## 4 Discussion and Future Perspective  
Despite the limitation of information retrieval from a single database and open-access articles, research on mercury contamination in NbS soils has gradually increased over the past decade, with a notable rise in publications addressing both treatment mechanisms and environmental implications. Spatially, the research has been dominated by countries in Asia, North America, and Europe, with strong thematic linkages observed between keywords such as \"biochar\", \"constructed wetlands\", and \"remediation\". These trends indicated a growing global interest in sustainable remediation approaches, though regional disparities in research focus suggest the need for expanded studies in underrepresented areas. Furthermore, significant knowledge gaps were observed, particularly regarding the long-term performance and reliability of NbSs in arid or semi-arid climate areas, where water availability and evapotranspiration rates may compromise treatment efficacy. Additionally, limited research exists on how NbS function under extreme weather events or shifting seasonal hydrology, which are increasingly relevant under climate change. Particularly, differences in NbS application between climate zones have also not been systematically examined. For instance, tropical regions may exhibit higher biological productivity and faster pollutant cycling, while temperate zones may offer more stable hydrologic regimes that support consistent treatment performance. The absence of comparative studies across climatic contexts limits the transferability of design principles and may lead to site-specific inefficiencies. These regional and climatic deviations highlight a critical research gap and should be more explicitly acknowledged in future investigations and policy guidance. The synthesis revealed that mercury contamination in soils originates from diverse sources, including industrial emissions, mining, agricultural runoff, and atmospheric deposition, with lakeside environments acting as significant sinks due to sedimentation and hydrological connectivity. In such aquatic-adjacent areas, mercury can accumulate in surrounding soils through periodic flooding and surface runoff, increasing the risk of methylation. However, key challenges remain in accurately quantifying diffuse mercury inputs and implementing nature-based remediation approaches that are efficient across heterogeneous land uses. Amidst uncertainties, NbSs have shown strong potential for mercury treatment through mechanisms like plant uptake, root retention, redox transformation, and sediment binding. Aquatic plants such as _Acorus calamus_, _Eichhornia crassipes_, and _Aquarius palifolius_ demonstrate high mercury removal efficiencies, often exceeding 70-90%, with certain species also capable of minimizing MeHg formation. However, the synthesis conducted in this study suggest that challenges still remain, particularly the risk of enhanced MeHg production under anaerobic conditions, which can offset the benefits of THg removal. Optimal design, plant selection, and substrate control were found critical for the long-term effectivity of NbSs. A more focused review revealed that biochar has strong potential as a treatment component for mercury-contaminated environments by reducing total mercury mobility, lowering plant uptake, and converting Hg into more stable, less bioavailable forms. However, under certain conditions, particularly in fluctuating redox environments, biochar can unintentionally enhance mercury methylation, increasing the production of toxic MeHg. These risks, along with variability in biochar properties and site-specific soil chemistry, highlight the need for careful selection, modification, and monitoring of biochar applications in real-world treatment systems. Overall, a potential is seen in NbSs in terms of offering a more cost-effective alternative to conventional Hg removal technologies.