Impact of organic carbon and iron bioavailability on the magnetic susceptibility of soils

Microorganisms are known to couple the degradation of hydrocarbons to Fe(III) reduction leading to the dissolution and (trans)formation of Fe minerals including ferro(i)magnetic Fe minerals such as magnetite. The screening of soil magnetic properties, in particular magnetic susceptibility (MS), has the potential to assist in locating and assessing hydrocarbon (e.g. gasoline) contamination in the environment. In order to evaluate this, it must be understood how changes in soil geochemistry and hydrocarbon input impact MS. To this end, we incubated microcosms with soils from six different field sites anoxically and followed the changes in soil MS. In parallel we simulated hydrocarbon (i.e., gasoline) contamination in the same soils under anoxic conditions. We found that in microbially active microcosms both with or without added gasoline, average changes in MS of 6.9 ± 2.6% occurred, whereas in sterile controls the changes were less than 2.5% demonstrating that microbial metabolism played a major role in the (trans)formation of ferro(i)magnetic minerals. The microcosms reached stable MS values after a few weeks to months in four out of the six soils showing an increase in MS while in two soils the MS decreased over time. After stable MS values were reached, further addition of labile organic carbon (i.e., lactate/acetate) did not lead to further changes in MS, but the addition of Fe(III) oxyhydroxides (ferrihydrite) led to increases in MS suggesting that the changes in MS were limited by bioavailable Fe and not by bioavailable organic carbon. In the control experiments without carbon amendment, we observed that natural organic matter was mobilized from the soil matrix by water or microbial growth medium (0.33-0.47 mL/g field moist soil) added to the microcosms, and that this mobilized organic matter also stimulated microbial Fe metabolism and thus also led to a microbially driven change in MS. This study shows that changes in MS after an increase of the amount of bioavailable organic carbon can occur in a variety of soils. It also suggests that whether MS increases or decreases depends on the initial MS of the soil and the extent of the MS change seems to depend upon the amount of bioavailable Fe(III).