ROOT EXUDATION OF ORGANIC-ACIDS - IMPORTANCE TO NUTRIENT AVAILABILITYAND THE CALCIFUGE AND CALCICOLE BEHAVIOR OF PLANTS

Many vascular plant species are unable to colonize limestone soils and the floristic composition of adjacent limestone and acid silicate soi ls differs greatly. Low-molecular organic acids (LOAs) in root exudate s may greatly affect plant availability of nutrients and it is hypothe sized that contrasting exudation of LOAs is a major mechanism controll ing the calcicole and calcifuge behaviour of plants. Rhizosphere soil solution from two calcicole and two calcifuge species, grown in a pH-i ntermediate soil, was expelled by high-speed centrifugation. The conce ntrations of LOAs in these solutions were determined by an application of ion chromatography using a supported liquid membrane enrichment te chnique. Concentrations of dicarboxylic (mainly oxalic) and tricarboxy lic (mainly citric) acids were much higher in the soil solution of the calcicole species, whereas there was no difference in monocarboxylic (mainly lactic + acetic) acids between rhizosphere soil solutions of t he two species categories. A consistent difference in the relative mol ar proportion of mono-, di- and tricarboxylic acids was also demonstra ted among all species, indicating a species specific exudation of LOAs from plant roots. The solubilizing effect of acetic, oxalic and citri c acid and their Na-salts on Fe, Mn and phosphate in two limestone soi ls and in the pH-intermediate soil was also tested. Citric acid and/or Na-citrate were powerful solubilizers of Fe and Mn and oxalic acid an d/or Na-oxalate of phosphate, whereas acetic acid and/or Na-acetate wa s quite weak in this respect. The results from this study strongly sup port the view that high exudation rates of di- and tricarboxylic LOAs is a major mechanism controlling calcicole behaviour of plants.