Microelement mobile forms in southern taiga landscapes of the Central Forest State Biosphere Nature Reserve (Russia)

We investigated the vertical and spatial distribution of chemical elements (ChEs) in four cross-sections within a catena formed in typical southern taiga on Retiosols , underlying loess  loams and carbonate moraine deposits. Catena located in the Tver' region (Russia). In plants (70 samples, 19 species) and soils (31 samples), the total content of the ChEs was determined by mass spectrometry. In soil samples, we measured pH, grain size and levels of ChE mobile fractions (exchangeable (F1), bound to organic complexes (F2) and bound to Fe and Mn hydroxides (F3).

In the A-horizons the average total concentration of Fe is 1,2%, Ti – 0,33%; Mn – 482 mg‧kg-1, Zr–292, Sr–90, Zn–39, Cr–21, Pb–21, Ni–9, Cu–8. The concentration of metal F1 diminishes in order: Fe>Mn>Sr>Zn, Pb>Ti, Cr, Ni, Cu, Co, Zr. The concentrations of F2 and F3 show the following order: Fe>Mn>>Ti, Zr, Pb>Co>Ni, Cu, Zn>Cr, Sr and Fe>Mn>Ti>Zn, Sr, Pb>Cr>Cu, Ni, Co>Zr, respectively.

In all studied Retisols, vertical distribution of the total Pb and Zr, F1 of Co, Fe, Mn, Pb and Zn, F2 of Cu, Fe, Pb and Zn, F3 of Pb accumulate in topsoil. For the total Co, Fe, Ni, Sr and Zn, F1 of Co, Cr, Cu, Mn, Pb, Zn and Zr, F2 of Co, Cr, Cu, Fe, Mn, Ni, Pb, Zn and Zr and F3 of Co, Cr, Cu, Ti, Zn, Zr the loss from the albic horizons and/or the accumulation in the argic horizons were registered.

Spatial distribution of the total concentration of ChEs increases in the A-horizon in the upper part of the catena slope position. In the A-horizons at footslope and toeslope positions, the concentration of F1 Ni, Cu, Sr and Zr, F2 Ni, Cu and Zn increases, and the concentration F2 of Co, Cr, Pb, Ti and Zn, F2 of Cr, Ti and Co, F3 of Mn, Ni, Zn, Pb, Zr decreases.

Ratios calculated on the basis of the total and mobile element content were applied to evaluate biogenic migration of ChEs with different biophilicity in the "plant-soil" system. According to soil-to-plant transfer ratios, Mn, Zn and Cd are actively involved in biological accumulation. Participation in biological accumulation of Mn and Zn was noted in many works (Avessalomova, 2007; Isachenkova, Tarzayeva, 2006, Kadata-Pendias, Szteke, 2015)

Mn and Zn have important physiological significance in plants; they actively migrate in plant tissues. Cd is not a necessary ChEs for plants but is easily absorbed by the root system and leaves (Kabata-Pendias, 2011). Cationic elements (Cd and Zn) have high mobility in the soils (Jen-How Huang, 2011). Our results indicate that in the reference forest communities, tree species play the major role in the uptake and turnover of biophilic microelements (Mn, Zn, Co) while sphagnum moss and grassy covers mostly absorb the elements with low biophilicity (Fe, Ti, Cr, Zr, Pb). Metabolic pathways carry out the absorption of Fe and Cr (Kabata-Pendias, 2011).