Legacy phosphorus in Alabama Hartsells soil after long-term amendment with broiler litter

Numerous studies have investigated effects of long-term manure application on total phosphorus (P) and inorganic P (P_i ), but few have evaluated soil organic P (P_o ). Little is known about crop management effects on P_o 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 P_i was relatively stable at 21%-49% at 0-5 cm and 25%-45% at 5-10 cm. Extractable P_i ranged from 0.69 to 2.4 mg g^-1 . BL increased total extractable P_i (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 P_o was classed in the order of monoesters > phytate and polyphosphates, where most was extractable with NaOH. BL increased extractable P_o in all fractions. Care should be taken when applying BL to highly weathered soils to avoid legacy P_o accumulation. Soybean rotations and cover crops could help remediate P-laden soils after repeated BL application.