RESUMEN
Numerous studies have investigated effects of long-term manure application on total phosphorus (P) and inorganic P (Pi ), but few have evaluated soil organic P (Po ). Little is known about crop management effects on Po in soils with varying minerology. In this study, sequential fractionation was used to characterize specific P forms after 25 years of broiler litter (BL) or ammonium nitrate (Con) applications to an Alabama Hartsells soil. Crops (corn [Zea mays L.], soybean [Glycine Willd.], and corn or soybean with a wheat [Triticum aestivum L.] cover crop) were under conventional tillage (CT) or no-tillage (NT). Regardless of crop, tillage, or fertilizer type, the proportion of extractable Pi was relatively stable at 21%-49% at 0-5 cm and 25%-45% at 5-10 cm. Extractable Pi ranged from 0.69 to 2.4 mg g-1 . BL increased total extractable Pi (p ≤ 0.001) at 0-5 cm and 5-10 cm. Total extractable P was influenced at 0-5 cm (p ≤ 0.006) by both tillage and fertilization type, but not at 5-10 cm or at either depth in soybean plots. Long-term BL application increased total extractable soil P at 0-5 cm. In corn systems, CT did not reduce P loading to topsoil or result in P leaching to lower soil depths, compared to NT. Soybean and soybean-wheat reduced P loading in BL plots, compared to corn and corn-wheat. Soil Po was classed in the order of monoesters > phytate and polyphosphates, where most was extractable with NaOH. BL increased extractable Po in all fractions. Care should be taken when applying BL to highly weathered soils to avoid legacy Po accumulation. Soybean rotations and cover crops could help remediate P-laden soils after repeated BL application.