Fractionation of Inorganic Phosphate in Calcareous Lake Sediments

Inorganic orthophosphate (inorganic P) added with NH₄F reagent to calcareous Wisconsin lake sediments was recovered in the NH₄F (0–32%), citrate-dithionite-bicarbonate (18–23%), and first HCl (38–63%) reagents of an inorganic P fractionation scheme. Calcium fluoride, formed during NH₄F extraction of calcareous materials, sorbed inorganic P, presumably resulting in the formation of a CaF₂-orthophosphate complex; inorganic P was also sorbed by CaCO₃ and CaF₂ in an NaOH system. Consequently, the values of NH₄F-P and first NaOH-P underestimated the amount of inorganic P released from secondary Al- and Fe-containing sediment components during the NH₄F 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 CaCO₃, the NH₄F reagent was omitted in subsequent fractionation studies. Added inorganic P sorbed from 0.1N NaOH solution by CaCO₃ 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.