The N2O product ratio of nitrification and its dependence on long-term changes in soil pH

The contribution of nitrification to the emission of nitrous oxide (N2O) from soils may be large, but its regulation is not well understood. The soil pH appears to play a central role for controlling N2O emissions from soil, partly by affecting the N2O product ratios of both denitrification (N2O/(N2+N2O)) and nitrification (N2O/(NO2−+NO3−). Mechanisms responsible for apparently high N2O product ratios of nitrification in acid soils are uncertain. We have investigated the pH regulation of the N2O product ratio of nitrification in a series of experiments with slurries of soils from long-term liming experiments, spanning a pH range from 4.1 to 7.8. 15N labelled nitrate (NO3−) was added to assess nitrification rates by pool dilution and to distinguish between N2O from NO3− reduction and NH3 oxidation. Sterilized soil slurries were used to determine the rates of chemodenitrification (i.e. the production of nitric oxide (NO) and N2O from the chemical decomposition of nitrite (NO2−)) as a function of NO2− concentrations. Additions of NO2− to aerobic soil slurries (with 15N labelled NO3− added) were used to assess its potential for inducing denitrification at aerobic conditions. For soils with pH⩾5, we found that the N2O product ratios for nitrification were low (0.2-0.9‰) and comparable to values found in pure cultures of ammonia-oxidizing bacteria. In mineral soils we found only a minor increase in the N2O product ratio with increasing soil pH, but the effect was so weak that it justifies a constant N2O product ratio of nitrification for N2O emission models. For the soils with pH 4.1 and 4.2, the apparent N2O product ratio of nitrification was 2 orders of magnitude higher than above pH 5 (76‰ and 14‰). This could partly be accounted for by the rates of chemodenitrification of NO2−. We further found convincing evidence for NO2−-induction of aerobic denitrification in acid soils. The study underlines the role of NO2−, both for regulating denitrification and for the apparent nitrifier-derived N2O emission.