Low‐Molecular‐Weight Organic Acids in Selected Forest Soils of the Southeastern USA

Abstract Organic acids can form stable complexes with metals and therefore can affect metal solubility and speciation. The low‐molecular‐weight aliphatic organic acids extracted in water and present in soil solution from O, A, Bh and Bt horizons from a group of forested Utlisols, Entisols and Spodosols were identified by high performance liquid chromatography ( hplc ). Oxalic acid was found in all samples and was present generally in the highest concentrations. Oxalate concentrations in soil solution ranged from 25 to 1000 µ M and were greater in the Bh and Bt horizon soils than in the A horizon soil. High concentrations of formic acid were also identified in most soils, ranging from 5 to 174 µ M in soil solution. Trace amounts of citric, acetic, malic, lactic, aconitic, and succinic acids were detected in some samples. In a greenhouse pot study, the concentrations of low‐molecular‐weight organic acids in the rhizosphere of slash pine ( Pinus elliottii Engelm.) seedlings growing in A, Bh, and Bt horizons from an Ultic Haplaquod were compared with the concentrations in the nonrhizosphere soil. The observed concentrations were approximately an order of magnitude greater than in native soils. Oxalate was the only low‐molecular‐weight organic acid identified in the non‐rhizosphere soil. The suite of organic acids identified in the rhizosphere was more complex than in the bulk soil. In the rhizosphere, high concentrations of both oxalate and formate were detected, along with trace amounts of citric, acetic, and aconitic acids. Since oxalate forms stable complexes with Al, the presence of large concentrations of oxalate may affect P availability in these soils.