## 3 Results
### Biochar as an NbS Component for Treating Mercury  
The application of biochar in mercury-contaminated soils has shown highly variable outcomes in terms of THg removal and MeHg production, with performance strongly influenced by biochar properties, system configuration, and prevailing redox conditions. As summarized in Table 4, a cross-study comparison of seven experimental systems showed THg removal efficiencies ranging from approximately 30% to over 95%, demonstrating biochar's potential as a stabilizing agent for mercury. In particular, Fe-modified rice straw biochar in mesocosoms achieved up to 98% THg removal, while also reducing MeHg by around 60% under moderately oxic conditions [58]. Similarly, column systems using combined oak woodchip and Fe-based biochar also achieved high THg removal (>90%) and suppressed MeHg formation by over 70% [59]. However, one consistent trend across the literature is that MeHg concentrations increased in a majority of the systems, particularly those operating under anoxic or fluctuating redox conditions. For instance, in field-based mesocosoms mimicking constructed wetland redox dynamics, aged rice husk biochar was associated with up to a twofold increase in MeHg levels [60]. This increase was attributed to changes in the microbial community, with enhanced activity of methylating organisms such as sulfate-reducing and iron-reducing bacteria. Likewise, [PERSON] et al. [61] and [PERSON] et al. [60] observed MeHg elevation in rice paddy or flooded soil systems amended with biochar. These findings indicate that the introduction of biochar into redox-unstable or anaerobic environments may unintentionally enhance MeHg production, especially in systems with high organic matter and microbial activity. The type of system used (microcosm, mesocosom, column, or pot experiments) was also observed to be critical in interpreting treatment outcomes. Microcosms and columns allowed tighter control of redox boundaries, enabling better THg immobilization under designed conditions. For example, pine-derived biochar in anaerobic sediment micro-cosms reduced aqueous THg concentrations significantly due to its high surface area and redox buffering capacity [62]. On the other hand, field-like systems and pot trials, though more realistic, exhibited greater microbial MeHg generation, suggesting that operational complexity and real-world variability increase risk when applying biochar in NbSs. Furthermore, the functional design of the NbSs, such as whether the wetland system is surface flow or vertical flow, influences Hg fate by dictating oxygen gradients, substrate contact time, and microbial niche development. In surface flow wetlands or flooded soils, biochar often encounters strongly reduced zones where methylating bacteria thrive, while aerated or vertically flowing systems may offer better control over microbial activity and redox interfaces. These distinctions could be critical, as the dual role of biochar, as both a sorbent for inorganic Hg and a potential stimulant of MeHg production, depends on how the system manages sulfate levels and organic matter decomposition. Altogether, the review findings emphasize that while biochar has strong potential to enhance Hg immobilization in NbSs, its deployment must consider not only the type of biochar and Hg species targeted but also the hydrologic regime, redox status, and microbial ecology of the system. Without such context-specific tuning, biochar amendments may exacerbate, rather than mitigate, mercury risks through unintentional stimulation of MeHg production pathways. The conceptual framework presented in Figure 6 synthesizes the major factors influencing Hg dynamics in NbSs, including sources, plant uptake mechanisms, and treatment interactions. Mercury inputs into NbS systems were found to commonly originate from mining activities, atmospheric deposition, and industrial emissions, particularly coal-related sources. Lakeside environments, especially undeveloped zones, often function as passive THz sinks.