Exploring the links between bacterial communities and magnetic susceptibility in bulk soil and rhizosphere of beech (Fagus sylvatica L.)

Soil magnetic properties are sensitive indicators of pedogenetic processes. Although many of the processes that increase soil magnetism are well documented, the role of microbial communities and the metabolic characteristics of Fe-reducing bacteria are still largely unknown. For this work, two soils with contrasting magnetic properties were identified in a beech forest on Monte Zuccarello (central Italy). Samples of bulk soil and rhizosphere were obtained from A (0-7 cm), AB (7-16 cm), Bw1 (16-22 cm) and Bw2 (22-29 cm) horizons of both soils, and were analysed for their physicochemical characteristics, the amount of magnetic minerals, and the composition of total and culturable bacterial communities, focusing on siderophore-producing bacteria (SPB). Analyses confirmed that the magnetic soil (MS) have a higher content of maghemite and magnetite association, compared to non-magnetic soil (NMS). Since the formation of maghemite can occur through different processes, we investigated on the possible role of soil bacteria in the formation of this magnetic mineral. As soil maghemite generally contains small amounts of Fe2+, the formation of which has been attributed to the combustion of organic matter, SPB have been isolated and identified. MS samples showed the highest number of SPB (mainly Micrococcaceae, Bacillaceae, and Pseudomonadaceae), suggesting a significant increase in Fe-reducing bacteria. High-throughput sequencing analysis revealed a separation in terms of the total composition of the bacterial community between bulk and rhizosphere of both MS and NMS, dominated by Proteobacteria, Actinobacteria, Acidobacteria, and Verrucomicrobia phyla. Interestingly, Gamma and Delta Proteobacteria, as well as Nitrospirae, were usually more abundant in MS than in NMS, confirming that the MS was predominantly characterized by groups of magnetotactic bacteria and SPB, and potentially contributing to enhance the soil magnetic properties.