EFFECTS OF GRAZING BY FLAGELLATES ON COMPETITION FOR AMMONIUM BETWEENNITRIFYING AND HETEROTROPHIC BACTERIA IN SOIL COLUMNS

The enhanced mineralization of immobilized nitrogen by bacteriophagous protozoa has been thought to favor the nitrification process in soils in which nitrifying bacteria must compete with heterotrophic bacteria for the available ammonium. To obtain more insight into this process, the influence of grazing by the flagellate Adriamonas peritocrescens on the competition for ammonium between the chemolithotrophic species Nitrosomonas europaea and the heterotrophic species Arthrobacter globi formis in the presence of Nitrobacter winogradskyi was studied in soil columns, which were continuously percolated with media containing 5 m M ammonium and different amounts of glucose at a dilution rate of 0.00 7 h-1 (liquid volumes). A. globiformis won the competition for ammoniu m. The grazing activities of the flagellates had two prominent effects on the competition between N. europaea and A. globiformis. First, the distribution of ammonium over the profile of the soil columns was mor e uniform in the presence of flagellates than in their absence. In the absence of flagellates, relatively high amounts of ammonium accumulat ed in the upper layer (0 to 3 cm), whereas in the underlying layers th e ammonium concentrations were low. In the presence of flagellates, ho wever, considerable amounts of ammonium were found in the lower layers , whereas less ammonium accumulated in the upper layer. Second, the po tential ammonium-oxidizing activity of N. europaea was stimulated in t he presence of flagellates. The numbers of N. europaea at different gl ucose concentrations in the presence of flagellates were comparable to those in the absence of protozoa. However, in the presence of flagell ates, the potential ammonium-oxidizing activities were four to five ti mes greater than those in the absence of protozoa.