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 approximate to 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 (H2Ore-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) H2Ore-PO4 should be quantified when measuring resin extractable P.