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The absence of nitrification in soils rich in organic matter has often been reported. Therefore, competition for limiting amounts of ammonium between the chemolithotrophic ammonium-oxidizing speciesNitrosomonas europaea and the heterotrophic speciesArthrobacter globiformis was studied in the presence ofNitrobacter winogradskyi in continuous cultures at dilution rates of 0.004 and 0.01 h−1 . Ammonium limitation ofA. globiformis was achieved by increasing the glucose concentration in the reservoir stepwise from 0 to 5 mM while maintaining the ammonium concentration at 2 mM. The numbers ofN. europaea andN. winogradskyi cells decreased as the numbers of heterotrophic bacteria rose with increasing glucose concentrations for both dilution rates. Critical carbon-to-nitrogen ratios of 11.6 and 9.6 were determined for the dilution rates of 0.004 and 0.01 h−1 , respectively. Below these critical values, coexistence of the competing species was found in steady-state situations. Although the numbers were strongly reduced, the nitrifying bacteria were not fully outcompeted by the heterotrophic bacteria above the critical carbon-to-nitrogen ratios. Nitrifying bacteria could probably maintain themselves in the system above the critical carbon-to-nitrogen ratios because they are attached to the glass wall of the culture vessels. The numbers ofN. europaea decreased more than did those ofN. winogradskyi . This was assumed to be due to heterotrophic growth of the latter species on organic substrates excreted by the heterotrophic bacteria.

Competition for Ammonium between Nitrifying and Heterotrophic Bacteria in Dual Energy-Limited Chemostats