Fractionation of Organic Phosphorus in Lake Sediments

The organic phosphorus of 10 Wisconsin lake sediments of diverse properties was fractionated on the basis of extractability and anion exchange properties. The fractionation procedure involved sequential extraction of the sediments with 1N HCl for 1 hour (HCl-Po), cold 0.3N NaOH for 16 hours (Cold NaOH-Po), and hot 0.3N NaOH for 8 hours (hot NaOH-Po) followed by anion exchange chromatography of the NaOH extracts into three fractions: fraction I, the 0.3N NaOH eluate, contained organic P that was not retained by the resin; fraction II, the 0.5M LiCl eluate, contained organic P with exchange properties similar to inorganic orthophosphate and monophosphate esters; fraction III, the 1M LiCl + 0.2N HCl eluate, contained organic P comparable to polyphosphate esters in exchange properties. Total organic P extracted by the fractionation method was comparable to that removed by the standard Mehta et al. extractants indicating that the fractionation method provided valid total organic P data as well as information on the forms of organic P present in sediments. HCl-Po constituted 6 to 17%, cold NaOH-Po 60 to 80%, and hot NaOH-Po 10 to 30% of the total organic P of the sediments. Anion exchange chromatography of the NaOH extracts showed that most of the organic P (48 to 80%) was present in fraction I, the fraction likely composed of high molecular weight humic-fulvic complexes. Fraction II contained 14 to 36% and fraction III less than 10% of the organic P in the NaOH extracts. Total inositol penta- and hexa-phosphates comprised less than 10% of the total organic P of the sediments. The total and major fractions of sediment organic P were related to organic C, total N and oxalate-extractable Al, but were not related consistently to any other sediment property or to the trophic level of the lake.