Rice grown in naturally selenium-rich soils may carry an unexpected health dilemma: the same geological conditions that enrich the grain with a potentially beneficial nutrient can also increase its burden of toxic cadmium. A study published in Plant and Soil now suggests that a carefully measured dose of sulfate fertilizer could help resolve both problems at once. In experiments spanning soil chemistry, rice plants and simulated human digestion, researchers found that potassium sulfate reduced cadmium concentrations in rice grains by 15 to 47 percent while increasing selenium concentrations by 28 to 54 percent. The combined effect lowered the study’s estimated carcinogenic risk from eating the rice by 31 percent, bringing it into a range the researchers classified as safe. The findings offer a possible strategy for producing selenium-biofortified rice without amplifying the risks posed by naturally occurring heavy metals.

Cadmium is a non-essential toxic metal that can accumulate in agricultural soils through geological processes, mining, fertilizers and industrial pollution. Rice is particularly vulnerable because it is often cultivated in flooded fields, where alternating waterlogged and drained conditions continually reshape the chemical forms of metals. Once absorbed by roots, cadmium can move through the plant’s vascular system and accumulate in edible grains. Long-term dietary exposure is associated with kidney damage, bone disorders and cancer risk. Selenium, by contrast, is an essential trace element required for selenoproteins involved in antioxidant defense, thyroid metabolism and other biological processes. Yet selenium follows a narrow nutritional path: too little can contribute to deficiency, while too much can become toxic. In selenium-rich rice-growing regions, the challenge is therefore not simply to add selenium, but to control its concentration and balance its benefits against the hazards of accompanying contaminants.

Yuanzhe Ma, Shangyan Hao, Chengxiang Li, Menghan Guo and Fuyong Wu of Northwest A&F University in China investigated whether sulfate application could influence that balance. Their experiments used potassium sulfate, written chemically as K₂SO₄, a fertilizer that supplies both potassium and sulfur in the form of sulfate. The researchers first conducted soil-incubation experiments with sulfur doses of 0, 100, 200 and 300 milligrams of sulfur per kilogram of soil. They then grew rice in pots treated with either no sulfate or 200 milligrams of sulfur per kilogram. To move beyond measuring total metal concentrations, the team also used a physiologically based extraction test, designed to mimic the chemical conditions of the human stomach and estimate how much cadmium and selenium could become bioaccessible during digestion. Finally, they applied an assessment framework that incorporated selenium’s potential anticancer contribution rather than treating cadmium exposure as the only relevant health factor.

The soil experiments revealed a mechanism that depends heavily on the chemistry of the paddy environment. At sulfate rates of 200 and 300 milligrams of sulfur per kilogram, the treatment reduced the amount of cadmium considered available for plant uptake during the drainage phase. The researchers attributed this change to the formation of iron–sulfur–cadmium ternary complexes and the incorporation of cadmium into crystalline iron oxide lattices. Iron oxides are important mineral surfaces in paddy soils because they can bind metals and limit their movement into soil water. When sulfate alters the local sulfur and redox chemistry, it can help redirect cadmium into more stable mineral-associated forms. But this soil-level immobilization was not universal. High background sulfur concentrations, a shortage of reactive iron oxides and alkaline soil conditions limited the treatment’s ability to lock cadmium into the soil matrix. The result was a reminder that a fertilizer’s effectiveness cannot be inferred from its chemical formula alone; soil mineralogy and pH can determine whether the intended reaction takes place.

The rice plants, however, showed a second layer of protection that remained important even when the soil chemistry did not strongly suppress cadmium availability. With 200 milligrams of sulfur per kilogram applied, the roots developed more crystalline iron plaque, a coating of iron and manganese oxides that forms on root surfaces when oxygen leaks from plant tissues into flooded soil. This plaque can act as a chemical filter, binding cadmium before it enters the root. The researchers described this as a plant-level compensatory mechanism: even where sulfate failed to immobilize large amounts of cadmium in the surrounding soil, the rice plant increased its capacity to trap the metal at the root–soil boundary. Sulfur may also support the production of thiol-containing compounds inside plants, including sulfur-rich molecules that bind cadmium and help sequester it in less mobile forms. Together, stronger root-surface iron plaque and intracellular cadmium fixation can restrict the metal’s movement toward stems and developing grains.

