Iron isotope fractionation in arable soil and graminaceous crops

Abstract

Soils contain large quantities of Fe, however, the Fe-solubility is very low. Plants have developed two efficient strategies to secure Fe uptake from soil under Fe-deficient conditions: (i) the sequential acidification-reduction-transport strategy (strategy I) and (ii) the chelation-based strategy (strategy II). All processes involved in the Fe cycle in soil-plant systems can fractionate stable Fe isotopes. Hence, I (i) conducted a systematic review about the state of Fe isotope research in plant studies and highlighted the research gaps. Then I supplemented this theoretical study by two experiments: I (ii) examined the effect of different Fe availabilities on Fe isotope fractionation in wheat plants under controlled conditions and I (ii) investigated the effect of 50 years of irrigation on Fe isotope fractionation in soils and cereals in a long-term field experiment. <br /> My review suggested that strategy I plants especially take up light Fe isotopes, while strategy II plants fractionate less towards light isotopes. Aboveground tissues usually show even lighter Fe isotope signatures than the roots, with flowers (δ56Fe: -2.15 to -0.23‰) being isotopically the lightest. I found that all reported strategy I plants consistently enriched light Fe isotopes under all growth conditions. Strategy II plants, however, could be enriched with either light or heavy Fe isotopes, depending on the growth conditions. Depending on the Fe speciation and concentration present in the growth medium, some strategy II plants like rice are able to adapt their uptake strategy as they also possess ferrous transporters and are hence also able to take up Fe(II) ions. <br /> In a greenhouse study, I cultivated summer wheat (Triticum aestivum L.) under Fe-sufficient (control, 0.0896 mM Fe-EDTA) and deficient (Fe-deficient, 0.0022 mM Fe-EDTA) conditions. Plants were sampled at different growth stages (vegetative and reproductive growth stages) and separated into different plant organs (root, stem, leaf, spike/grain). All samples were analyzed for their Fe concentrations and δ56Fe isotope compositions. The results showed that Fe-deficiency reduced the whole plant Fe mass by 59% at vegetative growth. During reproductive growth, Fe mass fluxes indicated different preferential Fe translocation pathways under different Fe supply. Under Fe-deficient conditions, Fe uptake from growth substrate increased whereas under Fe sufficient conditions Fe was preferentially redistributed within the plant. Under Fe-sufficient conditions increasingly lighter δ56Fe values from older to younger plant parts were found, but no indications that the chelation-based uptake strategy was activated. However, with serious shortage of Fe, the shift towards lighter δ 56Fe values was reduced. This suggested that Fe isotope ratios can reflect both wheat growth conditions and ages. <br /> In a field study, I sampled wheat plants and Retisol soil cores down to a depth of 100 cm from a long-term irrigation treatment at Berlin-Thyrow. The irrigated plots had higher Feavail concentrations than the non-irrigated plots in the top 40 cm of soil, but there were no changes in δ56Fe values. Due to the research site being one of the driest areas in Germany with hardly a meaningful water percolation, the maximum difference of δ56Feavail values between 40 to 50 cm and 70 to 100 cm was explained soil pedogenesis rather than irrigation treatment. The wheat plants grown in both irrigated and non-irrigated plots were slightly enriched in light Fe isotopes, exhibiting similar δ56Fe values to those of the respective topsoil. I concluded that the overall δ56Fe signature of wheat was regulated by plant homeostasis and specific on-site soil characteristics, whereas irrigation had little if any significant effect on the Fe isotopes in the crops. <br /> Overall, my study showed that the Fe isotope compositions of wheat plants were not affected by Fe availabilities in substrate until the anthesis stage. However, during the reproductive growth phase with sufficient Fe supply, δ56Fe values of different plant organs showed significant Fe fractionation. The former processes were hardly affected by irrigation.