Characteristics And Mechanism Of Iron Isotope Fractionation In Soil-rice System

Iron is the fourth most abundant element on earth,and its geochemical behavior is of great significance as an important part of the circulation of soil elements.Iron is also one of the essential trace elements in the normal life activities of plants,and participates in many biological metabolic processes,such as photosynthesis,respiration,nitrogen assimilation and fixation,and hormone synthesis in organisms.Iron isotope methods have been widely used to investigate the biogeochemical processes of iron,such as dissimilatory iron reduction,ferrous and abiotic oxidation of ferrous iron,and adsorption,precipitation and other processes.In previous study of iron isotope fractionation during soil-plant systems,iron uptake in plants with different mechanisms for iron uptake showed distinguish magnitude and behavior of iron isotope fractionation.In the plant of Strategy I,Fe（III）is reduced into Fe（II）and it is uptaken into the plant.The Fe isotope composition becomes light and magnitude of fractionation is large inside the plant（-0.13‰^-1.64‰）.In the plant of Strategy II,Fe（III）is transported into the plant by process of chelation.The Fe isotope composition becomes heavy and magnitude of fractionation is small inside the plant（-0.11‰⁰.17‰）.Therefore,Fe isotope fractionation is a promising approach for revealing the mechanism of iron transport in soil-rice systems.In this study,the variations and mechanism of iron isotope fractionation of paddy soil,pore water and rice plants during different paddy and maturity stages in soil-rice system were investigated.This study has important significant in understanding the transport mechanism of iron in soil-rice system.The main conclusions of this study are as follows:1.Pore water enriched in light iron isotopes relative to soil(Δ⁵⁶Fe_{water-bulk soil}=-0.29±0.01‰),which is due to the fact that iron in pore water mainly resulted in the dissolution of iron in soil,and both coordination and reductive dissolution give priority to the release of light iron isotope.The iron isotope fractionation factor between iron plaque and pore water(Δ⁵⁶Fe_{iron plaque-water}=1.47±0.02‰)is identical within error with the iron isotope fractionation factor（1.5‰）in the process of ferrous oxidation to ferrihydrite.This result shows that the iron plaque is formed by the oxidation of ferrous and deposition of ferrihydrite in pore water.Rice plant enriched in heavy iron isotope(Δ⁵⁶Fe_{rice plant-water}=0.66±0.05‰)relative to pore water,indicating that rice plant mainly transports iron through the chelation process of strategy II as mentioned above.However,different from the typical mechanism II plants,the degree of iron isotope fractionation in rice plant is up to 1.39±0.05‰,indicating that the form of iron will change during the transport in rice plant.For example,the fractionation factor of-0.72+0.05‰(Δ⁵⁶Fe _{after uptake-cortex})occurs during the transport of iron from cortex to stem,which may be caused by the reduction of trivalent iron.The enrichment of light iron isotopes in stems relative to roots(Δ⁵⁶Fe_stem-root=-1.39‰±0.05‰)is due to the transformation of ligands complexing with iron from phytosiderophore（PS）and nicotinamine（NA）to citrate.The enrichment of heavy iron isotopes in leaves relative to stems(Δ⁵⁶Fe_leaf-stem=0.58±0.07‰)is due to the fact that iron binds with iron transport protein（ITP）to form Fe-ITP complexes and stored in chloroplasts.With the increase of leaf height,the iron isotope composition becomes lighter,and followed a Rayleigh fractionation trend during Fe transport from leaves to seeds,with an enrichment factor（ε）of 0.33±0.17‰.2.Fe isotope compositions of the soil samples in the two periods were identical within error（0.05%）.With the increase of rice growth and iron accumulation,iron accumulation in the mature stage is higher than that in the jointing stage.However,Fe isotope compositions of soil and rice did not change greatly asδ⁵⁶Fe was 0.48±0.09%and 0.52±0.05%respectively,indicating that the fractionation of soil-rice system was probably equilibrium fractionation,and the chemical form affects the isotope fractionation among organs,while the process of iron transport has little effect on the composition of iron isotope.