Determination of water-soluble low-molecular-weight organic acids in soils by ion chromatography

Abstract

Low-molecular-weight organic acids (LOAs) widely occur in soils and affect the soil properties and processes. We developed a simplified and relatively rapid method to identify and quantify the water-soluble LOAs in soils by using ion chromatography with an exclusive column for organic acids, an ion exchange column for anions, and an electric conductivity detector. These combinations enabled to separate more than ten kinds of LOAs. Freeze-drying and passage through Sep-Pak Plus reverse-phase cartridge columns (Waters Corp., Milford, MA) were introduced as pretreatments to concentrate and purify the soil-water extracts. By applying this method, we investigated the vertical distribution of LOAs in two different adjacent soil profiles in Hamatonbetsu, Hokkaido, Japan. One profile consisted of a Sand-dune Podzol (Typic Haplorthods) and the other of a Sand-dune Regosol (Typic Udipsamments). Formic, lactic, oxalic, malic, and citric acids were detected in all the water-extracts from horizon-wise soil samples. In contrast, propionic, butyric, and succinic acids were not detected in any samples. Acetic acid was identified only in the surface horizons. The total amounts of LOAs ranged from 4.7 to 37.2 µmol kg-1 with a predominance of formic and oxalic acids. The total amounts of LOAs decreased with the increase in the depth in the Regosol. LOAs were abundant in the spodic Bh and Bsl horizons as well as in the surface horizon in the Podzol, suggesting that LOAs playa critical role in the podzolization process. The amounts of water-soluble LOAs were closely correlated with the soil acidity. These results indicated that further attention should be paid to the water-soluble LOAs determined by the simplified method using ion chromatography in the present study, in relation to their role in the soil formation processes and soil acidification.