Compositional Differences Affecting Phosphorus Dynamics In Bh and Bt Horizons.

                   Phosphorus (P) transported from agricultural fields can be a major contributor to eutrophication in aquatic systems. Loss of P via surface and subsurface movement is an environmental concern. Vertical movement of P through the soil profile results in its contact with the Bh horizons of Spodosols or the Bt horizons of Ultisols. Compositional differences between Bh and Bt horizons result in contrasting P sorption behavior and P loss risks with excess P application. Phosphorus sorption isotherms are widely used to determine potential risk of P loss to runoff or drainage. The objectives of this study were to i) determine the discrete difference between the components, organic versus inorganic and noncrystalline versus crystalline, that are associated with P retention for Bh and Bt horizons, respectively; and ii) compare the P sorption-desorption characteristics of Bh and Bt horizons. Soils were collected from 6 sites (3 Bh and 3 Bt) located within Florida. The soils were analyzed for dithionite-, oxalate- and pyrophosphate-extractable Fe and Al. The P sorption capacity of soils was determined by traditional Langmuir isotherms. Comparison of dithionite, oxalate and pyrophosphate extractable Fe and Al indicates the dominance of crystalline iron oxides in Bt horizons whereas Bh horizons were dominated with organically complexed- Fe and Al. Maximum P retention capacity, Smax was significantly correlated to dithionite extractable -Fe and -Al, oxalate extractable-Al and pyrophosphate extractable-Al for Bt horizons, whereas for Bh horizons it was significantly correlated with all forms of Al (dithionite, oxalate and pyrophosphate) but not with Fe. Although Bh and Bt horizons sorb comparable amount of P, dominance of crystalline iron oxides in Bt horizons accounts for greater P retention against desorption.