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 peritocrescen s and of fertilization with ammonium on the competition for ammonium b etween the chemolithotrophic ammonium-oxidizing species Nitrosomonas e uropaea and roots of Plantago lanceolata were studied in pots with gam ma-sterilized soil in the presence of Nitrobacter winogradskyi. Nitros omonas europaea was the weaker competitor for the available ammonium i n this study. In the planted pots, the potential nitrifying activities and the mineral N concentrations were almost zero, whereas the number s of nitrifying bacteria were reduced 200-fold in the presence of plan ts than without them. No significant effects of grazing and fertilizat ion on the nitrification rate were found in the presence of plants, as the roots were more competitive for ammonium than Nitrosomonas europa ea. 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 o n the nitrification rate. Numbers of Nitrosomonas europaea and Nitroba cter winogradskyi were not significantly different under the four trea tments without plants. However those of the former species agreed well with the observed potential ammonium-oxidizing activities. The mechan ism behind the stimulation of the nitrification process and the availa bility of ammonium in the pots to the plant roots and to the nitrifyin g bacteria are discussed. It is concluded that roots of Plantago lance olata were more competitive for limiting amounts of ammonium than Nitr osomonas europaea cells. Grazing by flagellates and fertilization with ammonium favoured the nitrification process only in the absence of pl ant roots, whereas a combination of the two treatments had an additive effect.