Ammonia oxidation in laboratory liquid batch cultures of autotrophic ammonia oxidizers rarely occurs at pH values less than 7, due to ionization of ammonia and the requirement for ammonium transport rather than diffusion of ammonia. Nevertheless, there is strong evidence for autotrophic nitrification in acid soils, which may be carried out by ammonia oxidizers capable of using urea as a source of ammonia. To determine the mechanism of urea-linked ammonia oxidation, a ureolytic autotrophic ammonia oxidizer, Nitrosospira sp. strain NPAV, was grown in liquid batch culture at a range of pH values with either ammonium or urea as the sole nitrogen source. Growth and nitrite production from ammonium did not occur at pH values below 7. Growth on urea occurred at pH values in the range 4 to 7.5 but ceased when urea hydrolysis was complete, even though ammonia, released during urea hydrolysis, remained in the medium. The results support a mechanism whereby urea enters the cells by diffusion and intracellular urea hydrolysis and ammonia oxidation occur independently of extracellular pH in the range 4 to 7.5. A proportion of the ammonia produced during this process diffuses from the cell and is not subsequently available for growth if the extracellular pH is less than 7. Ureolysis therefore provides a mechanism for nitrification in acid soils, but a proportion of the ammonium produced is likely to be released from the cell and may be used by other soil organisms.

European legislation imposes tough restrictions on the quality of landfill leachate discharges, of which a major component is ammonia. Thus, there is a pressing need, particularly for oper ators who will have to meet these discharge standards, for a greater understanding of the origin and transformations of ammonia and other nitrogenous compounds from landfills. Moreover, with the concept of 'sustainability' being applied to landfilling, the industry will need to meet the challenge of accelerating the rate of refuse decomposition. Ammonia is both a potentially toxic product of refuse degradation and an essential nutrient for the bacteria responsible for this. The quantities present in municipal solid waste (MSW) are known but prediction of the amounts released during decom position is hampered by the lack of long term data on amino nia concentrations in leachate either from landfills or exper imental systems with a measured initial nitrogen content. For the treatment of ammonia from landfill regimes designed to accelerate refuse degradation, it is vital to under stand the nitrogen requirements of the degradation processes, in order to minimise potential ammonia toxicity while ensur ing sufficient concentrations to support the rate of decompo sition. The authors found little experimental evidence to sup port current views on the nitrogen transformations that occur in MSW, although new experimental data support the hypothesis that denitrification occurs in landfill sites. It is generally considered that the high ammonia concentrations in leachate provide evidence that ammonia is released from the decomposition of protein in refuse, even though the con centrations of the various nitrogenous components in refuse during decomposition are not known. Although the concen trations of nitrogenous components in leachate have been characterised, these do not necessarily reflect the degradation of MSW.