Soil organic matter and phosphate sorption on natural and synthetic Fe oxides under in situ conditions

In redoximorphic soils, iron (Fe) and manganese (Mn) oxides undergo reduction with subsequent oxidation of their reduced counterparts (Fe²⁺ and Mn²⁺) impacting nutrient sorption and the stability of soil organic matter (SOM). One tool to investigate the soil redox status is the indicator of reduction in soils (IRIS) method. Thereby, synthetic Fe and Mn oxides are coated onto polyvinyl chloride (PVC) bars, which are typically installed for an operator-defined period in the soil. After removal of the bars we studied organo-mineral associations, which have been formed under field conditions on the surface of the coated bars.

In this study, each one Mn and Fe oxide-coated redox bar were installed for 30 days in a Mollic Gleysol. A previous study revealed, that the Mn oxide coating facilitated a non-enzymatic redox reaction under anoxic conditions, while Fe²⁺ from the soil solution is oxidized to Fe³⁺ along the Mn oxide coating and Mn²⁺ is removed from the PVC surface [1]. In consequence, in situ Fe oxides formed along the Mn oxide coatings and were further considered as ‘natural’ Fe oxides. This enables us to differentiate between sorption occurring onto the surfaces of ‘synthetic’ Fe oxides from the Fe bar versus ‘natural’ formed Fe oxides along the Mn bar. They were analysed by nanoscale secondary ion mass spectrometry (NanoSIMS) to study the distribution of Fe (⁵⁶Fe¹⁶O⁻), SOM (¹²C¹⁴N⁻), and phosphorus (³¹P¹⁶O₂⁻). NanoSIMS is a spectromicroscopic technique offering a high lateral resolution of about 100 nm, while having a great sensitivity for light elements. In contrast to classic bulk analysis, it offers the possibility to examine the spatial distribution of SOM and phosphorous at the microscale within the intact organo-mineral matrix. 

Image analysis of individual Fe oxide particles revealed a close association of Fe, SOM, and P resulting in coverage values up to 71% for synthetic and natural iron oxides. Furthermore, ion ratios between sorbent (⁵⁶Fe¹⁶O⁻) and sorbate (¹²C¹⁴N⁻; ³¹P¹⁶O₂⁻) were smaller along the natural oxides when compared with those for synthetic Fe oxides. We conclude that both natural and synthetic Fe oxides rapidly sequestered SOM and P (i.e., within 30 days) but that newly, natural formed Fe oxides sorbed more SOM and P than synthetic Fe oxides.

 

[1] Dorau, K.; Eickmeier, M.; Mansfeldt, T. Comparison of Manganese and Iron Oxide-Coated Redox Bars for Characterization of the Redox Status in Wetland Soils. Wetlands 2016, 36, 133–144.