Wheat is one of the most important staple foods worldwide. It is all the more important to understand whether and from where this grain absorbs potentially harmful substances such as metals. Until now, it was unclear whether such metals come from the soil or get into the wheat grains from the fertilizers used. Researchers have now re-examined this question and found that the toxic metals are primarily due to mineral fertilizers.
Plants not only absorb nutrients through their roots, but also toxic metals. It was previously unclear whether they came from the soil or the fertilizers applied. “It is known from field and greenhouse studies that the type of fertilization, i.e. mineral or organic, has an influence on the metal content in food plants,” says co-author Marie Muehe from the Helmholtz Center for Environmental Research (UFZ) in Leipzig. “But until now it was not known whether the metals absorbed by the plants came from the soil or directly from the fertilizers applied.” This is what the research team led by first author Robert Hill from Duke University and Muehe wanted to find out with the current study.
Strontium isotopes as a “fingerprint”
To do this, the team used soil samples and wheat grains from the static fertilization experiment at the UFZ Bad Lauchstädt research station – one of the oldest long-term experiments in the world. Test fields there were fertilized differently over more than 120 years: sometimes not at all, sometimes exclusively mineral or organic, sometimes in combination. As a result, the soil properties have developed significantly apart, for example the pH value or the organic matter content. The extensively archived sample material from this long-term experiment provided the researchers with ideal conditions for their analyses.
Hill and his colleagues examined samples from the past 20 years and first determined the metal content in soil, wheat grains and fertilizers used. To find out whether the metals contained in the wheat grains come from the soil or the fertilizer, the researchers analyzed the ratio of the strontium isotopes 87Sr and 86Sr in the samples. “Because the ratio of these two isotopes is different in every soil, it is essentially something like a fingerprint,” explains Hill.
“If the ratio in the wheat grain is the same as in the soil in which the plant grew, you can conclude that it absorbed the strontium from the soil. However, if the isotope ratio in the grain corresponds to that of the fertilizer, this is a clear indication that the strontium entered the grain via the fertilizer,” Hill continued. And since it is known that plants absorb strontium and cadmium via similar pathways, conclusions can also be drawn about cadmium.
Mineral fertilizers increase metal contamination
The analyzes showed: The toxic metals in the wheat grains come from the fertilizers and not from the soil. In addition, the metal contents were significantly higher with mineral fertilization than with organic fertilization. “To classify these results, it is important to know that our studies were carried out in a region with very fertile black earth soils. These effects could be even more evident in sandy or acidic soils,” says co-author Alexandra Pieńkowska from UFZ. It is therefore important to counteract soil acidification, for example by liming the soil.
But not all metals in mineral fertilizers are undesirable. Zinc, for example, is an important metallic trace element. The researchers therefore tested whether its uptake by wheat plants could be promoted without introducing more toxic metals at the same time. “By combining the fertilizers, the zinc content in the wheat grain could actually be increased without increasing the cadmium content,” says Muehe. “We therefore recommend combined fertilization or the alternation of mineral and organic fertilizers. On the one hand, this saves fertilizer costs and, on the other hand, the grain is of higher quality.”
In further studies, the researchers now want to investigate how metal levels in crops behave when environmental conditions change, for example due to climate change.
Source: Helmholtz Center for Environmental Research – UFZ; Specialist article: Environment International, doi: 10.1016/j.envint.2025.109851