Nitrogen and oxygen isotope ratios of nitrous oxide emitted from soil and 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 (metric potentials of -0.1, -1.0 and -5.0 kPa), from soil amended with sodium succinate and sodium ethanoate, and produced by pure cultures of the nitrifying bacteria Nitrosomonas europaea and Nitrosolobus multiformis, and by the denitrifying bacterium Pseudomonas putida, has been determined in laboratory experiments. N2O from all sources was depleted in the 15N and 18O isotopes relative to the conventional references [atmospheric N2 and standard mean ocean water (SMOW), respectively]. N2O from soil was depleted in 15N and 18O to increasing 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 produced by P. putida was similar to that emitted from wetter soil (matric potential of -0.1 kPa). N2O emitted from the wetter soil was enriched in 15N and 18O compared with that emitted from the drier soil. The differences in isotopic composition between N2O from the wetter and drier soil were attributed principally to isotopic fractionation during N2O reduction to N2 in the terminal step of denitrification. The effect of both sodium succinate and sodium ethanoate amendment was to increase the overall rate of N2O emission, much of which arose from denitrification as revealed by incubation in 100 kPa O2. In addition, in the sodium ethanoate amended soil N2O reduction to N2 did not occur, as revealed by incubation in 10 kPa C2H2. The N2O from the sodium ethanoate amended soil was depleted in 15N to a greater extent than the sodium succinate amended soil, which is consistent with the observation that N2O reduction to N2 leaves residual N2O relatively enriched in 15N.