Metals and metalloids are crucial for human, soil and plant health. They are ubiquitous in soils, originating from both natural geological and human-made sources. Some of the metals and metalloids, such as arsenic and cadmium, can accumulate in food crops to levels that are deemed unsafe for human health. Others, such as cobalt, chromium, copper, nickel and lead, can harm plant growth and ecosystems when present at elevated concentrations in soils.
The study by Hou et al. analyzed 796,084 data points of the concentrations of the above seven metals and metalloids from 1,493 regional soil studies. Using machine learning techniques, they mapped soil pollution risks at the global scale.
Soil pollution is defined as metal concentrations exceeding the agricultural thresholds or human health and ecological thresholds. As both types of regulatory thresholds vary widely among different countries, the researchers used the 25 percentile values of all thresholds from 11 countries in Europe, USA, and China. They estimate that on 14 to 17% of cropland, at least one toxic metal exceeds agricultural thresholds.
Exceedance of the agricultural threshold for cadmium is the highest at 9%, followed by nickel (5.8%), chromium (3.2%), arsenic (1.1%), and cobalt (1.1%). The global exceedance of the human health and ecological thresholds for the seven metals or metalloids is estimated to be 6.8%, lower than the exceedance of the agricultural thresholds due to less stringent values. Based on these data, they estimated that between 0.9 and 1.4 billion people live in regions of heightened public health and ecological risks.
A striking feature of the global soil metal pollution map is the intercontinental “metal-enriched corridor” in low-latitude Eurasia and across southern Europe, the Middle East, South Asia, and southern China (Figure 1). This corridor is attributed to a combination of anthropogenic and environmental factors.
Based on their machine learning model, Hou et al. found climate, topography, mining activity, and irrigation to be the key drivers for soil metal exceedance. Near-surface temperature, precipitation, and potential evapotranspiration have the strongest effects on soil metal exceedance. Increasing temperature and precipitation accelerate the weathering processes that release metals from soil parent materials and enhance the enrichment of metals in clay minerals and iron oxides. Hot and wet climate also enhances the plant-pumping effect through root uptake of metals from deeper soil, thus bringing the metals to the surface soil through litter. The soil metal exceedance rate of mountainous areas, due to their low percentages of flat areas and high percentages of steep slopes, is nearly double the global average. Mining activities and the proportion of irrigated land are also strong predictors of metal exceedance in soil.
Aggregate distribution of exceedance of agricultural thresholds of arsenic, cadmium, cobalt, chromium, copper, nickel, and lead in soil.
The study also highlights the limitations of existing data, with many remote areas lacking sufficient soil sampling. The purposeful exclusion of data points of local contaminated sites would underestimate the impact of such hot spots of pollution. The use of a single threshold of total concentration to define pollution can cause substantial uncertainty, as the availability and toxicity of metals and metalloids depend on soil properties, such as pH, the contents of soil organic matter, clay, and iron oxides. For example, the agricultural threshold of 1 mg/kg cadmium used in the study can mean high risk in acid soils but little risk in neutral to alkaline soils. As admitted by the authors, the 25 percentile threshold values used in the study are relatively conservative. Changing the percentile threshold values to, for example 50 percentile, would drastically reduce the exceedance rate.
The study gives a comprehensive global overview of the scale of soil metal pollution and the potential risk to human and ecosystem health. The findings come at a time of growing concern over soil degradation, particularly as demand for critical metals increases due to the transition to renewable energy technologies. The researchers call for urgent international action, including targeted funding for pollution monitoring and soil restoration.
