Critical Aspects of Sedimentary Phosphorus Chemical Fractionation

Sequential leaching procedures have been widely used as an environmental tool to quantify sedimentary P reservoirs. Nevertheless, they still present important issues that need to be investigated such as readsorption of leached ions and organic P hydrolysis. Bottom sediments of a lagoonal environment were analyzed following some selected extraction procedures that are based on chelating agents at the same pH as the sediment (GB) or a mixture of reducing and acid extractants (AAC and SEDEX). Readsorption was estimated by washes with MgCl₂ after each extraction, and turned out to be significant in the step of the SEDEX sequence that quantifies the authigenic Ca-bound P. Here, readsorbed P accounted for up to 46% of the fraction. In contrast, in the GB sequence readsorbed P accounted for only 1.7 to 4.3% of the Fe-bound P and Ca-bound P. The AAC and SEDEX sequences yielded relative organic P contents ranging from 24.5 to 30.5% of the total P, and the GB sequence yielded values of 41.3 to 50.7%. Despite the large difference between the two sequences, the relative proportions among the samples were consistent. The difference may result from the combination of hydrolysis sedimentary organic P by some extractants (mostly citrate dithionite bicarbonate [CDB] and HCl) in the AAC and SEDEX sequences, and of incomplete dissolution of Ca-bound phases by the relevant chelating agent (Na-EDTA [ethylene dinitrilo tetracetic acid]) in the GB sequence. In fact, when detrital apatite of magmatic origin was used as an analogue phase, only about 48% of it was leached during two Na-EDTA extractions. The undissolved detrital apatite was therefore wrongly attributed to the organic P reservoir and produced values of organic P overestimated by 2.9 to 8.8%. The organic P values obtained with the different sequences, although corrected for this effect, still showed differences of 5.4 to 13.9%, that were attributed to the hydrolysis of labile organic compounds. These results point out that the best sequence to use should be chosen taking into account the peculiarities of each sequence and the contents of detrital apatite and organic matter of the sediments under study.