Study of the Transformation of Nano- and Macrodisperse MnO2 Forms in the Soil–Plant System Using Synchrotron Methods

The transformation processes, absorption mechanisms, and toxicity of nano- and macroforms of manganese oxide (MnO2) in ordinary carbonate chernozem on yellow-brown clay loams (Haplic Chernozem (Loamic, Pachic)) and spring barley (Hordeum sativum L.) plant tissues were studied in a model experiment using a combination of X-ray absorption fine structure (XAFS) spectroscopy, X-ray diffraction (XRD), and chemical analysis. The mobility of Mn compounds appears to depend on the particle size distribution. According to X-ray fluorescence (XRF) analysis, Mn concentrates in the root tissues, while the lowest content was observable in steams. XAFS analysis shows that the initial MnO2 compounds are transformed into more reactive species—Mn3O4 and MnO, which potentially increases the availability of toxic Mn2+ forms. The application of nanodisperse compounds has a negative effect on the soil–plant barrier function, thereby leading to significant Mn accumulation in barley plant organs. According to the synchrotron analysis techniques, the studied Mn species undergo the following sequence of transformations: nanoparticles/macrodisperse MnO2 (Mn4+) particles → reductive dissolution → release of Mn2+/Mn3+ ions → complexation with the formation of mobile Mn complexes available to plants.