Multidimensional Drivers of Mercury Distribution in Global Surface Soils: Insights from a Global Standardized Field Survey

Global Distribution of Mercury in Foliage Predicted by Machine Learning

Foliar assimilation of elemental mercury (Hg0) from the atmosphere plays a critical role in the global Hg biogeochemical cycle, leading to atmospheric Hg removal and soil Hg insertion. Recent studies have estimated global foliar Hg assimilation; however, large uncertainties remained due to coarse accounting of observed foliar Hg concentrations, posing a substantial challenge in constraining the global Hg budget. Here, we integrated a comprehensive observation database of foliar Hg concentrations and machine learning algorithms to predict the first spatial distribution of foliar Hg concentrations on a global scale, contributing to the first estimate of global Hg pools in foliage. The global average of foliar Hg concentrations was estimated to be 24.0 ng g-1 (7.5-56.5 ng g-1), and the global total in foliar Hg pools reached 4561.3 Mg (1455.2-9062.8 Mg). The spatial distribution showed the hotspots in tropical regions, including the Amazon, Central Africa, and Southeast Asia. A range of 2268.5-2727.0 Mg yr-1 was estimated for annual foliar Hg assimilation accounting for the perennial continuous assimilation by evergreen vegetation foliage. The first spatial maps of foliar Hg concentrations and Hg pools may aid in understanding the global biogeochemical cycling of Hg, especially in the context of climate change and global vegetation greening.

Mercury supply limits methylmercury production in paddy soils

The neurotoxic methylmercury (MeHg) is a product of inorganic mercury (IHg) after microbial transformation. Yet it remains unclear whether microbial activity or IHg supply dominates Hg methylation in paddies, hotspots of MeHg formation. Here, we quantified the response of MeHg production to changes in microbial activity and Hg supply using 63 paddy soils under the common scenario of straw amendment, a globally prevalent agricultural practice. We demonstrate that the IHg supply is the limiting factor for Hg methylation in paddies. This is because IHg supply is generally low in soils and can largely be facilitated (by 336-747 %) by straw amendment. The generally high activities of sulfate-reducing bacteria (SRB) do not limit Hg methylation, even though SRB have been validated as the predominant microbial Hg methylators in paddies in this study. These findings caution against the mobilization of legacy Hg triggered by human activities and climate change, resulting in increased MeHg production and the subsequent flux of this potent neurotoxin to our dining tables.

Spatio-temporal trends of mercury levels in alluvial gold mining spoils areas monitored between rainy and dry seasons in the Peruvian Amazon

Artisanal and small-scale gold mining (ASGM) in the Amazon has degraded tropical forests and escalated mercury (Hg) pollution, affecting biodiversity, ecological processes and rural livelihoods. In the Peruvian Amazon, ASGM annually releases some 181 tons of Hg into the environment. Despite some recent advances in understanding the spatial distribution of Hg within gold mine spoils and the surrounding landscape, temporal dynamics in Hg movement are not well understood. We aimed to reveal spatio-temporal trends of soil Hg in areas degraded by ASGM.,. We analyzed soil and sediment samples during the dry and rainy seasons across 14 ha of potentially contaminated sites and natural forests, in the vicinities of the Native community of San Jacinto in Madre de Dios, Peru. Soil Hg levels of areas impacted by ASGM (0.02 ± 0.02 mg kg-1) were generally below soil environmental quality standards (6.60 mg kg-1). However, they showed high variability, mainly explained by the type of natural cover vegetation, soil organic matter (SOM), clay and sand particles. Temporal trends in Hg levels in soils between seasons differed between landscape units distinguished in the mine spoils. During the rainy season, Hg levels decreased up to 45.5% in uncovered soils, while in artificial pond sediments Hg increased by up to 961%. During the dry season, uncovered degraded soils were more prone to lose Hg than sites covered by vegetation, mainly due to higher soil temperatures and concomitantly increasing volatilization. Soils from natural forests and degraded soil covered by regenerating vegetation showed a high capacity to retain Hg mainly due to the higher plant biomass, higher SOM, and increasing concentrations of clay particles. Disturbingly, our findings suggest high Hg mobility from gold mine spoil to close by sedimentary materials, mainly in artificial ponds through alluvial deposition and pluvial lixiviation. Thus, further research is needed on monitoring, and remediation of sediments in artificial to design sustainable land use strategies.

A novel cross-disciplinary approach for estimating spatio-temporal pattern and drivers of pollution transfer caused by industrial transfer processes

Food security is the basic requirement for human health and social stability. However, food pollution risk caused by economic development models, e.g., industrial transfer, is poorly understood. In this study, we developed and tested a point-area-domain cross-disciplinary approach to quantitatively estimate patterns and processes of soil and rice heavy metal (Pb, Hg and Cd) pollution risk caused by the fourth global industrial transfer process in CG Province of China. Our results indicate that the industrial transfer policy caused obvious cross-regional soil Pb, Hg and Cd pollution and industrial transfer. Industrial transfer process largely exacerbated rice Hg pollution risk in industrial transfer undertaking areas, mainly in the west (29.18%), periphery (28.38%) and south (25.88%) regions. The concentration increase in the west ranged from 0.0001 to 0.0775 mg/kg (mean±S.D. 0.00327 ± 0.00447, median 0.0026 mg/kg). The most important industries regarding rice Hg pollution in industrial transfer undertaking areas included special equipment (26.53%)>metal processing (11.93%)>communication equipment (11.25%)>chemical product (10.90%)> electrical machinery and equipment (9.59%). The pollution transfer was mainly attributed to environmental regulations, factor endowment and industrial structure. Our study highlights the importance for the integration of industrial transfer into food security and poverty reduction plans in the developing regions. ENVIRONMENTAL IMPLICATION: Food security is the basic requirement for human health and social stability. However, food pollution risk caused by economic development models, e.g., industrial transfer, is poorly understood. We developed and tested a point-area-domain cross-disciplinary approach to quantitatively estimate patterns and processes of soil and rice heavy metal pollution risk caused by the fourth global industrial transfer process. Our results suggest that industrial transfer process largely exacerbated soil and rice Hg pollution risk in the southeast coastal areas of China. Our study highlights the importance for the integration of industrial transfer into food security strategies in developing countries.