COMPETITION FOR AMMONIUM BETWEEN PLANT-ROOTS AND NITRIFYING AND HETEROTROPHIC BACTERIA AND THE EFFECTS OF PROTOZOAN GRAZING

The competition for limiting amounts of ammonium between the chemolith otrophic ammonium-oxidizing species Nitrosomonas europaea, the heterot rophic species Arthrobacter globiformis and roots of Plantago lanceola ta (Ribwort plantain) was studied in a series of model systems of incr easing complexity, i.e. energy-limited continuous cultures, non-water- saturated continuously percolated soil columns and pots with gamma-ste rilized soil planted with axenic P. lanceolata seedlings. The effects of bacterial grazing by the flagellate species Adriamonas peritocresce ns on the competition for ammonium were also investigated in the three model systems. It was found that N. europaea was a weaker competitor for ammonium than either A. globiformis or plant roots of P. lanceolat a. It is assumed that the heterotrophic bacteria have a higher affinit y for ammonium than the nitrifying bacteria, whereas growing plant roo ts have a greater capacity to exploit the soil for ammonium than the i mmobile nitrifying bacteria. It is not very likely that allelochemical s were involved in suppressing the nitrification process. Four reasons are given for this assumption. Presence of the flagellates strongly s timulated the potential nitrification rate in all model systems. It is assumed that there is a more even distribution over the soil of eithe r nitrifying bacteria or their substrate ammonium in the presence of f lagellates. In addition to the distribution effect, there is a stimula tion of the potential ammonium oxidation rate. The results are discuss ed in the light of the function of nitrate as nitrogen sink in the bio geochemical nitrogen cycle.