Influence of drought on the production of mineral nitrogen in a typical Argiudol of the pampas

A typical Argiudol of Pergamino (Buenos Aires province) was sampled in August 1988 and April l989. The samples were air-dried at room temperature. Samples were analyzed at different times with regard to: (a) real and potential ammonification, i.e. NH4+(-N) at sampling time and after 60 days at 28 degrees C at 75% equivalent humidity (e.h.). Ammonifier counts were obtained by the most-probable-number method (MPN). (b) real and potential nitrification: determination of (NO2(-) + NllO3(-)N at sampling time and after 60 days at 28 degrees C and 75% e.h. MPN of ammonium and nitrite oxidizers were also determined. (c) Denitrification: N2 O was determined by gas chromatography and denitrifying bacteria by MPN. (d) Determination of water-stable aggregates at the beginning and end of the assay. Potential nitrification and denitrification increased as a function of the drying period, at variance with potential ammonification. Nitrifying and denitrifying bacteria appear to be able to grow under hygroscopic humidity. In both runs performed, numbers of nitrite-oxidizing bacteria were higher than that of ammonium-oxidizing bacteria. Correlation of the curves corresponding to MPN variations between denitrifiers and nitrite-oxidizers during drying was observed. This result supports the theory of cyclic reduction of NO3(-) to NO2(-) in anaerobic microsites; the latter then diffuses to aerobic sites, where it is reoxidized to NO3(-). No relationship was observed between bacterial numbers and ammonifying or nitrifying activity. It is proposed that enzymes from bacteria that die during drying could remain protected by clays or organic matter in the soil. These enzymes could act together with those present in living bacteria, thus leading to an increase of potential nitrification and denitrification.