At the same time, sulfate treatment increased selenium delivery to the grain. Sulfate and selenate, the oxidized forms of sulfur and selenium, are chemically similar enough to interact with some of the same transport pathways in plants. The study indicates that increased sulfur supply promoted sulfur–selenium co-transport, allowing more selenium to reach the rice grain. This process is biologically complex: plants do not simply absorb sulfate and selenate indiscriminately, but regulate them through transport proteins and sulfur-assimilation pathways. Once inside plant tissues, selenium can enter metabolic routes related to sulfur, becoming incorporated into selenium-containing compounds or stored in forms that influence bioaccessibility. The researchers’ findings suggest that sulfate application shifted this network in a way that increased grain selenium without increasing its estimated digestibility disadvantage. Grain selenium concentrations rose by 28 to 54 percent, while the treatment’s effects on cadmium moved in the opposite direction.

The simulated digestion experiments added an important human-health dimension. Total cadmium in a grain does not necessarily equal the amount that enters the body, because chemical binding can prevent some of the metal from dissolving under gastric conditions. In the study, higher sulfur concentrations in the rice grain reduced gastric cadmium bioaccessibility through thiol-mediated immobilization. Thiol groups contain sulfur atoms with a strong affinity for soft metal ions such as cadmium. When cadmium binds to thiol-containing proteins and other macromolecular organic components, it becomes less soluble and less likely to be released during the stomach phase simulated in the laboratory. Crucially, the researchers reported that sulfur reduced cadmium bioaccessibility without reducing selenium bioaccessibility. That distinction matters because a grain could appear safer based on total cadmium measurements while still exposing consumers to a substantial fraction of the metal if it remains readily digestible. The results point to a layered form of protection involving soil minerals, root surfaces, plant metabolism and grain chemistry.

The study’s risk analysis combined these pathways rather than relying solely on the reduction in cadmium concentration. According to the researchers, sulfate application reduced the carcinogenic risk associated with the rice by 31 percent and lowered it to a safe level under their assessment framework. Selenium’s potential anticancer effect contributed to that overall benefit, alongside the reduction in cadmium content and its gastric bioaccessibility. But the authors also emphasized a critical limit: excessive selenium biofortification could offset the advantage. Selenium is beneficial within an appropriate range, yet high exposure can produce toxicity and other adverse effects. The finding makes the treatment a matter of dose optimization rather than a blanket recommendation to apply more sulfur. It also highlights why food-safety assessments should account for interactions between nutrients and contaminants. A field intervention that changes two elements at once may alter both hazard and benefit, and those effects can move in opposite directions depending on concentration.

The work does not establish that potassium sulfate will solve cadmium contamination in every selenium-rich paddy field. The experiments were conducted in controlled soil-incubation and pot systems, and the soil-level mechanism was constrained under several conditions, including alkaline pH and inadequate iron oxide content. Field soils experience fluctuating water levels, microbial activity, fertilizer histories and weather patterns that can change sulfur and cadmium chemistry over time. Drainage is especially important because oxidation can transform iron and sulfur minerals and potentially remobilize previously immobilized cadmium. The study therefore supports site-specific testing of sulfate rates, soil pH, iron mineral abundance, background sulfur and grain selenium concentrations before large-scale application. The authors make clear that excessive selenium must be avoided, and that measurements of bioaccessibility are valuable alongside conventional chemical analyses. Data generated and analyzed in the study are available from the corresponding author on reasonable request.

Despite those limitations, the findings offer a striking example of how agricultural chemistry can be used to manage a soil–plant–human problem as a connected system. Instead of treating cadmium cleanup, selenium enrichment and dietary risk as separate challenges, the researchers traced how one fertilizer altered mineral reactions in soil, defensive structures on rice roots, transport pathways inside the plant and the behavior of metals during digestion. That systems approach could be especially valuable in regions where selenium-rich farmland is economically important but naturally associated cadmium threatens the safety of the harvest. Potassium sulfate is already familiar as an agricultural input, which could simplify its integration into management programs compared with entirely new remediation technologies. Yet the study’s central message is one of precision: sulfur can make rice both less contaminated with cadmium and richer in selenium, but only when the chemistry of the field and the nutritional limits of the crop are carefully respected.