## 3 Results
### Sources of Mercury in Soils  
Various land uses were identified as the subject of research on mercury contamination in the reviewed studies. The frequency distribution chart shown in Figure 4b visualized how land use types have been represented in studies on mercury in NbS contexts. Undeveloped lakeside areas such as forests and natural land were most frequently studied, comprising 32.0% (N = 24) of the total number of studied sites, followed closely by peatlands at 30.7% (N = 23), where natural biogeochemical processes are often evaluated. Mining and lakeside (commercial) areas were each included in 9.3% (N = 7) of the studies, while undeveloped land (10.7%; N = 8) and industrial zones (8.0%; N = 6) were addressed less often. This pattern suggests that research has been concentrated in relatively natural or transitional landscapes, where mercury behavior and NbS performance can be observed under less urbanized conditions. Figure 4: Summary of the sources of mercury in soils visualized with a (**a**) box plot of soil THg concentration across different land use types and (**b**) frequency distribution of land use types reported in the reviewed studies (data from 13 studies, 75 sites). The box plot in Figure 4a displays the distribution of total mercury (THg) concentrations in soils across various land uses, particularly industrial (IN), mining (MI), peatland (PL), undeveloped land (UD), lakeside commercial (LC), and lakeside undeveloped (LU), while Table 1 presents a summary of the reviewed studies on mercury contamination and pollution in soils from various land uses. Among these categories, lakeside areas (both LC and LU) exhibited the highest THg levels, particularly in LC sites where the range extends to nearly 1.2 mg/kg. This aligns with findings from subarctic and boreal studies, which indicated that land uses with hydrological connectivity to water bodies, such as riparian zones, peatlands, and undisturbed catchments, tend to promote mercury retention and transformation [19; 20; 21]. LU sites likely accumulate mercury through natural deposition and limited disturbance, while LC sites may reflect historical or diffuse anthropogenic input. Contrastingly, mining and industrial soils presented lower median and mean THg values despite their known associations with heavy metal pollution. The relatively narrow interquartile ranges and modest outlier values in MI sites may reflect remediation efforts, natural attenuation, or localized rather than widespread contamination, consistent with studies from [PERSON] et al. [22] and [PERSON] et al. [23], which observed significant spatial variability and historical mining footprints confined to specific key accumulation zones. Peatlands and undeveloped areas showed moderate mercury concentrations, likely influenced by organic content and historical deposition without the amplifying effects of intensive land use or clear-cutting. In [PERSON] et al. [24], mercury in lake catchment soils across England was primarily sourced from modern anthropogenic inputs, including urban runoff, waste disposal, and soil erosion exacerbated by human disturbance. While atmospheric emissions have declined, current Hg loads in sediments reflect active transfer from polluted catchment soils. For example, elevated Hg were linked to direct contamination from domestic waste, industrial runoff, and land management practices, with erosion and sedimentation facilitating Hg transport into the lakes. The study by [PERSON] et al. [25] focused on agricultural zones and identified mercury inputs mainly from atmospheric deposition and irrigation using contaminated water. The research highlighted that modern agrochemical practices, including fertilizer and pesticide use, may indirectly contribute to soil Hg accumulation and that plowing and irrigation accelerate mercury mobilization into deeper soil layers. In [PERSON] et al. [26], the spatial distribution of mercury in Lake Taihu sediments was strongly linked to urban and industrial pollution.