A NanoSIMS study on the distribution of soil organic matter, iron and manganese in a nodule from a Stagnosol

The development of Stagnosols is the consequence of perched water tables, which induce periodic oxidizing and reducing conditions. These cause the spatial distribution of iron (Fe) and manganese (Mn) between the soil matrix and ferromanganese concretions or nodules. Since oxides of these metals may interact with organic matter, we studied their spatial distribution in bulk material from the Bg horizon of a Stagnosol and in a nodule separated from the horizon. We used wet-chemical analyses and X-ray diffractometry together with microscopic techniques and nano-scale secondary ion mass spectrometry (NanoSIMS), the latter allowing for a submicrometre-scale spatial resolution. X-ray diffractometry revealed the presence of quartz, clay minerals, micas and feldspars as the dominant minerals and indicated the presence of lepidocrocite. Relative to the bulk horizon material, the nodule was strongly enriched in organic C (by a factor of 31) and pedogenic (dithionite-extractable) Fe and Mn (by factors of 2.2 and 62). We selected two regions on a thin section of the nodule for NanoSIMS investigations after studying the element distribution by scanning-electron microscopy (SEM): one was located in an almost closed pore, the other one along an elongated pore. The NanoSIMS measurements allowed a clearer distinction of Fe- and Mn-accumulation zones than SEM-EDS. The evaluation of the NanoSIMS measurements by unsupervised classification revealed that zones containing silicates and Mn oxides and the transitional zones between Fe and Mn oxides were particularly enriched in soil organic matter, while, with one exception, the pure Fe-accumulation zones did not indicate the presence of soil organic matter.