Stable isotope fingerprinting of metal uptake by higher plants

The vegetation that covers the land surface is a key player in the cycling of elements through the Earth surface. Higher plants, with microorganisms in soils, take up large amounts of metallic nutrients like Ca, Mg, K, micronutrients like Fe, Cu, Zn, and metalloids like Si. They return these nutrients in the form of plant litter to soils. Minor shifts in the composition of the “non traditional” isotopes fingerprint these cycles. We have calibrated the stable isotope shifts of the 56Fe/54Fe (iron-56/iron-54) ratio in uptake experiments. We grow higher plants in controlled environments on soils or nutrient solutions. We also explore techniques to characterise the “plant-available” iron fraction in soils.

Here we see how “strategy I” plants (left, bean plant) reduce the Fe(III) contained in the growth solution. This reduction favours the light isotope 54Fe (shown in ‰ difference of the 56Fe/54Fe ratio) to that of the growth solution. The total plant hence contains lighter Fe than the growth solution. Reduction and associated isotope fractionation also accompanies the translocation of Fe within the bean plant. Hence the younger parts of the plant contain lighter Fe than the older parts. A “strategy II” plant (maize, right) chelates Fe(III) without reduction. Chelation is not associated with large isotope fractionation. Hence strategy II plants are similar in isotope composition to the growth medium. A minor shift to light Fe is caused by the constitutive reducing mechanism that strategy II plants use in addition to chelation. Such isotope fractionations by higher plants have now been observed for many metals, amongst them Mg, Ca, Cu, Zn, and Mo.

Guelke, M., von Blanckenburg, F., 2007. Iron isotope fractionation in higher plants. Environmental Science and Technology.
von Blanckenburg, F.; von Wirén, N.; Guelke, M.; Weiss, D. J.; Bullen, T. D. (2009): Fractionation of Metal Stable Isotopes by Higher Plants. Elements, 5, 6, 375-380.
Guelke, M., von Blanckenburg, F., Staubwasser, M., Schoenberg, R., Stuetzel, H., 2010. Determining the stable iron isotope signature of plant-available iron in soils. Chemical Geology 277, 269-280.
Guelke-Stelling, M.; von Blanckenburg, F. (2012): Fe isotope fractionation caused by translocation of iron during growth of bean and oat as models of strategy I and II plants. Plant and Soil, 352, 1-2, 217-231.