Phosphate elution from anion-exchange membranes in soil analysis

Resin-extractable phosphate (PO₄-P) is a widespread sink-based measure of soil bioavailable phosphorus (P) used in biogeochemistry and soil fertility. However, acid elution of P may hydrolyze organic P and thus compromise measurement of PO₄-P. We evaluated sodium bicarbonate (NaHCO₃) as an alternative to sulfuric acid (H₂SO₄) for PO₄-P elution. We found 0.5 M NaHCO₃ achieved ≈100% PO₄-P recovery at higher initial PO₄-P concentrations (20 and 30 mg P/L), compared to 95%–98% with 0.25 M H₂SO₄. For 24 soil samples representing all 12 USDA soil orders, NaHCO₃ elution yielded 9.6% higher resin extractable PO₄-P than H₂SO₄. Additionally, the PO₄-P remaining in water extract after resin removal (H₂O_re-PO₄) was consistently quantifiable, and constituted up to 46% of total extractable PO₄-P. These results demonstrate (i) NaHCO₃ is a more effective eluent for resin extractable P than H₂SO₄, and (ii) H₂O_re-PO₄ should be quantified when measuring resin extractable P.