Low molecular weight dicarboxylic acids in rhizosphere soil of durum wheat

A gas chromatographic (GC) method was developed for the determination of water extractable and HCl/MeOH extractable low molecular weight dicarboxylic acids in rhizosphere soils of durum wheat (Triticum turgidum var. Durum L.). Rhizosphere soils were collected after 2 weeks of plant growth by first removing the bulk soil from the root system and then by washing off the rhizosphere soil that adhered to the root surface with water. After shaking of the rhizosphere/water mixtures, dicarboxylic acids were concentrated on anion exchange membranes. Rhizosphere soils were then freeze-dried and the samples shaken with 0.5 M HCl in MeOH to remove any remaining dicarboxylic acids adsorbed to the soil particles after the water extraction. The efficiency of HCl/MeOH extraction from three different soils ranged from 85.9 to 106.3% for most acids; however, the recovery of oxalic acid varied with soil type. Oxalic, fumaric, and succinic acids were found in both the water and HCl/MeOH extracts of rhizosphere soils of durum wheat, while malonic acid was present only in the HCl/MeOH extracts. Dicarboxylic acids detected by GC in the water extracts were further identified by ion exclusion and ion exchange liquid chromatography. Washing soil from roots of durum wheat provided an efficient method for the collection of rhizosphere soil. Extraction of these soils with water and HCl/MeOH provided estimates of soluble and non-water-soluble low molecular weight dicarboxylic acids, respectively, using GC. Determination of these acids will aid in further understanding of their role in rhizosphere processes.