## 5 Future Research Opportunities
### Long-Term Observation Studies  
This study also highlighted the lack of long-term research needed to fully understand the benefits of biochar amendment in GRs. Most biochar-based GRs reviewed in this study were examined over a short study period. For example, the results in [71; 79] were only from two and three simulated rainfall events, respectively. [PERSON], [PERSON], [PERSON], [PERSON], [PERSON] and [PERSON] [73] evaluated the hydrological responses of biochar-amended GRs through the use of soil columns using only one simulated rainfall. Eight rainfall events within six months were simulated in [72] to study both the quantity and quality of runoff from biochar-amended GRs. As a result, the long-term performance of biochar-amended green roofs could not be assessed. On the other hand, a limited number of studies with extended monitoring periods were also identified. The performance of GRs amended with biochar during two seasons was observed in [64]. A long monitoring period of roughly a year was adopted in a few studies [74; 77; 88]. Studies involving multiyear experiments were very limited. For example, a five-year experiment reported in [90] measured phosphorous levels in the discharge from several extensive GRs, with biochar added to one of the GRs during the final two years. [PERSON], [PERSON] and [PERSON] [61] tracked the leaching of TP from biochar-amended GR substrates for a period of seven years. Their long-term study revealed a strong correlation between the age of GRs and the effectiveness of biochar. The studies [88; 89] found a significant decline in the leaching of nutrients towards the end of the experiments. Similarly, [PERSON], [PERSON], [PERSON], [PERSON], [PERSON] and [PERSON] [101] observed a negative correlation of GR aging with concentrations of TN and dissolved P, K, and Mg over 3 years. Biochar-amended GR substrates of Vavrincova, Pipiska, Urbanova, Fristak, Hornik, Machalova and Soja [95] indicated first-flush behavior with higher TN and TP concentrations at the beginning. After 30 months, there were no significant differences between biochar and non-biochar treatments. Therefore, biochar from sewage sludge was concluded to be a sustainable substrate additive for extensive GR substrates. This nutrient-rich biochar gradually released nutrients and did not greatly increase nutrient concentrations in GR runoff when compared to the conventional GRs. The decrease in nutrient concentrations over time could be attributed to well-established plants enhancing nutrient uptake and stabilizing the root substrate. Although the concentrations of TPin runoff from biochar-amended GRs were higher than those from conventional GRs during the experiment, a downward trend of TP leaching over time was identified in [90]. Additionally, [PERSON], [PERSON] and [PERSON] [61] studied the leaching of phosphorous (P) and nitrogen (N) and reported similar findings related to biochar and GR aging. They found that biochar consistently reduced P and N leaching until an increase in P levels was noted 6 years after GR installation, which was attributed to fertilization. Since the pollutant load is calculated by multiplying the runoff volume and pollutant concentration, the long-term effects of biochar on runoff quality become more pronounced due to its substantial impact on reducing runoff volume. An excellent example is the study of [PERSON], [PERSON], [PERSON] and [PERSON] [64]. In some events, concentrations of TP and TN in runoff from biochar-amended GRs were higher than those from non-biochar GRs. However, results from a nonlinear regression analysis in this study revealed positive impacts of biochar on the cumulative load of TP due to a substantially lower runoff volume. Therefore, regardless of the higher pollutant concentrations and loads reported in some studies, it can be concluded that biochar has long-term potential to mitigate the leaching of pollutants from GRs to the environment.