## 4 Barriers Limiting Biochar Amendment in GRs
### Environmental Concerns  
Biochar has proven effective in agriculture and has recently been introduced in green infrastructure strategies like GRs. Nonetheless, an awareness about the potential of biochar in GRs remains lacking. Additionally, many questions remain unanswered regarding the environmental impacts of biochar-amended GR systems, such as biochar loss and leaching of contaminants [116]. [PERSON], [PERSON] and [PERSON] [126] found that 94% of unprocessed biochar was lost from GRs due to wind and water erosion after only 2 years. Additionally, waste biomass is often used to manufacture biochar, raising concerns about contaminants such as nutrients and heavy metals leaching from biochar-amended GRs [134]. Nevertheless, some studies have concluded that heavy metals in waste-based biochar feedstocks are converted to stable forms and thus the use of biochar has lower environmental risks than the direct use of waste [135; 136]. Both positive and negative findings regarding runoff quality from biochar-amended GRs have been reported and discussed earlier in this review. This suggests significant variability in GR performance across different systems and pollutants, underscoring the need for long-term studies to mitigate environmental risks associated with the application of biochar in GRs on a large scale.  
This review has also identified a few studies on environmental benefits of biochar in GRs to increase carbon sequestration. Biochar has the potential to combat global warming by capturing several greenhouse gases (GHGs) and enhancing carbon storage. Unlike carbon sequestration, other environmental impacts of biochar in GRs have not been thoroughly explored. Limited information on the Life Cycle Assessment (LCA) of biochar use in GRs was also noted in this review. LCA is a commonly used tool to estimate potential environmental impacts of a product during its life cycle. For example, [PERSON], et al. [137] attempted to explore the impacts of biochar on the environment by applying the LCA of an extensive GR and other urban applications of biochar. All biochar applications demonstrated improved environmental performance in terms of carbon sequestration. However, in other impact categories (natural resource use, human toxicity, and ecotoxicity), biochar could be either more or less environmentally friendly, depending on the biochar supply chain and material substitution. Biochar demand was predicted to increase rapidly, which raises environmental concerns regarding its end-of-life impact.  
In a broader context, LCA has been carried out for other soil-related applications. Similar findings were found in a comprehensive review on the application of biochar in soil systems by [PERSON], [PERSON] and [PERSON] [134]. Despite high variability in methodologies and contexts across the literature, the positive impacts of biochar in storing stable carbon in soils and reducing GHG emissions from agricultural production are apparent. Those benefits outweigh GHG emission from pyrolysis systems, feedstock processing, and handling. However, our knowledge of the effects on other impact categories relating to human and ecotoxicology remains limited and these require further investigation. Another review, by [PERSON], et al. [138], raised environmental concerns about biochar's impacts on human health and ecosystems. The authors also highlighted the effectiveness of biochar in decreasing emissions of GHGs such as carbon dioxide and nitrous oxides. On the other hand, they called for future studies on the long-term effects of biochar in soil structures to ensure its sustainability in soil applications. Thorough investigations into biochar production and applications in the long term are essential.