Isotope fractionation of zinc in the paddy rice soil-water environment and the role of 2’deoxymugineic acid (DMA) as zincophore under Zn limiting conditions

Non-traditional stable isotope systems are increasingly used to study micronutrient cycling and acquisition in terrestrial ecosystems. We previously proposed for zinc (Zn) a conceptual model linking observed isotope signatures and fractionations to biogeochemical processes occurring in the rice soil environment and we suggested that 2’deoxymugineic acid (DMA) could play an important role for rice during the acquisition of Zn when grown under Zn limiting conditions. This proposition was sustained by the extent and direction of isotope fractionation observed during the complexation of Zn with DMA synthesised in our laboratory. Here we report a new set of experimental data from field and laboratory studies designed to further elucidate the mechanisms controlling Zn isotope fractionation in the rice rhizosphere and the role of DMA. First, we present acidity (pKa) and complexation (logK) constants for DMA with H+ and Zn2+, respectively, using synthetic 2’deoxymugineic acid and show that they are significantly different from previously published data using isolates from plants. Our new set of thermodynamic data allows for a more accurate calculation of the formation of ZnDMA complexes over pH ranges typically found in the rhizosphere of flooded lowland rice soils and in rice plant compartments (xylem, phloem). We show that at pH > 6.5, Zn is fully complexed by DMA and at pH 0) detected during adsorption of Zn on goethite in alkaline solutions or between root and soil solution in rice grown in alkaline soils. We show that removal of different Zn species from solution and changes in the Zn coordination control extent and direction of isotope fraction during adsorption. Using the new set of results and combining it with recent findings from the literature, we present a refined conceptual model linking biogeochemical processes to Zn isotope fractionation in the rice soil system. Our results confirm the importance of root induced chemical changes in the rhizosphere of rice growing in soils with low Zn availability, demonstrate the unique ability of isotope signatures to deconvolve geochemical processes and conditions in the plant-soil environment and support the hypothesis of an important role of DMA in Zn acquisition under Zn limiting conditions