The first 30 centimeters of soil are the foundation of life. This foot-deep slice of the pedosphere is the vital space for most plant roots. When roots go deeper, it’s to anchor the plant, not to nourish it. Within this narrow band, bacteria, fungi, nematodes, and countless other microscopic organisms form the so-called biological crust, which in turn supports the larger life forms above. Now, a review of thousands of studies — and many more soil samples — reveals that this same 30-centimeter layer also contains toxic concentrations of metals in agricultural soil used to grow the food humans eat. The massive study, published Thursday in Science, estimates that up to 17% of farmland worldwide contains excessive levels of one or more metals and metalloids.
A team of researchers from the U.S., Europe, and China reviewed thousands of existing studies on the presence of metals in the soil. They found over 82,000 papers. After applying a series of filters — such as focusing on 21st-century research, limiting the scope to the uppermost soil layer, and including only studies that measured metal concentrations in soil samples — they narrowed it down to about 1,500 studies. These provided data from nearly 800,000 locations in populated regions across the globe.
Using a machine learning system, a field of artificial intelligence, they modeled and estimated the global extent of excessive contamination from seven specific metals: arsenic (technically a metalloid and a known carcinogen), cadmium (linked to various cancers and prone to accumulating in grains and fruits, especially rice), chromium (in its highly toxic hexavalent form, often released by leather tanning and pigment industries), cobalt (essential for lithium batteries, and thus a driver of exploitation and conflict in Central Africa), copper (a natural dietary component that can disrupt endocrine function in excess), nickel (important for plant growth but stunts it when overabundant), and lead (harmful to children’s neurological development and cognitive abilities).
The researchers found that between 14% and 17% of the global cropland contains dangerously high concentrations of at least one of these metals. In terms of area, the upper estimate represents about 242 million hectares. “We also show that between 900 million and 1.4 billion people [roughly 11% to 18% of the world’s population] live in areas with contaminated soils. That’s a lot of people,” says Jerome Nriagu, professor emeritus of Environmental Chemistry at the University of Michigan and senior author of the study.
It’s important to distinguish between contamination and high concentration. “Contamination” typically refers to human-caused pollution, such as from mining or industrial disasters like Spain’s Aznalcóllar spill. “High concentration,” by contrast, can stem from natural processes — environmental forces (rain, sun, ice, solar radiation…) acting on the pedosphere.
Zooming in to the regional level, 19% of China’s agricultural land shows elevated concentrations of heavy metals, much of it linked to human-caused pollution. Even higher percentages are observed across large parts of northern and central India. In Europe, the authors draw on data from the LUCAS program — an initiative led by the European Commission’s Joint Research Centre to monitor the condition and evolution of land use across the EU. Based on thousands of periodically collected soil samples, up to 28% of soils in EU member states contain excessive levels of at least one metal. However, these figures reflect the entire land area, not just land used for farming.
Among the metals, the most widely distributed on the map is cadmium, which is present in toxic concentrations in 9% of soils. It is followed by nickel and chromium, with significant concentrations in the Middle East and northern Russia. Next comes arsenic, whose distribution overlaps with polluted groundwater zones across large areas of China, but also in several parts of South America. The list ends with cobalt, found in high levels in countries such as Zambia and the Democratic Republic of the Congo — pollution closely linked to mining activities — along with copper and lead, the most toxic of all, which can cause harm even in tiny amounts.
“The widespread distribution of cadmium contamination comes from both natural and anthropogenic sources,” explains Deyi Hou, lead author of the study and researcher at Tsinghua University’s School of Environment in Beijing, in an email. “Geochemically, certain parent rock materials [substrate below the ground], such as black shales, contain high levels of cadmium, leading to elevated concentrations in the soil due to weathering.”
Anthropogenic activities further exacerbate this problem, “particularly the use of cadmium-containing phosphate fertilizers, wastewater irrigation, industrial emissions from mining, smelting, and e-waste processing, as well as atmospheric deposition from coal combustion,” Hou adds. This combination of human-caused pollution and natural background levels is what deeply concerns scientists.
Mapping the presence of metals (see image) reveals a band of elevated concentrations that the researchers call the “metal-rich corridor.” This belt stretches from northern Italy to southeastern China, cutting across Greece, Anatolia, the Middle East, Iran, Pakistan, and the northern and central regions of the Indian subcontinent. These are densely populated areas with deep historical roots, and the researchers link today’s contamination to human activity dating back to ancient times.
“These regions largely overlap with the core areas of early human civilizations, including ancient Greek and Roman civilizations, Persian culture, ancient Indian societies, and the Yangtze River Civilization in China,” Hou recalls.
Previous work with ice cores extracted from Greenland and Siberia detected anomalous lead concentrations dating back more than 2,000 years. This metal is key in silver metallurgy.
“While natural factors such as weathering of parent rock material and phytoextraction [absorption by roots] play a role, millennia of intense human activity, particularly mining and smelting, have been key factors,” says Hou. “This corridor reflects the enduring legacy of human impact on the Earth’s surface and provides compelling evidence of the Anthropocene as a new geological era.”
However, the study does not assign blame to either natural or human causes. That wasn’t its objective, and pinpointing the origin of these metals on a global scale is no easy task. The different timescales involved also complicate matters. A spill like the one in Aznalcóllar, for instance, happened in just a few hours on April 25, 1998, whereas the natural introduction of metals into the pedosphere is a much slower process. The formation of new soil occurs at a rate of just three millimeters per century. Events as gradual as the end of the last Ice Age, which took around 10,000 years for the ice to retreat, illustrate this contrast. Looking at the map again, it is clear that in areas north of the 50th parallel (which runs across Germany from west to east) there are hardly any high concentrations of metals.
As researcher Manuel Delgado Baquerizo of Spain’s Institute of Natural Resources and Agrobiology of Seville (IRNAS) explains, “periods of glaciation have a very strong impact on soil biochemistry; when the ice disappears, the soil completely disappears, leaving the parent rock completely exposed.” And with the soil goes the metal load.
Delgado Baquerizo, an expert on the environmental impact of soil contamination, points out that “heavy metals in general are quite toxic, but they have to reach high levels.” “These researchers have looked at the soil, not at the actual food we might consume,” adds Baquerizo, who was not involved in the study.
For him, the real challenge is setting thresholds — knowing exactly what concentration of a metal per kilogram of soil becomes harmful to the soil ecosystem, its inhabitants, and human health. “There are no established standards,” he says. The authors of the study used maximum limits set by 10 different countries and calculated an average — but that doesn’t fully capture the full scope of the problem.
Baquerizo concludes: “The problem is that many heavy metals have a cumulative effect. You may be exposed to a small amount, but if you’re exposed to that small amount over a long period of time, it can have an impact on your health.”
The most obvious example is lead. Ever since the Romans began using it in their pipes, it has continued to be used for over 2,000 years to distribute running water.
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