PHOSPHORUS TRANSFORMATIONS AND AVAILABILITY UNDER CROPPING AND FERTILIZATION ASSESSED BY ISOTOPIC EXCHANGE

In fertilized agricultural soils, both fertilizer and soil P are partl y depleted by crop export, while residual fertilizer P reacts with the soil. As a result, P availability changes in complex ways that cannot be described by an available P pool. Our objective was to characteriz e the P availability in two long-term field experiments on a Mollisol and an Alfisol (near Lethbridge and Breton, Canada), by using the soil solution P (C-P) and the time-dependent isotopically exchangeable P ( EI). The relationship of C-P to E(t) was further evaluated for short-t erm reactions by incubating field samples for 1 d with increasing P ra tes. Both C-P and and were related to changes in the soil P budget res ulting from continuous cropping and wheat (Triticum aestivum L.)-fallo w rotations with and without fertilization, with lower C-P values in t he more depleted soils. In the Alfisol, field variations in pH and tex ture affected both and and C-P, obscuring relationships between the fi eld P budgets and availability parameters. In the Mollisol, one single equation described the relationship between and and C-P for both incr eases of C-P due to short-term additions and decreases due to long-ter m crop export, indicating that transformations of available P were ful ly reversible.