Toxic metals in wheat grain: do they come from the soil or the fertilizer?

“From field and greenhouse studies, it is known that the type of fertilisation – i.e. mineral or organic – has an influence on the metal concentration in crops”, says Prof. Dr Marie Muehe, head of the Plant Biogeochemistry working group at the UFZ and co-leader of the study. “But until now, it was not known whether the metals absorbed by the plants came from the soil or directly from the fertilisers”. This is what the German–American research team aimed to find out with their current study.

The scientists used soil samples and wheat grains from the static fertilisation experiment at the UFZ research station in Bad Lauchstädt. The static fertilisation experiment was established in 1902 and is one of the most valuable long-term experiments worldwide, largely because of its exceptional duration. “Some plots have not been fertilised for over 120 years while others have been fertilised with only mineral fertiliser, only organic fertiliser – such as manure from neighbouring farms – or a mixture of both”, says Muehe. Over the long duration of the trial, the soil properties (e.g. the pH value or the organic matter content) have diverged considerably. The sample material from this long-term trial was therefore the perfect basis for the analyses. Because soil samples and harvested wheat grains from all trial plots are archived each year, we have an excellent data basis.

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 fertilisers used”, explains Aleksandra Pieńkowska, UFZ doctoral candidate and co-lead author of the study. In order to find out whether the metals contained in the wheat grains originate from the soil or the fertiliser, the researchers used the strontium isotope signature method, which is based on the fact that the chemical element strontium (Sr) occurs in two different isotopes, ⁸⁷Sr and ⁸⁶Sr. “Because the ratio of these two isotopes differs in each soil type, it is essentially a fingerprint”, explains Robert Hill, doctoral candidate at Duke University and co-lead author of the study. “If the ratio in the wheat grain is the same as in the soil on which the plant has grown, it can be concluded that the plant absorbed the strontium from the soil. However, if the isotope ratio in the grain corresponds to that of the fertiliser, this is a clear indication that the strontium got into the grain via the fertiliser”. And because it is known that plants absorb strontium and cadmium via similar pathways, conclusions can also be drawn for cadmium.

The results show that the toxic metals in the analysed wheat grains originate from the fertiliser rather than from the soil. In addition, the metal contents are particularly high when mineral rather than organic fertilisation is used. “To interpret these results, it is important to note that our investigations were carried out in a region with very fertile chernozem soils. These effects could become even more evident in sandy or acidic soils”, says Pieńkowska. Given that long-term mineral fertilisation also leads to soil acidification, measures to stabilise pH levels (e.g. liming) become even more important”.

However, not all metals contained in mineral fertilisers are undesirable. For example, zinc is an important metallic trace element for nutrition. But can the absorption of zinc in wheat grain be promoted while preventing the absorption of toxic metals? “Our investigations have also shown that by combining the fertilisers, it was possible to increase the zinc concentration in the wheat grain without increasing the cadmium concentration”, says Muehe. “We therefore recommend combined fertilisation or alternating applications of mineral and organic fertilisers. This not only reduces fertiliser costs but also improves grain quality”.

In subsequent studies, the researchers aim to examine how metal levels in crops respond to changing environmental conditions, including those driven by climate change.