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SOIL CHEMISTRY NOTE

Total Phosphorus Determination in Colloid-Containing Soil Solutions by Enhanced Persulfate Digestion

^a Institute of Soil Science and Land Evaluation, Biogeophysics Section, Univ. of Hohenheim, D-70593 Stuttgart, Germany
^b Department of Soil Science, Institute of Ecology, Berlin Univ. of Technology, Salzufer 11-12, D-10587 Berlin, Germany

^* Corresponding author (jan.siemens{at}tu-berlin.de).

It is unclear whether persulfate digestions completely liberate P associated with mineral colloids in soil solutions and aqueous soil extracts. We tested a modified persulfate digestion using aqueous soil extracts and suspensions of goethite, gibbsite, illite, and montmorillonite that were exposed to 24 µmol P L^–1 as orthophosphate (ortho-P) or myo-inositol hexaphosphate (IHP). Digestion was performed by autoclaving (121°C, 60 min) 5 mL of extract or suspension after the addition of 1 mL of solution, which contained 150 mmol L^–1 K₂O₈S₂ and 180 mmol L^–1 H₂SO₄. Subsequently, 0.7 mL of 188 mmol L^–1 ascorbic acid was added and the sample was heated in a water bath at 95°C for 60 min. For the samples where P was sorbed onto goethite and gibbsite, recoveries of total P were 97.2 ± 3.7% (ortho-P) and 102 ± 3.7% (IHP). For the samples where P was sorbed onto clay minerals, average total P recoveries were 99.2 ± 5.1% (ortho-P) and 106 ± 3.0% (IHP). No differences between total P concentrations measured in persulfate-digested soil extracts or with an additional digestion with HF and HNO₃ were detected.

Abbreviations: IHP, myo-inositol hexaphosphate • ortho-P, ortho-phosphate • PE, polyethylene