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

Abstract

Nitrous oxide can be produced by nitrification, denitrification or dissimilatory NO 3 − reduction to NH 4 + (DNRA), whereas N 2 can only be produced by denitrification. Nitrite is a common intermediate in all three processes, high pH favouring NO 2 − accumulation and DNRA. During denitrification the mole fraction of N 2O decreases as pH increases, but little is known about the effect of soil pH on the production of N 2O 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 N 2O and N 2 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 NO 3 − (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 NO 3 − was labelled at 40 atom% excess 15N. Automated isotope-ratio mass spectrometry was used to determine the fluxes of N 2 and N 2O, and the source of the N 2O. The flux of N 2 increased with pH but the effect of pH on the flux of N 2O was inconsistent. The maximum flux of N 2O occurred at pH 6.5 and the minimum fluxes at pH 6.0 and 8.0. The dominant source of N 2 and N 2O was from the NO 3 − pool. There was evidence that DNRA was occurring as well as denitrification particularly at the higher pH values. At pH 8 NO 2 − accumulated and there was a direct relationship between N 2O flux and NO 2 − concentration. The 15N content of the NO 2 − pool was similar to that of the NO 3 − pool, and the NH 4 + pool became significantly enriched. At pH 6.5 DNRA was probably also occurring, because the NH 4 + pool was significantly enriched, but NO 2 − did not accumulate. Nitrification occurred in all treatments and contributed 23% of the N 2O 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 NO 3 − 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 N 2O production may be much more important that presently realised. The relative contributions of denitrification and DNRA to N 2O production are impossible to quantify using only 15N-labelled NO 3 −. Confirmation of simultaneous fermentation and denitrification would require bacterial identification and enumeration.