## 4 Barriers Limiting Biochar Amendment in GRs
### Unpredictability Due to Inconsistent Research Findings  
Despite varying substrate mixes, feedstocks, and pyrolysis conditions, most studies in Table 2 demonstrated the positive impacts of biochar on runoff retention in GRs. However, this review identified contradicting findings in few studies. For instance, the study by [PERSON], [PERSON], [PERSON] and [PERSON] [72] showed no significant improvement in runoff retention from a biochar-amended GR. The authors reported retention rates of 72.54% and 72.08% from an unmodified GR and a biochar-modified GR, respectively. Results from the study by [PERSON], et al. [127] illustrated that biochar performed differently with varying biochar types and conditions of vegetation. Biochar improved runoff retention and reduced peak discharge of only test beds (3.24 m\({}^{2}\)) with native plants that had a low vegetation cover and density during small rainfall events. Conversely, biochar was observed not to have positive impacts on GR test beds with sedum that had a high vegetation cover and density. In general, most reviewed studies showed a positive influence of biochar on the runoff retention rate of GRs. The improvements could be negligible or significant depending on the feedstock, pyrolysis temperature, substrate compositions, substrate depths, biochar amendment rates, and other biochar variables. Furthermore, concerns about the application of biochar include the reduction in air-filled porosity (AFP), which can lead to waterlogging during high-intensity rainfall events. This, in turn, may adversely impact plant health and the ecosystem services provided by GRs [128]. An effective GR system not only excels in water retention but also has an appropriate water-releasing capability to avoid waterlogging [100]. In contrast to runoff quantity, results from this review indicate a more inconsistent effectiveness of biochar in terms of GR runoff quality, which is in accordance with findings in the study by [129]. The effectiveness of biochar strongly depends on its physio-chemical characteristics, which vary according to the type of feedstock, pyrolysis conditions, and types of pollutants [90]. Hence, inconsistent research outcomes were identified in Table 3. Among numerous pollutants, the leaching of TP was prominent in most studies [61,72,74,90,94,95]. For example, [PERSON], [PERSON], [PERSON] and [PERSON] [74] reported that loads of TP were higher in runoff from all tested biochar-amended substrates with different depths (5, 10, 15, and 20 cm). In contrast, positive effects were observed for other parameters such as TN. Similar findings relating to loads of TP were also found in other studies [72,94]. Another notable example of biochar degrading runoff quality is the study presented in [90]. Both oat-hull and wood-derived biochar contributed significantly to the TP in the runoff. The event mean concentration (EMC) of TP in runoff from unmodified extensive GRs fluctuated from 0.05 to 1.47 mg/L during the study period of 2 years. Conversely, biochar-amended GRs negatively influenced runoff quality by elevating the EMC of TP, ranging from 0.18 to 2.09 mg/L. This research outcome highlights the need to evaluate the effects of substrate additives prior to their application. Additionally, [PERSON], [PERSON], [PERSON], [PERSON], [PERSON] and [PERSON] [80] also reported a decline in runoff quality following the addition of biochar, with the increases in the concentration of TN, chemical oxygen demand (COD), and biochemical oxygen demand (BOD). The impacts of biochar on plant performance have also been uncertain in some studies. Biochar had insignificant influences on the coverage area of native species and stonecrops in the study by [PERSON] [63]. They were examined in 64 failed extensive GR trays (0.24 m\({}^{2}\)) using different biochar treatments (processed and unprocessed biochar, mixing and top-layer application methods).