COMPETITION FOR AMMONIUM BETWEEN NITRIFYING BACTERIA AND PLANT-ROOTS IN SOIL IN POTS - EFFECTS OF GRAZING BY FLAGELLATES AND FERTILIZATION

The effects of grazing by the soil flagellate Adriamonas peritocrescens and of fertilization with ammonium on the competition for ammonium between the chemolithotrophic ammonium-oxidizing species Nitrosomonas europaea and roots of Plantago lanceolata were studied in pots with gamma-sterilized soil in the presence of Nitrobacter winogradskyi. Nitrosomonas europaea was the weaker competitor for the available ammonium in this study. In the planted pots, the potential nitrifying activities and the mineral N concentrations were almost zero, whereas the numbers of nitrifying bacteria were reduced 200-fold in the presence of plants than without them. No significant effects of grazing and fertilization on the nitrification rate were found in the presence of plants, as the roots were more competitive for ammonium than Nitrosomonas europaea. The involvement of allelochemicals originating from plant roots is discussed and appeared to be unlikely. In the unplanted pots, larger amounts of nitrate and greater potential ammonium-oxidizing activities were found in the presence of flagellates and NH4+ fertilizer than in their absence, although ammonium was present in all unplanted pots. A combination of flagellates and fertilization had an additive effect on the nitrification rate. Numbers of Nitrosomonas europaea and Nitrobacter winogradskyi were not significantly different under the four treatments without plants. However those of the former species agreed well with the observed potential ammonium-oxidizing activities. The mechanism behind the stimulation of the nitrification process and the availability of ammonium in the pots to the plant roots and to the nitrifying bacteria are discussed. It is concluded that roots of Plantago lanceolata were more competitive for limiting amounts of ammonium than Nitrosomonas europaea cells. Grazing by flagellates and fertilization with ammonium favoured the nitrification process only in the absence of plant roots, whereas a combination of the two treatments had an additive effect.