Iron-manganese nodules of soils from natural landscapes of the South of Russian Far East

The structure, composition, and specificity of microelements accumulation by the rounded nodules of soddy-brown-podzolic gleyic soils (Gleyic Luvisol (Manganiferric)) from nature reserves and a national park in the south of the Far East were studied using advanced analytical methods and noninvasive techniques. The nodules are characterized by pronounced differentiation into external (brown and ocher-brown, Fe-enriched, dense) and internal (dark brown, Mn-enriched, loose) zones. According to the Mn compounds distribution in the internal zone, two types of nodules were identified: with an undifferentiated internal zone and with a core(s). The cores contain C-enriched microzones, which are centers of Fe and Mn presipitation. The stages of the co-precipitation of Fe and Mn and the stages with predominant precipitation of one of the elements were identified in the nodules. The nodules consist of a complex of minerals inherited from soils as well as nodule-specific minerals (goethite, feroxyhyte, and birnessite). The Fe content in the nodules was on average 4 times higher than the soil content, the Mn content was 21.9 times higher, and the C content was 3.6 times higher. In the nodules, Pb accumulates most intensively (EF 5.53–12.14); its accumulation is determined by the combined participation of C- and Mn-containing compounds. Nickel (EF 0.89–5.81) and Cr (EF 1.22–2.60) accumulate less actively; V (EF 0.85–1.88) and Sr (EF 0.58–1.43) accumulate weakly. The phases accumulating Ni, Cr, V, and Sr are compounds of nodules containing Fe and C. Zinc does not accumulate in nodules. A comparison of the microelement water-soluble form concentrations indicates a decrease in the mobility of Cr, Pb, Ni, V, and Sr in nodules compared to soils.