Soil pH affects the processes reducing nitrate to nitrous oxide and di-nitrogen

Nitrous oxide can be produced by nitrification, denitrification or dissimilatory NO3− reduction to NH4+ (DNRA), whereas N2 can only be produced by denitrification. Nitrite is a common intermediate in all three processes, high pH favouring NO2− accumulation and DNRA. During denitrification the mole fraction of N2O decreases as pH increases, but little is known about the effect of soil pH on the production of N2O during DNRA, and on the relative importance of nitrification, denitrification and DNRA to N-gas fluxes. We investigated the processes responsible for the production of N2O and N2 in one soil at pH values of 5.6, 6.0, 6.5 and 8.0. A range of aeration conditions were created by 24 h incubations at 20°C with factorial combination of two rates of NO3− (1.0 and 4.0 μmol N g−1 oven-dry soil), two rates of C (20 and 80 μmol C g−1 oven-dry soil), and water to attain three soil moisture contents (50, 65, 80% WFPS). The added NO3− was labelled at 40 atom% excess 15N. Automated isotope-ratio mass spectrometry was used to determine the fluxes of N2 and N2O, and the source of the N2O. The flux of N2 increased with pH but the effect of pH on the flux of N2O was inconsistent. The maximum flux of N2O occurred at pH 6.5 and the minimum fluxes at pH 6.0 and 8.0. The dominant source of N2 and N2O was from the NO3− pool. There was evidence that DNRA was occurring as well as denitrification particularly at the higher pH values. At pH 8 NO2− accumulated and there was a direct relationship between N2O flux and NO2− concentration. The 15N content of the NO2− pool was similar to that of the NO3− pool, and the NH4+ pool became significantly enriched. At pH 6.5 DNRA was probably also occurring, because the NH4+ pool was significantly enriched, but NO2− did not accumulate. Nitrification occurred in all treatments and contributed 23% of the N2O flux in the treatments with the lowest contents of moisture and C. The flux due to nitrification was low (<0.1 μmol N g−1 h−1) under these conditions and was not detectable in other treatments because of the much higher fluxes from NO3− reduction. Many agronomic practices, such as liming, urea fertilisation and organic manure addition, result in soil pH values >6.5 at microsites so DNRA as a process for N2O production may be much more important that presently realised. The relative contributions of denitrification and DNRA to N2O production are impossible to quantify using only 15N-labelled NO3−. Confirmation of simultaneous fermentation and denitrification would require bacterial identification and enumeration.