The competition for limiting amounts of ammonium between the chemolithotrophic ammonium-oxidizing species
Nitrosomonas europaea, the heterotrophic species Arthrobacter globiformis and roots of Plantago lanceolata
(Ribwort plantain) was studied in a series of model systems of increasing complexity, i.e. energy-limited continuous
cultures, non-water-saturated continuously percolated soil columns and pots with y-sterilized soil planted with
axenic P. lanceolata seedlings. The effects of bacterial grazing by the flagellate species Adriamonas peritocrescens
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. lanceolata. It is assumed that the heterotrophic bacteria have a higher affinity for ammonium than the nitrifying
bacteria, whereas growing plant roots have a greater capacity to exploit the soil for ammonium than the immobile
nitrifying bacteria. It is not very likely that allelochemicals were involved in suppressing the nitrification process.
Four reasons are given for this assumption.
Presence of the flagellates strongly stimulated the potential nitrification rate in all model systems. It is assumed
that there is a more even distribution over the soil of either nitrifying bacteria or their substrate ammonium in
the presence of flagellates. In addition to the distribution effect, there is a stimulation of the potential ammonium
oxidation rate. The results are discussed in the light of the function of nitrate as nitrogen sink in the biogeochemical
nitrogen cycle
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