Determining the stable Fe isotope signature of plant-available iron in soils

The isotope composition of iron in soils can display the environmental conditions that formed this soil. But plants extract only the mobile iron from soil, which is a small fraction of the soilsʹ total iron. Yet this fraction is notoriously difficult to extract experimentally. Here we provide evidence that this signature is provided readily in the form of strategy II plants (grasses). We determined the stable Fe isotope signature of iron pools in two agronomic soils with two different sequential extraction methods. The Fe isotopic composition of the following soil mineral pools was measured: exchangeable iron, iron of poorly-crystalline (oxyhydr)oxides, iron in organic matter, iron of crystalline oxides and silicate bound iron. We found variations of about 1 per mil in δ56Fe (δ56Fe/[‰] = [(56/54Fesample/56/54FeIRMM-014) − 1]·103) in the iron isotopic composition between the different soil mineral pools. The pools that contribute most to plant nutrition are water-extractable- and exchangeable iron, iron in organic matter and iron of poorly-crystalline (oxyhydr)oxides. These fractions are about 0.3 per mil lighter than the bulk soils. Silicates in our soils have a δ56Fe of up to 0.4‰, suggesting preferential loss of light Fe during weathering. We compared the isotope composition of the plant-available Fe to that of typical strategy I and strategy II plants, grown on the soils. While redox and other transformation processes in the rhizosphere enrich strategy I plants to varying degrees in light Fe isotopes, strategy II plants exhibit a uniform Fe isotopic composition and are only slightly enriched in the heavier iron isotopes by about 0.3‰. Therefore these plants may record the Fe isotope composition of plant-available iron in soils, to which the composition of strategy I plants can be compared to.