Impaired Reduction of N(2)O to N(2) in Acid Soils Is Due to a Posttranscriptional Interference with the Expression of nosZ

Accumulating empirical evidence over the last 60 years has shown that the reduction of N(2)O to N(2) is impaired by low soil pH, suggesting that liming of acid soils may reduce N(2)O emissions. This option has not gained much momentum in global change research, however, possibly due to limited understanding of why low pH interferes with N(2)O reductase. We hypothesized that the reason is that denitrifying organisms in soils are unable to assemble functional N(2)O reductase (N(2)OR) at low pH, as shown to be the case for the model strain Paracoccus denitrificans. We tested this by experiments with bacteria extracted from soils by density gradient centrifugation. The soils were sampled from a long-term liming experiment (soil pH 4.0, 6.1, and 8.0). The cells were incubated (stirred batches, He atmosphere) at pH levels ranging from 5.7 to 7.6, while gas kinetics (NO, N(2)O, and N(2)) and abundances of relevant denitrification genes (nirS, nirK, and nosZ) and their transcripts were monitored. Cells from the most acidic soil (pH 4.0) were unable to reduce N(2)O at any pH. These results warrant a closer inspection of denitrification communities of very acidic soils. Cells from the neutral soils were unable to produce functional N(2)OR at pH values of ≤6.1, despite significant transcription of the nosZ gene. The N(2)OR expressed successfully at pH 7.0, however, was functional over the entire pH range tested (5.7 to 7.6). These observations lend strong support to our hypothesis: low soil pH diminishes/prevents reduction of N(2)O, primarily by precluding a successful assembly of functional N(2)O reductase.