08.12.2025
Plants not only absorb nutrients through their roots, but also toxic metals such as cadmium. It was previously unclear whether the toxic metals came from the soil or the fertilizers applied.
Under the leadership of the Helmholtz Center for Environmental Research (UFZ) and Duke University (USA), a research team examined wheat grains to clarify the question and, using a special isotope signature, found that the majority of the toxic metals come from the mineral fertilizer.
A combination of mineral and organic fertilization would not only reduce levels of toxic metals, but also increase levels of metals important for human nutrition. This is what the researchers concluded in their study, which was published in the specialist magazine Environment International.
“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 Prof. Dr. Marie Muehe, head of the plant biogeochemistry working group at the UFZ and co-study leader of the publication.
“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 German-American research team wanted to find out with their current study.
To do this, the scientists used soil samples and wheat grains from the static fertilization experiment at the UFZ Bad Lauchstädt research station. The static fertilization experiment was set up in 1902 and is one of the most valuable long-term experiments in the world, particularly due to its long duration. “Some test fields have not been fertilized for over 120 years, others only with mineral fertilizer or with organic fertilizer such as manure from neighboring farms or with a mixture of both,” explains Marie Muehe.
Over the long duration of the experiment, the soil properties have developed significantly apart, for example the pH value or the organic matter content. The sample material from this long-term test was therefore the perfect basis for the investigations. There is also an excellent database, as soil samples and harvested wheat grains from all test areas are archived every year.
For their study, the researchers took a closer look at samples from the past 20 years. “We first determined the respective metal contents in the soil, in the harvested wheat grains and in the fertilizers used,” explains Aleksandra Pienkowska, UFZ doctoral student and co-first author of the study. To find out whether the metals contained in the wheat grains come from the soil or the fertilizer, the researchers used a special procedure: the so-called strontium isotope signature method. It is based on the fact that the chemical element strontium (Sr) occurs in two different forms – so-called isotopes – namely 87Sr and 86Sr.
“Since the ratio of these two isotopes is different in every soil, it is essentially something like a fingerprint,” explains Robert Hill, a doctoral student at Duke University (Durham, USA) and co-first author of the study. “If the ratio in the wheat grain is the same as in the soil in which the plant grew, one can conclude that it has 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.” And since it is known that plants absorb strontium and cadmium via similar pathways, conclusions can also be drawn about cadmium.
On the one hand, the results show that the toxic metals in the wheat grains examined come from the fertilizer applied and not from the soil. On the other hand, the metal contents of mineral fertilization are particularly high compared to 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.
In sandy or acidic soils, these effects could be even more noticeable,” says Alexandra Pienkowska. Given that long-term mineral fertilization also causes the acidification of the soil, measures to stabilize the pH value, such as liming the soil, become even more important.
However, not all metals contained in mineral fertilizers are undesirable. Zinc, for example, is a metallic trace element that is important for nutrition. But can the absorption of zinc into the wheat grain be promoted while at the same time preventing the absorption of toxic metals? “Our studies have also shown this: By combining fertilizers, the zinc content in the wheat grain could actually be increased without the cadmium content increasing,” says Marie 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 has a higher quality.” In further studies, the researchers want to investigate how metal levels in crops behave when environmental conditions change, for example due to climate change.
» Original publication
Source: Helmholtz Center for Environmental Research (UFZ)
