Determination of organic acids in soil extracts by ion chromatography

Low molecular-weight organic acids play a determinant role in nutrient cycling and plant growth in agricultural and forest ecosystems. Our study was made to develop a sensitive method to identify and quantify organic acids in soil extracts from the rhizosphere of quackgrass grown under greenhouse conditions. The procedure involves ion-exchange chromatographic separation using step gradient, and quantification by UV absorption for aromatic acids and by conductivity for aliphatic acids. In a single chromatographic run, 14 aliphatic acids were separated in 55 min and 14 aromatic acids in 25 min. The methods have high precision with relative standard deviations of peak areas ranging from 1.50 to 7.99% for aliphatic acids and from 0.65 to 6.37% for aromatic acids. The method for aliphatic acids is recommended for mono- or dicarboxylic acid analysis, whereas the method for aromatic acids constitutes a major improvement over previous procedures. We identified 13 organic acids in the quackgrass rhizosphere and 18 organic acids in soil containing decomposing residues of this plant. The largest concentrations were found in soils containing quackgrass residues. Aliphatic acid concentrations were generally larger than those of aromatic acids. Formic and acetic acids dominated the aliphatic group, whereas benzoic and p-hydroxybenzoic acids were found in largest amounts among aromatic acids.