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Fractionation of Inorganic Phosphate in Calcareous Lake Sediments¹

ABSTRACT

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.

¹ Contribution from the Dept. of Soil Sci. & the Water Chem. Lab., Univ. of Wis., Madison. Supported in part by Federal Water Quality Admin. Proj. no. WP-01470-01 and Office of Water Resources Res. Proj. no. 14-01-0001-1961 (B-022WIS), administered through the Univ. of Wis. Water Resources Ctr. Approved for publication by the Director of the Res. Div., College of Agr. & Life Sci., Univ. of Wis., Madison. Acknowledgment is also made of the cooperation and support of the Eng. Exp. Sta.

² Former Visiting Assistant Professor, Associate Professor, Associate Professor, and Assistant Professor, respectively. The address of the senior author is Canada Centre for Inland Waters Burlington, Ontario, Can.

Received for publication August 24, 1970. Accepted for publication December 22, 1970.