Acid–Base Characteristics and Clay Mineralogy in the Rhizospheres of Norway Maple and Common Spruce and in the Bulk Mass of Podzolic Soil

Acid–base characteristics and composition of clay minerals were estimated in the rhizospheres of Norway maple (Acer platanoides) and common spruce (Picea abies) and in the corresponding bulk soil samples taken in five replicates from the AELao horizon of podzolic soils. On the plot under spruce forest, both rhizospheric and nonrhizosheric soils were found to be more acid than those on the plot with a considerable part of maple trees in the forest stand. No reliable differences in pH values were found between the maple rhizosphere and corresponding bulk soil, while the rhizospheric soil under spruce forest had significantly lower pH values as compared with the enclosing soil. The rhizospheric soil under both tree species was found to contain reliably more illite minerals in clay-sized fraction, which could be due to the intensification of illitization and physical disintegration of micas and illites in coarse fractions. Under spruce forest stand, the clay fraction in both rhizospheric and nonrhizosheric soils contained more expandable minerals and less kaolinite and illites as compared with those under maple parcel. These differences can be explained partly by the spatial variability of clay composition in the parent material (mantle sandy loam) and, partly, by the dissimilarities in the functioning of different tree species and associated microbial communities causing lower pH values in soils under spruce forest. In the maple rhizospheric soil, pedogenic chlorites were characterized by a higher degree of aluminization in comparison with the enclosing soil owing to more favorable acid–base conditions. A tendency for a deeper transformation of illites into expandable clay minerals was revealed in the spruce rhizospheric soil as compared with the bulk soil, which can be explained by a more acid reaction facilitating the mobilization of aluminum.