Oxalate extraction and determination by ionic chromatography in calcareous soils and in mycorrhized roots environment
Piombo, G. | Babre, D. | Fallavier, P. | Cazevieille, P. | Arvieu, J.C. | Callot, G.
Secreted by mushrooms, oxalic acid plays a basic role in calcareous soils by trapping calcium (Ca) and releasing phosphorus (P). The assessment of this organic anion requires an extraction step before analysis. A theoretical study, with GEOCHEM software, emphasizes the complexing properties of oxalate towards Ca and Al at pH > 1.5, and especially for ferric iron (Fe III) at very acidic pH (< 0.5), thus complicating its quantitation; it also specifies the experimental conditions for extracting soil calcium oxalate and avoiding the Fe(III) interference. A simple and quick method is proposed for the calcium oxalate solubilization by mixing for 1 hour 5 grams of soil with boiling 1M HCl, the volume depending on sample calcareous, Al and Fe content, and adding a reducing agent (hydroxylamine) of ferric iron. A fast stirring with a H+-resin puts the resulting solution in good conditions for ionic chromatography analysis. The efficiency of this method is measured by the rate of recovery (93 to 101%) of known amounts of calcium oxalate added (0.8%) to soils having various calcareous contents (28 to 50%); it allows to detect, with a good reproducibility (VC < 5%), amounts of oxalate as low as 0.01% in 30% calcareous soil. This method has been successfully applied to calcareous soils sampled around mycorrhized roots.
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