar amendment in GRs has been uneven, with the majority of studies focusing on runoff retention, improved runoff quality, enhanced plant performance, and microbial diversity. Certain other benefits of biochar for GRs, such as thermal reduction, carbon sequestration, and control of soil erosion, have received relatively less attention. Future studies should address this imbalance to provide a more comprehensive understanding of biochar's full range of benefits. 4. Biochar-related variables such as particle sizes, processed forms, amendment rates, and application methods play an important role in determining the performance of biochar-amended GRs. However, contradictory findings in the literature highlight the need for further research to fully understand the impacts of these variables on the performance of green roofs. 5. The understanding of the long-term effects of biochar on GRs remains incomplete, likely due to the limited number of studies that conducted analysis with multiyear observations. 6. While biochar is well recognized to effectively sequester CO2, the integration of biochar into life cycle assessments (LCAs) is recommended to comprehensively understand its long-term environmental impacts. 7. It is also recommended to apply LCA to evaluate the cost aspects of biochar in GRs. This can provide deeper insights into the economic feasibility of biochar-amended GRs. 8. The utilization of biochar in GRs faces several barriers. In addition to inconsistent and limited research information, major challenges include high manufacturing costs, policy-related constraints, and environmental concerns. Addressing these issues is crucial for the widespread adoption of biochar in GRs. Conceptualization, C.N.N., A.K., A.C. and N.M.; methodology, C.N.N. and N.M.; software, C.N.N. and N.M.; validation, H.-W.C., A.K., A.C. and N.M.; formal analysis, C.N.N. and N.M.; investigation, C.N.N. ; resources, C.N.N. and N.M.; data curation, C.N.N. ; writing--original draft preparation, C.N.N. ; writing--review and editing, C.N.N., H.-W.C., A.K., A.C. and N.M.; visu  
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