BALANCE AND IMMOBILIZATION OF ((NH4)-N-15)2SO4 IN A SOIL AFTER THE ADDITION OF DIDIN AS A NITRIFICATION INHIBITOR

We studied the effect of a dicyandiamide-based nitrification inhibitor (Didin) on the kinetics of N transfer in soil. Incubation at various temperatures was carried out in the laboratory after adding ammonium sulfate labelled with N-15. Adding Didin increased the incorporation of mineral N in a form that could not be extracted by KCl. The temperature modified the incorporation kinetics, but after 1 year, no difference remained between the two treatments in which the inhibitor had been added, and in which the total N immobilized was 26% of the N-15 added compared with 13% in the control treatment. Losses of N-15, measured by difference, were greater in the control treatment (approximately 14%) than in the treatments containing Didin (10%). The addition of a nitrification inhibitor did not lead to the development of denitrifying microflora, the losses being mainly attributable to volatilization of NH3 encouraged by the high pH of the soil. The immobilized N was mainly found in the NH4+ form, adsorbed at the beginning of the incubation and subsequently in the amino acids. It seemed that the Didin effect on microflora was indirect, arising from longer maintenance of the tracer in NH4+ form. This hypothesis was borne out by computing the gross phenomena. The variation in results reported in the literature can therefore be attributed to modifications in immobilization kinetics.