Iron(III) Reduction in Anaerobically Incubated Suspensions of Highly Calcareous Agricultural Soils

The frequent presence of Fe chlorosis in plants grown on calcareous soils is influenced by the forms of soil Fe present and their contents. Previous studies suggest that temporary soil flooding may increase Fe phytoavailability. To study flooding effects in relation to microbial activity in greater depth, we incubated soil slurries in the laboratory under anoxic conditions and monitored changes in Fe mineralogy using wet chemical extractions and diffuse reflectance spectroscopy. Twenty‐four calcareous soils from southern Spain ranging widely in their properties were chosen for this purpose. Slurries of sterilized and native soils were compared with those of native soils with different amendments. In contrast to the sterilized controls, most of the slurries containing native soils released substantially increased amounts of Fe(II) to the solution after 6 wk of incubation; also, the extent of Fe(II) production correlated well with native contents of dissolved organic C. Indeed, the addition of organic acids typically found in root exudates resulted in a pronounced increase in Fe(II) production, and a similar effect was observed in soil slurries additionally inoculated with Geobacter sulfurreducens , a well‐known Fe(III)‐reducing bacterium. Microbial Fe(III) reduction mobilized poorly crystalline and crystalline Fe oxides. The critical extractable Fe value required for Fe nutrition of tolerant plants was reached in 18 of the slurries of native soils and in 22 of the native soils amended with organic acids. Temporary flooding seems to stimulate microbial Fe(III) reduction (especially in the presence of readily available organic C), thereby effectively increasing Fe phytoavailability in calcareous soils.