Rock magnetic, chemical and bacterial community analysis of a modern soil from Nebraska

Detailed rock-magnetic and bacterial community analyses have been conducted on a 2.2 m-long soil profile developed over loess in Nebraska. Results show magnetic enhancement in the A-horizon of the soil profile, in part due to increased concentration of pedogenic superparamagnetic (SP) ferrimagnetic minerals (i.e., magnetite, maghemite). The magnetic enhancement correlates well with higher microbial biomass and lower iron hydroxide concentrations (here determined using a goethite-specific magnetic method), therefore supporting the idea of bacterially-mediated topsoil magnetite formation through iron-reduction of iron hydroxides. Cultivation-independent bacterial community analysis biased towards the Geobacteraceae (common dissimilatory iron-reducing bacteria) failed to show the involvement of this phylogenetic group in SP magnetite formation at this site. Although preliminary in nature, our failure to implicate dissimilatory Geobacteraceae with magnetic enhancement in modern Nebraskan soil and Dearing et al. (Dearing, Hannam, Anderson, Wellington, Geophysical Journal International 144 (2001) 183–196) earlier failure to detect magnetotactic bacteria in enhanced British soils points to the need to focus further multidisciplinary investigations on the possible roles of other dissimilatory iron reducing bacteria in contributing to magnetic enhancement. Many authors have produced strong circumstantial evidence of a microbial role in causing magnetic enhancement in topsoils of temperate climate zones, yet a precise role of specific cultured microbes in enhancing soil magnetism is still lacking.