Phosphate elution from anion‐exchange membranes in soil analysis

Abstract Resin‐extractable phosphate (PO 4 ‐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 PO 4 ‐P. We evaluated sodium bicarbonate (NaHCO 3 ) as an alternative to sulfuric acid (H 2 SO 4 ) for PO 4 ‐P elution. We found 0.5 M NaHCO 3 achieved ≈100% PO 4 ‐P recovery at higher initial PO 4 ‐P concentrations (20 and 30 mg P/L), compared to 95%–98% with 0.25 M H 2 SO 4 . For 24 soil samples representing all 12 USDA soil orders, NaHCO 3 elution yielded 9.6% higher resin extractable PO 4 ‐P than H 2 SO 4 . Additionally, the PO 4 ‐P remaining in water extract after resin removal (H 2 O re ‐PO 4 ) was consistently quantifiable, and constituted up to 46% of total extractable PO 4 ‐P. These results demonstrate (i) NaHCO 3 is a more effective eluent for resin extractable P than H 2 SO 4 , and (ii) H 2 O re ‐PO 4 should be quantified when measuring resin extractable P.