Influence of parent material and soil type on the root chemistry of forest species grown on acid soils

The dissimilarities in soil pH and in aqueous Ca/Al and Mg/Al molar ratios in acid forest soils derived from either granite (GR) or amphibolite (AMP) materials are expected to have an influence on the root chemistry of forest species. Samples were taken from the A1 and A3 horizons of three GR soils (Typic Dystrudepts), under stands of Quercus robur L., Pinus radiata D. Don., and Eucalyptus globulus Labill., whereas A1 and B horizons were sampled from three nearby AMP soils (Lithic Hapludands, Andic Dystrudept) under the same tree species. Fine roots were sequentially extracted with NH4Cl (1 M) and HCl (1, 10, and 10 mM), and also digested with HNO3. The distribution of Ca, Mg, and Al at the root exchange sites was mainly related to aqueous Ca and Mg concentrations in the rhizosphere, which in turn, were related to the type of parent material. This was more evident in the deeper horizons where the effect of the mineral solid phase on the soil solution composition was more intense. The fraction of Ca-extractable with HCl comprised 36–51% of the sum of HCl and NH4Cl-extractable Ca, and that of Mg comprised 49–76% of the sum of HCl- and NH4Cl-extractable Mg. A large fraction of the Al was more strongly bound to the root in an HCl soluble form (78– 92%). Values of total Al concentration in roots (after HNO3 digestion) indicated that less Al accumulated in roots of the GR A1 horizons than in those of AMP A1 horizons. The GR A1 horizons had a more acidic (pH<4.2) and Al-undersaturated organic matter, which may have negatively affected the precipitation of Al compounds in roots. The acid forest soils studied have provided us with an opportunity to investigate the influence of a range of soil solutions under natural conditions (varying in pH, Ca/Al, and Mg/Al) and to confirm previous findings obtained under greenhouse conditions.