Fractionation of Inorganic Phosphate in Calcareous Lake Sediments

Abstract Inorganic orthophosphate (inorganic P) added with NH 4 F reagent to calcareous Wisconsin lake sediments was recovered in the NH 4 F (0–32%), citrate‐dithionite‐bicarbonate (18–23%), and first HCl (38–63%) reagents of an inorganic P fractionation scheme. Calcium fluoride, formed during NH 4 F extraction of calcareous materials, sorbed inorganic P, presumably resulting in the formation of a CaF 2 ‐orthophosphate complex; inorganic P was also sorbed by CaCO 3 and CaF 2 in an NaOH system. Consequently, the values of NH 4 F‐P and first NaOH‐P underestimated the amount of inorganic P released from secondary Al‐ and Fe‐containing sediment components during the NH 4 F and first NaOH extractions, whereas the values for “reductant‐soluble P” (citrate‐dithionite‐bicarbonate‐extractable) overestimated the amount of P occluded within Fe oxides, and the P extracted subsequently by HCl overestimated the amount of acid‐extractable Ca‐P present. Because of the side reactions with CaCO 3 , the NH 4 F reagent was omitted in subsequent fractionation studies. Added inorganic P sorbed from 0.1 N NaOH solution by CaCO 3 in pure form or in calcareous sediments was essentially quantitatively recovered in a subsequent citrate‐bicarbonate extraction. A proposed inorganic P fractionation scheme for calcareous materials, based on successive single extractions with NaOH, citrate‐dithionite‐bicarbonate, and HCl reagents, removed approximately 90% of the total inorganic P in the sediments. Native inorganic P released in the NaOH extraction originated from Fe‐ and Al‐bound P but not from Ca‐bound P, and P released in the subsequent citrate extraction was largely, if not wholly derived from P resorbed during the preceding NaOH extraction. Phosphorus released in the following HCl extraction was derived from Ca‐bound P. Previous inorganic P fractionation schemes have underestimated nonoccluded Fe‐ and Al‐bound P and overestimated Cabound P and, frequently, occluded Fe‐bound P in calcareous soils and sediments.