The population dynamics of the chemolithoautotrophic nitrifiers Nitrosomonas europaea and Nitrobacter winogradskyi were\ud studied in gnotobiotic microcosms fed with ammonium in response to the presence or absence of the emergent macrophyte\ud Glyceria maxima and the heterotrophic denitrifying bacterium Pseudomonas chlororaphis. By subjecting the plants to different\ud day lengths, the effect of possibly limiting factors (i.e. oxygen and ammonium) on the interactions between the nitrifiers and\ud denitrifying bacterium could be analysed. The presence of the plant had no effect on the growth of nitrifiers suggesting that, in\ud addition to radial oxygen loss from the roots, other non-plant sources of oxygen (e.g. diffusion from the water layer) were\ud important for nitrification. Potential nitrifying activities were suppressed by G. maxima due to ammonium uptake by the\ud plants. Elongation of the day length in combination with the presence of G. maxima led to an increase in the number of P.\ud chlororaphis. The presence of P. chlororaphis suppressed the growth of N. winogradskyi, but the growth of N. europaea and the\ud potential nitrifying activities were not significantly affected. Potential denitrifying activities were stimulated by the plant, but\ud showed no correlations with nitrifier activities or numbers. Apparently ammonium, and not oxygen, was the limiting factor for\ud nitrification in the root zone of G. maxima. However, when the plant did not deplete the ammonium pool, P. chlororaphis could\ud repress the nitrifiers indicating the latter's poor competitive status with respect to oxygen when the presence of root exudates\ud allows for heterotrophic oxygen consumption
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