Ureolytic nitrification at low pH
Laboratory studies of ureolytic nitrification were carried out to determine whether the ability of ammonia oxidisers to hydrolyse urea could explain their persistence and activity in acid soils. Ammonia oxidising bacteria were isolated from a number of acid soils, using previously described and novel techniques, and isolates tested for their ability to hydrolyse urea. None of the 17 isolated strains were found to be ureolytic, nor were they active below pH 7, indicating the persistence of neutrophilic ammonia oxidisers in acidic soils. The failure to isolate ureolytic and acidophilic strains suggested either their absence in these soils or inadequacies with the isolation procedure. Ten strains of ammonia oxidisers, previously isolated by other workers, were also tested for ureolytic activity and two were found to be ureolytic, Nitrosospira sp. (NPAV) and Nitrosospira sp. The growth of Nitrosospira sp. (NPAV) in liquid batch culture was studied in buffered and unbuffered media revealing that, in the presence of urea, growth and activity could be maintained in media with a pH value of 4-7 whereas growth on ammonium sulphate only occurred at or above pH 7. This suggested that ureolytic strains were capable of growth and activity in acidic conditions if urea was present, providing an explanation for the nitrification in acid soils. The oxidation of urea to nitrite by cultures was incomplete and ammonium accumulated. Growth appeared to inhibited at pH 8 in some media suggesting inhibition of growth by urea in these conditions. The growth and activity of Nitrosospira sp. (NPAV) was studied in continuous flow columns at low pH. Activity could be initiated in continuous flow columns by medium containing urea at pH 4 whereas ammonia was only oxidised at or above pH 6 when medium containing ammonium sulphate was supplied. When effluent nitrite production was constant and a steady state had been established, urea was completely hydrolysed by Nitrosospira sp., causing an increase in the pH, indicating the formation of NH3.