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Naturally elevated levels of As have been observed in some iron- enriched 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 soilhorizons (Ah 0-15, Bg 15-35, CrBg35-70 cm) developed from fluvial loam overlaying sand with reductomorphic properties (2Cr, +70 cm). The soil has petrogleyic properties due to enrichment of oxidic Fe in the Bg and CrBg horizons (275 and 390 g kg-1, 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-1, respectively) and were lowest in the sand (12 mg kg-1). 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 μg 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. [ABSTRACT FROM AUTHOR]