Stoppe, N., Amelung, W., Horn, R.
Original Research Article, Special Edition: International
Iron oxides promote aggregation, adsorb nutrients and pollutants and serve as electron acceptor; hence it is desirable to understand the composition of the soil and sedimentary iron pool. Here we tested if existing chemical extraction approaches using solely ammonium oxalate and sodium dithionite deliver consistent information on the allocation of amorphous and crystalline iron oxides for coastal sediments, which experience alternating redox conditions. For that purpose iron was extracted using citrate dithionite solution buffered with sodium bicarbonate and acid ammonium oxalate solution (pH 3.25). The content of dithionite extractable iron (Fed) and oxalate extractable iron (Feo) ranged from 1.7 to 7.4 g kg-1 and 1.0 to 11.3 g kg -1, respectively. We calculate the content of crystalline iron oxide as usual from the difference between Fed and Feo, however, we failed for nearly half of the investigated soil samples, because the Feo content exceeded Fed. It is assumed that the oxidation of Fe(II) phases into oxalate extractable Fe(III) phases as well as the catalyzed dissolution of Fe(III) minerals by Fe(II) oxalate complexes sophisticate the results gained by chemical extractions using only ammonium oxalate and dithionite as leachates, but that these interferences do not account