Purpose Properties of Fe oxides are poorly understood in soils with fluctuating water tables and variable redox conditions. The objective of this research was to (a) characterize the mineralogical composition of Fe oxides and (b) determine the relationship to the stable Fe isotope ratio in a soil with temporally and spatially sharp redox gradients.
Materials and methodsThe lowland Gleysol (Petrogleyic) is in Northwest Germany and consists of oximorphic soil horizons (Ah 0-15, Bg 15-35, and CrBg 35-70 cm) developed from Holocene fluvial loam overlaying glaciofluvial sand with reductomorphic properties (2Cr horizon, +70 cm). Field measurements during the course of 28 months included the monitoring of groundwater table, soil redox potential, and analysis of the soil solutions. Solid Fe phases were studied by room temperature and cryogenic 57 Fe Mössbauer spectroscopy, and stable Fe isotope compositions by multiple collector inductively coupled plasma mass spectrometry. Results and discussion The groundwater table ranged from −83 cm below to +8 cm above soil surface (median −27 cm). Permanent reducing conditions occurred in the 2Cr horizon with dissolved Fe concentrations of 44.8 mg L −1 (median). The duration of oxidizing conditions increased in the order CrBg < Bg < Ah. Total Fe increased from 50 (Ah) over 316 (Bg) up to 412 g kg −1 (CrBg) and was lowest in the 2Cr horizon (7 g kg −1 ). Ferrihydrite (51% of total Fe) was dominant over goethite (24%) in the Ah horizon. Conversely, nanogoethite dominated both the Bg (94%) and CrBg (86%) horizons. Iron in siderite amounted to 7% in the CrBg horizon. Iron isotope compositions yielded a range of δ 57 Fe values from +0.29‰ (Ah horizon) to −0.30‰ (Bg horizon). In contrast to the overlying CrBg (δ 57 Fe=−0.19‰) and Bg horizons, the 2Cr horizon is characterized by a relatively high δ 57 Fe value of +0.22‰. Conclusions Lasting water saturation and frequent reducing conditions lead to the enrichment of goethite in subsoil. Once formed, goethite remains stable compared to ferrihydrite because it is less available for microbial mediated reductive dissolution. High δ 57 Fe values in the topsoil primary result from fast ferrihydrite precipitation during aeration immediately after reducing conditions. In contrast,
Naturally elevated levels of As have been observed in some ironenriched lowland soils of the southern Münsterland, Germany. To determine whether As is mobilized by the reductive dissolution of As-hosting Fe oxides at reducing soil conditions, the release of arsenate and arsenite was investigated in the field for 24 mo and in laboratory experiments. The grassland Gleysol consists of oximorphic soil horizons ( , respectively). Most of the petrogleyic Fe belonged to nanosized goethite, whereas ferrihydrite was dominant in the topsoil. Arsenic levels of 149 mg kg −1 were found in the topsoil. Levels peaked in the oximorphic soil horizons (626 and 999 mg kg ). Sequential fractionation revealed that 84 to 96% of the As in the oximorphic horizons was associated with Fe oxides. Water saturation in combination with soil temperatures above 5 to 10°C resulted in a fast decrease of the redox potential (up to −120 mV) and release of As (up to 35 mg L −1 ) with Fe into the soil solution only in the Ah horizon. Although the petrogleyic horizons were mostly water saturated with reducing conditions, no As release was observed. A lack of As mobilization was confirmed in experiments performed under fixed redox conditions; neither Fe nor As was released into solution at an appropriate redox potential. The As species distribution showed redox disequilibrium because arsenate was detected under reducing conditions and arsenite under oxidizing conditions. We conclude that microbial-mediated reductive dissolution of Fe oxides pushes the As mobilization in the topsoil; water saturation and soil temperature were key factors. The dominance of goethite, which is more resistant to microbial reduction, and/or the possible readsorption of released As onto remaining or newly formed Fe oxide surfaces is responsible for the absent As mobilization in the petrogleyic horizons.
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