Fertilization Strategies Affect Phosphorus Forms and Release from Soils and Suspended Solids

The release of phosphorus from soils in surface runoff is strongly influenced by fertilizer inputs and contributes significantly to agriculturally driven eutrophication. This work evaluated the forms and availability of P in bulk soils and suspended solids (SS) produced by a water dispersion test that mimics the action of rain events and/or irrigation. This test was applied on soils cultivated with maize and fertilized with mineral N, P, and K (NPK); mineral P and K (PK); bovine slurry and P (S); or manure and P (M) for 15 yr. The P surplus in the treated soils was in the order NPK < PK < S < M. Forms and availability of P were analyzed in bulk soils, and their respective SS (< 20 mu m) by the Hedley sequential P fractionation method and the isotopic exchange kinetics. The labile forms increased according to P surplus and represented up to 15 and 25% of total P in the bulk soil and in the SS, respectively, indicating a selective enrichment of the more labile P forms in the erodible particles. Exchangeability of P from SS was rapid and intense as a result of a shift of P solution equilibrium at the increased water/solid ratio and a larger accumulation of more labile P in the detached particles than in the bulk soil. Phosphorus saturation of iron and aluminum oxides and the enrichment of fertilizer-derived P salts in the suspended solids control P forms and exchangeability for mineral fertilizer treatments, whereas in M soil carbon content assumed a key role.