## 4 Barriers Limiting Biochar Amendment in GRs
### Limited Understanding of Optimal Biochar-Based Variables  
This review paper highlighted the importance of biochar application methods. For example, the application methods of biochar were investigated in [64]. Birchwood biochar was applied at the bottom (buried biochar) and at the top (surface biochar) of the GR substrate. In both sedum and meadow GRs, buried biochar outperformed surface biochar in terms of runoff retention and runoff quality improvement. Although surface biochar in [63] continued having less impact on runoff retention as compared to mixing biochar, it was still recommended due to the lower labor cost. Processed biochar was suggested to be used against unprocessed biochar on the substrate surface to mitigate potential biochar loss. Moreover, biochar was also applied in other ways. The biochar layer was in the middle of the substrate in the studies by [61; 62]. However, the mixing method remains the most researched biochar application method [66; 67; 68; 69; 73; 81; 83; 89]. Processed biochar has recently attracted the attention of researchers [89; 126; 128; 130]. Compared to unprocessed biochar, processed biochar is well documented to have a lesser impact on water and nutrient retention. However, its long-term effects are more pronounced due to higher resistance to biochar and substrate losses [131]. [PERSON], [PERSON] and [PERSON] [89] investigated different particle sizes of unprocessed biochar and biochar processed into a granulated form and made similar conclusions. Unprocessed biochar had higher runoff and nutrient retention rates, but biochar granules improved plant performance and had better long-term effects due to less biochar loss. Granulated biochar also performed remarkably well in [126; 130] by attenuating biochar and substrate losses and enhancing plant growth. It is widely accepted that small biochar particles have better retention of water and nutrients due to their higher porosity and larger specific surface area [89]. On the other hand, large (coarse) biochar particles are less subject to water and wind erosion [132]. Some researchers recommended using small biochar particles amended to medium to coarse textured soils to obtain multiple benefits [89; 128; 133]. [PERSON], [PERSON] and [PERSON] [130] suggested the use of granulated biochar with a medium particle size (2-2.8 mm) to enhance plant performance while achieving an acceptable retention capacity. Using very fine biochar results in a higher water retention capacity, whereas the drainage speed is reduced. It helps prevent waterlogging, which can negatively affect plant health. Three particle sizes of biochar including fine (<2 mm), coarse (2 mm), and a mix of fine and coarse were investigated in [128]. Fine biochar particles achieved the highest water retention, but they reduced infiltration and AFP, causing waterlogging in extreme rainfall events. [PERSON], [PERSON], [PERSON], [PERSON] and [PERSON] [59] further highlighted the importance of fast drainage of GRs. While the biochar itself slightly improved the air-entry value (AEV) and maximized saturated water content (SWC), the combination of biochar and fiber was more effective. This combination reduced AEV and achieved an acceptable SWC. Enhanced drainage, characterized by a relatively low AEV, helps GRs to maintain adequate water storage for handling frequent rainfall. A greater amendment rate of biochar was mostly observed to increase the water retention capacity of GRs. For example, the maximum improvement in annual rainfall retention was achieved with the highest biochar amendment rate of 40% (\(v/v\)) in [75]. Significantly higher water retention by increasing the amount of biochar was also achieved in some studies [62; 79; 98]. Despite its benefits, some studies found that higher amendment rates of biochar did not always lead to increased runoff retention. [PERSON] [77] observed an increased amount of runoff from GRs with 10% and 15% (\(w/w\)) biochar, as compared to that with 5% (\(w/w\)) biochar.