Magnetic, geochemical and DNA properties of highly magnetic soils in England

A range of mineral magnetic, Mossbauer, geochemical, microscopy and molecul ar biological techniques are applied to a small set of bulk and fine fracti ons of highly magnetic English topsoils that overlie weakly magnetic sedime ntary geologies. Results show that the ferrimagnetic component of highly en hanced surface soils is dominated by superparamagnetic (SP) grains with a m inor proportion of larger stable single-domain/pseudo-single-domain (SSD/PS D) grains that may derive from magnetosomes and magnetic inclusions. DNA sc reening of the soils by polymerase chain reaction (PCR) shows that the conc entration of viable magnetotactic bacteria is too low (normally < 10(2) bac teria g(-1)) to explain the high concentrations of ferrimagnetic minerals o bserved, There does not appear to be any strong causative relationship betw een the presence or concentration of Magnetospirillum sp. and soil magnetic properties. Microcosm experiments were able to show that the destructive e ffects of waterlogging on secondary ferrimagnetic mineral (SFM) formation a re rapid and associated with significant changes in bacterial populations. The combined results are used to examine alternative explanations for SFM f ormation and are consistent with previous findings (Dearing el al. 1996b, 1 997) that ferrihydrite may be an important precursor of bacterially mediate d magnetite in strongly magnetic temperate soils-a process driven by the ra te of Fe flux to the biologically active surface soil.