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

• Water-extractable organic P (P o ) was underestimated in resin extraction. • 29–99 % of water-extractable P o (H 2 O-P o ) can also be extracted by resin. • Combining P o in residual water and resin extract better reflects H 2 O-P o. • P o hydrolysis is insignificant in resin extraction and elution after 19 h. • NaHCO 3 yielded 16–48 % higher resin extractable P o recovery in elution than H 2 SO 4. 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 (H 2 O-P o ) remains unclear. We (1) quantified the recovery efficiency of P o species during AEM extraction and subsequent elution by H 2 SO 4 and NaHCO 3 , (2) evaluated the extent of P o hydrolysis when (a) dissolved in H 2 O, H 2 SO 4 , or NaHCO 3 solutions and (b) eluted from AEM, and (3) evaluated an approach to measure H 2 O-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 (H 2 O re -P t ) cannot be assumed to represent the H 2 O-P o fraction. In 24 soils spanning 12 USDA orders, combining H 2 O re -P o and AEM extractable P o (AEM-P o ) provided a more representative estimate of H 2 O-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 NaHCO 3 than for H 2 SO 4 for model P o species and or soils of varying total P o concentration. These findings clarify the limitations of using H 2 O re -P t alone to estimate H 2 O-P o , illustrate how eluent choice affects P o quantification via hydrolysis, and demonstrate that same-day analysis, along with combining H 2 O re -P o and AEM-P o fractions, is needed when quantifying soil labile P i and P o by AEM-based methods.