IMMOBILIZATION AND MINERALIZATION KINETICS OF A NITROGEN-FERTILIZER IN CALCAREOUS CLAYEY SOIL (RENDZINA)

A field experiment was performed on a rendzina to study the fate of a N-15 labelled nitrogenous fertilizer under spring barley cultivation. The applied nitrogen was quickly and intensively immobilized. The maximum immobilization was reached after 2 months and was higher in the cultivated soil (46.7%) than in the bare soil (38.7%). Then during a period spanning from the end of stem elongation of the barley to the end of the following winter, the mineralization of the nitrogenous fertilizer reached 5.4% in the cultivated soil against 1.7% in the bare soil. These kinetics were then studied on the same soil in a pot experiment. The soil was incubated for 1 month at 28-degrees-C after the application of a nitrogenous fertilizer labelled with N-15. The mineral nitrogen was removed prior to rye-grass cultivation. This experiment made it possible to quantify the immobilization and then the contribution to subsequent mineralization of the different organic fractions in which the nitrogenous fertilizer had been immobilized.