The N 2O product ratio of nitrification and its dependence on long-term changes in soil pH

Abstract

The contribution of nitrification to the emission of nitrous oxide (N 2O) from soils may be large, but its regulation is not well understood. The soil pH appears to play a central role for controlling N 2O emissions from soil, partly by affecting the N 2O product ratios of both denitrification (N 2O/(N 2+N 2O)) and nitrification (N 2O/(NO 2 −+NO 3 −). Mechanisms responsible for apparently high N 2O product ratios of nitrification in acid soils are uncertain. We have investigated the pH regulation of the N 2O 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 (NO 3 −) was added to assess nitrification rates by pool dilution and to distinguish between N 2O from NO 3 − reduction and NH 3 oxidation. Sterilized soil slurries were used to determine the rates of chemodenitrification (i.e. the production of nitric oxide (NO) and N 2O from the chemical decomposition of nitrite (NO 2 −)) as a function of NO 2 − concentrations. Additions of NO 2 − to aerobic soil slurries (with 15N labelled NO 3 − added) were used to assess its potential for inducing denitrification at aerobic conditions. For soils with pH⩾5, we found that the N 2O 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 N 2O product ratio with increasing soil pH, but the effect was so weak that it justifies a constant N 2O product ratio of nitrification for N 2O emission models. For the soils with pH 4.1 and 4.2, the apparent N 2O 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 NO 2 −. We further found convincing evidence for NO 2 −-induction of aerobic denitrification in acid soils. The study underlines the role of NO 2 −, both for regulating denitrification and for the apparent nitrifier-derived N 2O emission.