Changes in the Soil Phosphorus Content of a Long-Term Fertilization Field Trial Studied in Laboratory Incubations

The importance of different soil phosphorus (P) compounds and their transformation influenced by several soil and other factors is well established. However, the dynamics of short-term processes taking part in the long-term changes of soil P including immobilization and mobilization is still not completely documented. Laboratory incubation experiments were carried out at 10 degrees C and 40 degrees C for studying the influence of incubation on the availability of residual and freshly applied P in samples of a long-term fertilization field trial conducted on a brown forest soil (U.S. taxonomy: Orthic Eutrochrept; FAO taxonomy: Eutric Cambisol). Samples showing three levels of P resulting from 10 years of intensive P fertilization (referred as P0, P1 and P2, respectively), were collected 30 years after fertilization ceased. Available P contents of soil samples were determined using three approaches: in water (modified Murphy-Riley method), sodium bicarbonate (Olsen, pH = 8.5), and ammonium lactate (AL, pH = 3.7) extract. Changes in the amounts of P were determined after 2 and 60 days of incubation in four freshly applied new treatments with increasing additions of P: 0, 100, 500, and 1000 mg of P2O5 per kg of soil, representing agronomic and extreme P rates. From the results of our experiments, it was suggested that after 2 days of incubation, at 10 degrees C, both agronomic and extreme P rates resulted in significant increases in P content in each extract. On the other hand, after 60 days, even higher values were obtained. Decreases found in water-P values after 60 days of incubation were considerable compared to either the Olsen-P or the AL-P values, indicating the decline of water-soluble P forms and further evidence of immobilization with increasing incubation time and temperature. Correlation between water-P, Olsen-P, and AL-P values were significant at both temperatures.