Influence of an Increased pH on the Composition of the Nitrate-Reducing Microbial Populations in an Anaerobically Incubated Acidic Forest Soil

Abstract

Change of soil pH greatly affects the rate and the percentual production of nitrite, NO, N2O, N2 and NH4+ 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. reducing nitrate to ammonia), and denitrifying (i.e. reducing nitrate to N2) 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 at pH 4. MPN counts of nitrate reducers at pH 7 were higher in complex versus defined mineral media. All (100%) of the nitrate reducers (n = 17) isolated at pH 4 had the properties of nitrate respirers. 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 N2O. All (100%) of a selection (n = 18) of isolated nitrate reducers were facultatively anaerobic and catalasepositive, most (72%) were Gram+ and 39% formed spores.