Iron dynamics in paddy soil chronosequence in view of stable iron isotope signature and iron speciation
Final Report Abstract
More than half of the world’s population feeds on rice, thus relying on a soil management where frequent redox cycles occur, leaving a potential fingerprint on Fe cycle. In this project, we first reviewed the processes involved in (semi)terrestrial Fe cycling, with specific focus on Fe isotope fractionation processes. We could show that particularly reductive dissolution preferentially removes light Fe isotopes from soils, thus leaving heavy Fe isotopes behind. With this knowledge, we wanted to investigate to what extent the redox oscillation of paddy soil would result in a variation of Fe isotope fingerprint of both the soil and the rice plants. Using a paddy chronosequence we demonstrated that after 2000 years of repetitions of flooding-drainage the paddy topsoil still contained the same Fe concentration as their nonpaddy counterparts. However, the Fe isotope composition of the remaining anaerobic bulk soil after removing soil water became isotopically heavy over time, which was more pronounced in the topsoil with ≥1000 years of rice cultivation. In the subsoil, Fe isotope composition showed variation among different depth, which could be attributed to downwards translocation and zonation of the isotopically light Fe dissolved from the topsoil. However, this isotopically light Fe was hardly detectable due to large reservoir effect of a large quantity of Fe in the bulk soil. Overall, changes in Fe isotope composition of the bulk soil was small and was not related to the age of rice cultivation. In contrast to bulk soil, Fe concentration of the plant-available Fe pool declined with increasing duration of the paddy land-use, accompanied by an enrichment of heavy with increasing cultivation age, likely reflecting loss of isotopically light Fe during drainage. The Fe isotope composition of plantavailable Fe varied along the soil depth, but this variation could not be scaled to the cultivation age. Except for the rice in the newly converted paddy fields, Fe concentration in the rice plants was larger when the rice grew in older paddy fields, and they tended to have isotopically heavier Fe than those grown in younger fields. The Fe concentration of rice roots increased along the age of rice cultivation, and their Fe isotope composition was significantly lighter than those of Fe plaques but correlated with the latter. Additional 54Felabelling experiments confirmed that Fe plaques played a “barrier” role in Fe uptake by rice, i.e., with the presence of Fe plaques on its surface, rice root seems not take up Fe from the nutrient solution (or soil solution) but from the Fe plaques directly. Surprisingly, Fe isotopes of soil-plant system are more resilient to environmental change than Fe concentration. Iron isotope fingerprint of redox oscillation could only be observed at millennial scale in soil layers that are directly affected by the long-term redox oscillation.
