FRACTIONATION AND SORPTION OF INORGANIC PHOSPHORUS IN LOUISIANA CALCAREOUS SOILS.

Understanding phosphorus-soil interactions is necessary for environmentally sound management of P. The focus of this study was to characterize the P forms and sorption properties of Louisiana calcareous soils and to investigate relationships between these variables and soil matrix properties. Five mildly calcareous soils (calcium carbonate equivalent [CCE] ranged from 8.9 to 48.3 g/kg) of different geological origins at two depths were evaluated. Soil P forms were sequentially fractionated by sodium bicarbonate, sodium hydroxide, citrate bicarbonate (CB), citrate bicarbonate dithionate (CBD), and HCl. Direct extractable P by Olsen, Bray II, and Mehlich III and ammonium oxalate were also determined. Phosphorus sorption was carried out with a 1:10 soil/solution ratio, and sorption parameters were derived from best-fit Langmuir and Freundlich models to the experimental data. Norwood (Red River alluvium) and Jeanerette (loess-derived) soils were dominated at both depths by HCl-P, presumably stable Ca/Mg phosphates, whereas the Commerce (Mississippi River alluvium) and Mer Rouge (Quachita River alluvium) surface soils also contained large percentages of [NaOH+CB]-P, primarily Fe phosphates. All chemically defined sequential P fractions, except for the HCl-P, were significantly (R2 = 0.42-0.85; P = 0.03-0.0002) correlated with different Fe fractions, especially amorphous and labile Fe. The HCl-P did not correlate with carbonate content or with any other major soil matrix component. Both the Langmuir sorption maximum (b) and the Freundlich distribution coefficient (Kd) were significantly correlated with ammonium oxalate-extractable Al, clay content, and labile Ca (R^2 = 0.47-0.79; P = 0.02-0.003). Overall, this study indicates a strong influence of Fe in P chemistry of mildly calcareous soils. It also reveals the importance of labile Ca and surface Al (at exposed edges of aluminosilicate clays) for further P sorption, possibly after available sites of Fe oxides in the calcareous soils are saturated.