Comparative aluminium speciation and quantification in soil solutions of two different forest ecosystems by ²⁷Al-NMR

A ²⁷Al-NMR method for aluminium speciation and quantification has been developed using standard solutions of aluminium complexes and has then been applied to lysimeter solutions of soils from two different forest ecosystems, a mature spruce and a mature beech stand. The data was compared to total aluminium concentrations obtained by ICP-AES and other soil data. Due to technical problems the direct spectroscopic analysis of aluminium complexes of humic and fulvic acids was not possible. Indirect determination of humic and fulvic aluminium species could be achieved by the comparison of total aluminium content determined by ICP-AES with the total amount of aluminium calculated by adding the concentrations of all complexes recorded in the ²⁷Al-NMR spectrum. Relevant low molecular complexes, such as the phytotoxic hexaaqua complex and its hydrolysis products, the equally toxic tridecameric form [AlO₄Al₁₂(OH)₂₄(H₂O) ₁₂]⁷⁺, and the nontoxic sulfate and oxalate complexes could be quantified with a very high selectivity and a sensitivity sufficient for most of the samples. The main advantage of this new method is the lack of any step which may influence the complex equilibrium in the original soil solution. It was found that samples from strongly acidified soils of the spruce stand contained significant amounts of the toxic monomeric aluminium fraction, expecting a serious damage to the root system. On the other hand, the nearly exclusive binding of aluminium to oxalate, humic and fulvic acid in the highly buffered soil from the beech stand indicated a low phytotoxic potential of the aluminium present in this soil.