Contrasting effects of inorganic and organic fertilisation regimes on shifts in Fe redox bacterial communities in red soils

Although the precipitation and dissolution of iron (Fe)-containing-minerals are driven by microbially-mediated iron-redox cycling, we still have a limited understanding of complex response of such microbial communities to fertilisation. Here, using chemical and synchrotron-based spectral analyses, we show that the distribution of poorly crystalline Fe minerals and crystalline Fe minerals is different in organically- and inorganically-fertilised soils, and that compared to no fertilisation (Control), Fe redox cycling bacteria were present at higher abundance and diversity in organically-fertilised soils but lower in inorganically-fertilised soil. During Fe(III) reduction, Geobacter were important active Fe(III) reducers, with a higher relative abundance in both organically- and inorganically-fertilised soils than in Control, and their higher abundance was responsible for greater dissolution of ferrihydrite in inorganically-fertilised soil than in organically-fertilised soil. However, during the Fe(II) oxidation, Pseudomonas and Anaerolinea were more abundant, and produced higher levels of poorly crystalline Fe oxides under organic fertilisation. Thus, for the first time, we demonstrate that inorganic and organic fertilisation regimes have contrasting effects on the Fe redox bacterial communities, which then influence Fe cycling in soils.