[Complete nitrification in soil columns perfused with nitrite]

Four columns of the same soil were put under the continuous flow of a NaNO2 solution [50 ppm N], so as to follow the evolution of nitrification. The soil, the columns as well as the solution continuous flow regulating system, were prepared as in the previous experiments. The columns, with the exception of No1 (control), contained also the following substances: No2 and 4: 2% de CaCO3 and No3 and 4:0.0015% SO - 4-S. Since the eight day of perfusion all the effluents had an important concentration of nitrate (Figure 1), exceeding in them, the sum of [No-2-N] an [NO-3-N] (Table 1) the concentration of th NO-2-N of the perfusion solution. Finally, the [NO-2-N] was reduced importantly in the effluents, and the (NO-3N), in all of them, stabilized itself as values nearing 50 ppm. Its suggested that at the beginning of the experiment, an endogenous source of NO-2 was functioning, which could explain the observed phenomenon, and that later it ceased to do so. The populational density of NO-2 oxidizers increased, during the experiment, 49 times (Table 2). An increase was also measured in the populational density of NH+4 oxidizers, which amounted to 41 times. This later fact contributes to support the existence of an endogenous provision of NO-2 during the initial part of the experiment.