NITROGEN AND OXYGEN-ISOTOPE RATIOS OF NITROUS-OXIDE EMITTED FROM SOILAND PRODUCED BY NITRIFYING AND DENITRIFYING BACTERIA

The isotopic composition at natural abundance levels of nitrous oxide emitted from a sandy loam, neutral pH soil under a range of soil water contents (matric potentials of -0.1, -1.0 and -5.0 kPa), from soil am ended with sodium succinate and sodium ethanoate, and produced by pure cultures of the nitrifying bacteria Nitrosomonas europaea and Nitroso lobus multiformis, and by the denitrifying bacterium Pseudomonas putid a, has been determined in laboratory experiments. N2O from all sources was depleted in the N-15 and O-18 isotopes relative to the convention al references [atmospheric N-2 and standard mean ocean water (SMOW), r espectively]. N2O from soil was depleted in N-15 and O-18 to increasin g extents with increasing soil water content. The isotopic composition of N2O produced by N. europaea and N, multiformis was similar to that emitted from drier soil (matric potential of -1.0 kPa) and the N2O pr oduced by P. putida was similar to that emitted from wetter soil (matr ic potential of -0.1 kPa). N2O emitted from the wetter soil was enrich ed in N-15 and O-18 compared with that emitted from the drier soil. Th e differences in isotopic composition between N2O from the wetter and drier soil were attributed principally to isotopic fractionation durin g N2O reduction to N-2 in the terminal step of denitrification. The ef fect of both sodium succinate and sodium ethanoate amendment was to in crease the overall rate of N2O emission, much of which arose from deni trification, as revealed by incubation in 100 kPa O-2. In addition, in the sodium ethanoate amended soil N2O reduction to N-2 did not occur, as revealed by incubation in 10 kPa C2H2. The N2O from the sodium eth anoate amended soil was depleted in N-15 to a greater extent than the sodium succinate amended soil, which is consistent with the observatio n that N2O reduction to N-2 leaves residual N2O relatively enriched in N-15.