Correlations between soil geochemical properties and Fe(III) reduction suggest microbial reducibility of iron in different soils from Southern China

• The microbial reducibilities of Fe(III) in different soils varied greatly.
• Microbial Fe(III) reduction rates had linear relation with soil weathering indices.
• Fe(III) in soils with lower weathering degree was more susceptible for reduction.
• Fe(III) had the similar reduction rates in soils from the same parent materials.

Fe(II) species are a group of active compounds during the soil geochemical processes. Various iron compounds show the different reducibility, solubility and extractability in soil, leading to the greatly different contribution of the iron compounds to Fe(II) concentrations. Therefore, it is very important to study the correlation of the total reducibility of iron and the soil geochemical properties in different soils for evaluating the activities of geochemical reactions. In this study, the Fe(III) reducibilities were investigated in 23 iron-rich soils with the different geochemical properties from Southern China. The results showed that the microbial reducibilities of Fe(III) in different soils with the same iron-reducing bacteria of Shewanella decolorationis S12 varied greatly. The soils derived from the same parent material had the similar microbial iron reduction rates. Furthermore, the soil Fe(III) reducibilities did not correlate well with any of the iron species in the soils except for Fe2O3. However, the soil weathering degree was found to be the most important factor in the microbial reducibilities of soil iron. All the weathering indices of studied soils were linearly correlated with Fe(III) reduction rate constants. The results illustrated the unique importance of soil geochemical properties on soil Fe(III) reduction susceptibilities and helped define the soil types to exhibit similar degrees of Fe(III) reduction under controlled lab conditions. In conclusion, the geochemical property of soil weathering degree was conducive to identify the iron reducibility and could become a powerful tool for characterizing the iron reducibilities in soils.