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 neu...
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