Oxalate distribution in soils under rhubarb (Rheum rhaponticum)

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Oxalate in soils may enhance phosphate availability, promote mineral dissolution, and increase the mobility of aluminium and heavy metal cations by complexation. Rhubarb (Rheum rhaponticum L.) has very high content of oxalate in leaves and petioles, and therefore the topsoil under rhubarb might have elevated contents of oxalate. Soil samples were collected at depths of 0–2.5 and 2.5–5 cm from 10 cm sections along 100 cm transects from rhubarb plants at four locations in Denmark, and from seven layers in a soil profile to 80 cm depth at one location. Oxalate was extracted from the soil with 0.2 M phosphate at pH 2 by reciprocal shaking for 24 h and then determined by a new fast capillary zone electrophoresis method with 300 mM KH2PO4 and 0.30 mM TTAB electrolyte adjusted to pH 7, developed and tested to analyse high-ionic-strength soil extracts. Rhubarb increases the oxalate content in soil under the leaves slightly. The average content of oxalate in the upper 0–5 cm soil was 444 µmol/kg at the Kaldred site, and 111–333 µmol/kg at the three other locations. In the soil profile, the content of oxalate decreased from 500 µmol/kg in 0–5 cm depth to 110 µmol/kg at 75–80 cm depth. No significant seasonal changes in oxalate contents were observed, while an annual variation of 100 µmol/kg could be observed at 0–2.5 cm depth. During plant decay in autumn, a slight increase in oxalate content was observed at 30 cm soil depth. In conclusion, the role of oxalate in weathering and metal transport appears to be limited in soils under rhubarb. Oxalate might stimulate microbiological growth and phosphate mobilisation in the rhizosphere, but concentrations observed are too low to impose any toxic effects to organisms.