Changes in soil microbial community structure associated with iron reduction and phosphate release in soil microcosms

Excess phosphorus inputs to surface waters, especially nonpoint source inputs like agricultural runoff, continue to be a significant cause of eutrophication of freshwater. To improve efforts aimed at reducing nonpoint source inputs, better models need to be used that incorporate a more holistic view of phosphorus transport and mobility in soil systems. An area that is currently lacking in these models is the interaction of microbial influences with other hydrological processes. The reduction of iron oxides in anaerobic soil has been shown to coincide with the release of soluble reduced iron as well as phosphate that was associated with the iron oxide surfaces. Most research has focused on correlating soil properties with the amount of ferrous iron and phosphate released, which has left a significant gap in knowledge about the microbial component of this process. To further study changes in the relative abundance of soil microbes during the reduction of iron oxides, soil microcosms were flooded with water to induce anaerobic conditions. Chemical analyses were performed to monitor ferrous iron, phosphate, dissolved organic carbon, pH, and electrical conductivity and soil samples were collected for nucleic acid extraction to monitor changes in the soil microbial community structure. Because of changes caused by gamma sterilization on control samples and the likelihood of resorption of soluble analytes, chemical data were not able to conclusively establish when/if microbial iron reduction was occurring. 16S rRNA gene sequencing of the soil microbial community taken during different time points after flooding showed an increase in the relative abundance of Pseudomonadaceae and Rhodocyclaceae families. Both of these families contain some species that are known to be capable of anaerobic respiration, which could explain their increased abundance at later time points once oxygen concentrations were lower. Additionally, Rhodocyclaceae contains the iv genera Ferribacterium and Rhodocyclus. Ferribactierum is known to use Fe as an electron acceptor, possibly contributing to some of the observed iron transformations, and Rhodoclycus is phylogenetically closely related to Candidatus Accumulibacter phosphatis—a known polyphosphate-accumulating organism—suggesting a possible influence on phosphorus mobilization. Evidence of changes in the microbial community demonstrate the need to account for the interaction of microbial influences, along with hydrological and chemical processes, in order to gain a more comprehensive understanding of phosphorus mobilization during iron reducing conditions brought on by soil saturation.