## 3 Results  
### Keyword Co-occurrence and Spatial Trends  
The keyword co-occurrence network map (Figure 2a) generated using VOSviewer revealed four main thematic clusters in mercury-contaminated soil and NbS research. The red cluster centered on \"bioremediation\" reflects microbial and plant-assisted detoxification. The green cluster, organized around \"methylmercury\" and \"wetlands\", represented studies on biogeochemical processes in aquatic systems. The blue cluster, highlighting \"soil\" and \"adsorption\", focused on remediation techniques in terrestrial environments. Moreover, the yellow cluster links \"biochar\" to soil stabilization approaches. The central positioning of \"mercury\" and its high connectivity across clusters underscores its interdisciplinary relevance, linking environmental, biological, and materials-focused research themes. In addition to the cluster analysis, an overlay visualization of the keyword co-occurrence illustrating temporal evolution from 2017 to 2021 was developed, as seen in Figure 2b. The overlay network map revealed that earlier studies, represented in darker blue tones, were primarily focused on foundational topics such as \"bioremediation\", \"soil pollution\", and \"methylmercury\", indicating a strong initial emphasis on microbial processes, contamination pathways, and wetland-based systems. In contrast, more recent keywords shown in yellow and green hues, such as \"biochar\", \"soil amendment\", \"paddy soils\", and \"charcoal\", indicated a growing research interest in the application of engineered natural materials for mercury stabilization and remediation. The term \"biochar\" appears as a prominent and relatively recent node, signifying its rapid emergence as a favored treatment component within NbS strategies. Overall, the timeline visualization highlighted a clear shift from classical bioremediation frameworks toward integrated, material-based approaches that support multifunctional ecosystem restoration. As visualized by the two network maps, the transition from microbially driven remediation to material-enhanced strategies like biochar reflects the field's shift toward multifunctional and system-integrated approaches on Hg remediation in NbSs. Figure 2: Keyword co-occurrence network related to mercury pollution in NbS soils: (**a**) cluster view showing thematic groupings, and (**b**) overlay visualization indicating the temporal evolution of keyword usage. The spatial trend of research on mercury in NbS soils is summarized in Figure 3. A contingency matrix in Figure 3a was developed to illustrate the co-occurrence between countries and frequently occurring keywords in the research field of mercury in NbS soils. The color gradients on the contingency matrix represent standardized residuals: red cells indicate a higher-than-expected frequency of keyword use in a given country, while blue cells indicate lower co-occurrence relative to the global average. For instance, the United States showed a strong focus on \"methylmercury\" and \"mercury (element)\", suggesting active engagement with microbial and chemical transformation pathways in mercury-contaminated systems. In contrast, Brazil, Colombia, and Indonesia were notably associated with \"phytoremediation\", reflecting an emphasis on plant-based remediation strategies. Moreover, research initiatives in China more evenly distributed, with slight emphasis on \"charcoal\" and \"soil\", aligning with the country's growing interest in material-enhanced soil treatments. These patterns reveal regional research priorities and potential areas of expertise or underexplored niches within the global research landscape. Figure 3: Spatial trends of research on mercury pollution in NbS soils using a (**a**) contingency matrix of keywords and countries, (**b**) number of articles per country from 2005 to 2024, and (**c**) heat map of the cumulative number of articles.