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Abstract

Change of soil pH greatly affects the rate and the percentual production
of nitrite, NO, N-2O, N-2 and NH-4+ from the reduction of nitrate. The
pH effect may be due to a change in the composition of the
nitrate-reducing bacterial community in soil. The pH of slurries of an
acidic (pH 4) soil from the Bavarian Forest was increased to pH 7 by the
addition of alkali and then incubated under anaerobic conditions for 30
h. Then, the soil slurries were assayed for the most probable number
(MPN) counts of nitrate-reducing bacteria and for the distribution of
nitrate-respiring (i.e. reducing nitrate no nitrite),
nitrate-ammonifying (i.e. redcing nitrate to ammonia), and denitrifying
(i.e. reducing nitrate to N-2) bacteria among randomly isolated
nitrate-reducing bacteria. As result, the MPN counts had increased by up
to 3 orders of magnitude in the slurries adjusted to pH 7 compared to
those left of pH 4. MPN counts had increased by up to 3 orders of
magnitude in the slurries adjusted to pH 7 compared to those left at pH
4. MPN counts of nitrate reducers at pH 7 were higher in complex versus
defined mineral media. All (1005) of the nitrate reducers (n = 17)
isolated at pH 4 had the properties of nitrate respirer. Most (86%) of
the nitrate reducers (n = 163) isolated at pH 7 were also nitrate
respirers, 15% were nitrate ammonifiers and 7% were denitrifiers. The
results indicate that the metabolic types of nitrate reducers became
more diverse upon neutralization of the acidic forest soil. However, all
of the isolated nitrate reducers were able to produce small amounts of
NO and N-2O. All (100%) of a selection (n = 18) of isolated nitrate
reducers were facultatively anaerobic and catalase-positive, most (72%)
were Gram+ and 39% formed spores.