Organic acid extraction from rhizosphere soil: effect of field-moist, dried and frozen samples

This study investigates the effect of soil treatment and storage on organic acid extraction. For this study one clayey-loamy (Typic Udochrept) and one sandy-loamy (Aquic Ustifluvent) soil were selected and used to grow Lupinus albus L. plants in a climate chamber. After 4 weeks the rhizosphere soil was sampled and divided into five portions: (a) field moist, no storage; (b) air-dried; (c) oven-dried, (d) field-moist at +4°C for 8 weeks; (e) field-moist at −20°C for 8 weeks. Organic acid extraction (1:4 w/v) was carried out for each soil portion both in water and in 10 mM NaH2PO4. Organic acid concentration was subsequently determined by reversed-phase high performance liquid chromatography (HPLC). Oxalic, fumaric, malonic and α-ketoglutaric acid were identified in the rhizosphere of both soils but the extractable concentration was significantly higher in the sandyloamy soil. For both soils NaH2PO4 extracted significantly higher organic acid concentrations than water. Oven drying increased the extractability of organic acids in both soils. Field moist samples (i.e. where no storage occurred) of the sandy-loamy soil showed a similar behaviour than −20° stored samples whereas the one of the sandy-loamy soil were more close to the air-dried samples. These results indicate that organic acid extraction strongly depends on soil storage as well as on the soil type. Sample storage seems thus to be a crucial issue for the determination of organic acids in rhizosphere soil and needs to be considered prior analysis.