[Nitrification in soil columns subjected to continuous flow]

Nitrification in columns of soil under continuous flow of substrate was studied. The soil was previously diluted with sterile sand (1 part of soil: 9 parts of sand; w : w) and admixed with 2% CaCO3. Soil columns, 15 cm high, were contained in glass cylinders with a cross sectional area of 39.6 cm2. In the main experiment, a soil column was subjected to the continuous flow of a KNO2 solution (100 ppm NO2- - N) at a flow rate of 46.0 cm3 h-1. An exponential increase of nitrate concentration in the column effluent was observed during the first 4 days (Fig. 1), suggesting an exponential growth of NO2- oxidizers in the soil column, with an apparent generation time of 1, 2 days. At the end of this phase, an almost complete conversion of nitrite to nitrate was reached followed by an important decrease in conversion and by a partial recovery with stabilization at 69 ppm NO3- - N in the column effluent. This phenomenon was presumably due to a rapid rate of O2 demand by the microorganisms which exceeded the supply, and to the subsequent adaptation of the NO2- oxidizers to a situation in which the O2 concentration was the limiting factor. At the end of the experiment, the average population density of NO2- oxidizers in the column was 1.5. 10(7) cells cm-3. In a preliminary experiment, a column of the same soil was continuously perfused with an (NH4)2SO4 solution with 140 ppm NH+4 - N, at a constant rate of 40.8 cm3 h-1 (Fig. 2.).(ABSTRACT TRUNCATED AT 250 WORDS)