Measuring the contributions of nitrification and denitrification to the flux of nitrous oxide from soil

The flux of N2O from soil can be due to nitrification or denitrification. Since aerobic and anaerobic microsites can develop within the same soil aggregate, nitrification and denitrification could be occurring at the same time. The contribution of nitrification and denitrification to the flux of N2O can be studied by differentially 15N-labelling the NO3− and NH4+ pools in soils. By periodically measuring and comparing the enrichments of the N2O, NH4+ and NO3− pools, the relative importance of the two processes can be quantified. The conclusions are based on calculations which assume that the 15N atom fractions of the nitrification and denitrification pools remain uniform throughout the incubation. The initial uniformity of the denitrification pool was tested by adding a nitrification-inhibitor, C2H2, at time zero and examining the 15N-distribution of the accumulated N2O at subsequent times. If the 15N distribution in the N2O is random it originated from one source, but ifthe 15N distribution is non-random the N2O originated from two or more sources. Two soil incubation experiments were conducted. In the first experiment fresh sieved soil was incubated over 10 days at 40, 50 and 60% moisture content with (NH2)2CO (70 μmol N g−1) and KNO3 (14 μmol N g−1) differentially labelled at 10 atom% excess 15N. The headspace was sampled daily for N2O before being refreshed with normal air. Every second day the sizes and enrichments of the NH4+ and NO3− pools were determined by destructive sampling. In the second experiment the assumption that the method of addition of label created only one denitrifying pool was tested by blocking nitrification with C2H2 (10 kPa). Fresh soil was incubated for three incubation times (6, 12 and 24 h) with differentially-labelled NH4NO3 (1.46 μmol N g−1) enriched to 20 atom% excess 15N, with glucose (42 and 83 μmol C g−1) to promote denitrification. In the first experiment the enrichment of the N2O did not match either the enrichment of the NH4+ or NO3− pools, showing that N2O was being produced by nitrification and denitrification. Quantification of the fractional contributions of nitrification and denitrification showed that denitrification was the dominant process in the first 2 days, but then nitrification became the dominant process for the rest of the incubation. More N2O was produced at 50 and 60% moisture than at 40% moisture, but the relative contributions of the two processes were the same at all moisture contents. Nitrification was responsible for 70% of the N2O flux. In the second experiment examination of the isotopic composition of the N2O showed that the 15N atoms were randomly distributed throughout the molecules. The N2O therefore orginated from one denitrifying pool, confirming that our method of addition of label initially created one NO3− pool for denitrification. There seems to be no feasible way at present to test the uniformity of the nitrification pool.