SOIL-PH AFFECTS THE PROCESSES REDUCING NITRATE TO NITROUS-OXIDE AND DI-NITROGEN

Nitrous oxide can be produced by nitrification, denitrification or dis similatory NO3- reduction to NH4+ (DNRA), whereas N-2 can only be prod uced by denitrification. Nitrite is a common intermediate in ail three processes. high pH favouring NO2- accumulation and DNRA. During denit rification the mole fraction of N2O decreases as pH increases, but lit tle is known about the effect of soil pH on the production of N2O duri ng DNRA, and on the relative importance of nitrification, denitrificat ion and DNRA to N-gas fluxes. We investigated the processes responsibl e for the production of N2O 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 i ncubations at 20 degrees C with factorial combination of two rates of NO3- (1.0 and 4.4 mu mol N g(-1) oven-dry soil), two rates of C (20 an d 80 mu mol C g(-1) oven-dry soil), and water to attain three soil moi sture contents (50, 65, 80% WFPS). The added NO3- was labelled at 40 a tom% excess N-15. Automated isotope-ratio mass spectrometry was used t o determine the fluxes of N-2 and N2O. and the source of the N2O. The flux of N-2 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 m inimum fluxes at pH 6.0 and 8.0. The dominant source of N-2 and N2O wa s from the NO3- pool. There was evidence that DNRA was occurring as we ll as denitrification particularly at the higher pH values. At pH 8 NO 2- accumulated and there was a direct relationship between N2O flux an d NO2- concentration. The N-15 content of the NO2- pool was similar to that of the NO pool, and the NH; 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 occ urred in all treatments and contributed 23% of the N2O flux in the tre atments with the lowest contents of moisture and C. The flux due to ni trification was low (< 0.1 mu mol N g(-1) h(-1)) under these condition s and was not detectable in other treatments because of the much highe r fluxes from NO3- reduction. Many agronomic practices, such as liming , urea fertilisation and organic manure addition, result in soil pH va lues > 6.5 at microsites So DNRA as a process for N2O production may b e much more important that presently realised. The relative contributi ons of denitrification and DNRA to N2O production are impossible to qu antify using only N-15-labelled NO3-. Confirmation of simultaneous fer mentation and denitrification would require bacterial identification a nd enumeration. (C) 1998 Elsevier Science Ltd. All rights reserved.