CHEMICAL FRACTIONATION OF ORGANIC PHOSPHORUS IN SELECTED HISTOSOLS1

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Abstract
It is often reported that mineralization of less stable forms of organic phosphorus (Po) in Histosols contributes to the P load in drainage waters entering the Water Conservation Areas of the biologically sensitive Everglades wetlands. Consequently, a detailed evaluation of the degree of Po stability in Histosols is required, and for this purpose, a Po chemical fractionation scheme was developed. The proposed scheme entailed an acid-alkali sequence of extractions with inclusion of a microbial biomass determination. Experiments conducted to evaluate the validity of the proposed scheme included an assessment of the hydrolysis of selected Po compounds [p-nitrophenyl phosphate (PNP), D-glucose 6-phosphate (GLU), glycerophosphate (GLY), and inositol hexaphosphate -phytic acid (PA)] in the extractants proposed for the fractionation scheme (0.5 M NaHCO3, 1 M HCl, and 0.5 M NaOH). Minimal hydrolysis (<6%) of all four Po compounds was caused by NaHCO3 extraction, and approximately 40% of the PNP and the GLU was hydrolyzed during extraction with 1 M HCl and 0.5 M NaOH, respectively. However, using the proposed scheme, PNP and GLU would have been removed as readily labile Po by the preceding NaHCO3 extraction. Significant effects of various soils extraction conditions on extractable P are also reported and include acid pretreatment and soil:extractant ratio. The proposed scheme was applied to three selected Histosols representing land cultivated for sugar cane, uncultivated land under pasture, and agricultural land converted to a wetland. The distribution of readily labile, moderately labile, and nonlabile pools of organic and inorganic P in the Histosols is discussed in relation to soil and land-use characteristics.