NUMBERS OF NITRITE-OXIDIZING BACTERIA IN THE ROOT-ZONE OF GRASSLAND PLANTS

The results of Most Probable Number determinations applying low and hi gh concentrations of nitrite reveal the presence of at least two diffe rent communities of potential nitrite-oxidizing bacteria in a number o f soil types. The effect of plant roots on these two communities was s tudied in pot experiments with soil from natural grassland in the pres ence or absence of either Festuca rubra or Plantago lanceolata. Both p lant species are dominant on the grassland soil used in this study. Pl ant roots had a stimulating effect on the numbers of nitrite-oxidizing bacteria determined with 0.05 mM nitrite in the enumeration medium as well as on the potential nitrite-oxidizing activity. On the other han d, plants roots, especially in younger plants, repressed the numbers o f nitrite-oxidizing bacteria enumerated with 5.0 mM nitrite in the cou nting medium. Pure culture studies with organotrophically grown Nitrob acter species clearly showed that this type of potential nitrite-oxidi zing bacteria could not have been responsible for the relatively high Most Probable Numbers observed in the root zones when applying 0.05 mM nitrite in the enumeration medium.