Low Molecular Weight Organic Acids and Their Aluminium Complexes in Forest Soils

Sammanfattning: The significance of low molecular weight (LMW) organic acids and their aluminium (Al) complexes in soil solutions of podzolized soils was examined. Methods for the analysis of these compounds and complexes were developed, and the impact of these substances on the weathering and podzolization processes was studied. The factors influencing Al concentrations in podzolic soils were also evaluated.A high performance liquid chromatography (HPLC) method that includes an extensive sample pretreatment procedure was developed for the determination of LMW aliphatic acids. Futhermore, a size exclusion chromatography (SEC) method was developed and used in combination with ultrafiltration for the determination of LMW organic Al complexes. A chemical equilibrium model was employed to compare the experimental results.The highest concentrations of LMW organic acids were observed in the organic (0) horizon. Citric acid was the dominant acid in the O and E horizons (20-250µM) and oxalic acid in the B horizon (3-80 µM). A number of other acids, suchas shikimic, formic, acetic, fumaric, malic, malonic, t-aconitic and lactic acid, were occasionally identified. These compounds commonly constituted 0-5% of the dissolved organic carbon (DOC) and 0-15% of the acidity in soil solutions from podzolic soils. In addition, these acids commonly bound 25-45% of the Al and 15-20% of the Fe in the O horizon. This fraction declined in the deeper layers.Weathering experiments showed the enhancing effect of organic acids on the weathering rate of primary minerals. The emperical results also demonstrate a significant interaction between pH and ligand-promoted i.e. at lower pH values, less enhancement of weathering was observed, for K-feldspars but not for plagioclases. Ultrafiltration studies suggested that LMW organic acids are important weathering agents.The present findings concern the impact of LMW organic acids on the weathering of soils, the transport of aluminium and iron in the soil profile, and the immobilisation of Al and Fe in the illuvial horizon of podzols. The experimental results support the theory that Al is immobilised in the form of imogolite-type materials after microbial decomposition of the organic ligands.