Accounting for water-extractable organic phosphorus in the anion-exchange resin membrane extractions of soils

Anion-exchange membrane (AEM) extraction is widely used to assess labile (a)biotic soil phosphorus (P) pools, but the potential of AEM extraction to quantify water-extractable organic P (H₂O-P_o) remains unclear. We (1) quantified the recovery efficiency of P_o species during AEM extraction and subsequent elution by H₂SO₄ and NaHCO₃, (2) evaluated the extent of P_o hydrolysis when (a) dissolved in H₂O, H₂SO₄, or NaHCO₃ solutions and (b) eluted from AEM, and (3) evaluated an approach to measure H₂O-P_o fraction in AEM extractions that (a) minimizes P_o hydrolysis and (b) accounts for P_o retention on AEM. The AEM extracted 29–99 % of water-soluble P_o species, meaning that total P remaining after AEM removal (H₂O_re-P_t) cannot be assumed to represent the H₂O-P_o fraction. In 24 soils spanning 12 USDA orders, combining H₂O_re-P_o and AEM extractable P_o (AEM-P_o) provided a more representative estimate of H₂O-P_o. Hydrolysis of P_o species immediately following extraction (16 h) and elution (3 h) was negligible, but increased by up to 40 % after 18 mo at 24 °C for glucose-6 phosphate (G6P), adenosine-5′-monophosphate (AMP), and phosphocholine. Elution efficiency of AEM-P_o was greater for NaHCO₃ than for H₂SO₄ for model P_o species and or soils of varying total P_o concentration. These findings clarify the limitations of using H₂O_re-P_t alone to estimate H₂O-P_o, illustrate how eluent choice affects P_o quantification via hydrolysis, and demonstrate that same-day analysis, along with combining H₂O_re-P_o and AEM-P_o fractions, is needed when quantifying soil labile P_i and P_o by AEM-based methods.