Mugineic acid (MA; (2S, 2′S, 3′S, 3″S)-N-[3-carboxy-3-(3-carboxy-3-hydroxypropylamino)-2-hydroxypropyl]-azetidine-2-carboxylic acid), a multidentate complexing agent, is exuded by the roots of graminaceous plants (e.g. barley) growing under Fe-deficient conditions. The nature of the interactions of MA with Fe (hydr)oxide minerals in soils is thus of considerable practical importance. To examine the availability of MA in dissolving Fe (hydr) oxides, we investigated the amounts of Fe dissolved from 24 samples of Fe (hydr)oxides by 100 μM MA (FeMA) and 200 mM NH4-oxalate (Feo). The Feo values were much larger than the FeMA ones, though the crystallinity of the Fe (hydr)oxides was a significant factor. For Fe (hydr)oxides consisting of goethite, hematite, ferrihydrite, and their mixture, FeMA increased with the increase of Feo, indicating that FeMA would be correlated with FeD, that is the amount of short-range ordered Fe minerals including ferrihydrite. For goethite, hematite, magnetite, maghemite, and lepidocrocite with a high crystallinity, MA was relatively ineffective in dissolving Fe, although a part of crystalline magnetite, maghemite, and lepidocrocite could be dissolved by NH4-oxalate. For soils containing significant amounts of magnetite, maghemite, lepidocrocite, it is unlikely therefore that Feo would be a useful indicator of plant-available Fe.
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